Unit-IQ) What is an Operating SystemAns From the user point of view

OS should be easy to use an application Hence Operating system is an interface between user and computer hardware

when it comes to the view point of a system OS needs to ensure that system resources are utilized efficiently Hence Operating system is resource manager

Abstract view of components of computer system are as shown below

Q) OS objectives and functions

Ans OS has 3 objectives or 3 functions

1)Convenience 2) Efficiency 3) Ability to Evolve

1) Convenience The OS provides following services to users to conveniently use the system

a) Program Creation OS provides editors debuggers etc These are actually part of OS but are accessible through OS

b) Program Execution To execute a program 1) it must be loaded into memory 2) IO devices files and other resources must be initialized The OS handles all these tasks

c) Access to IO devices Each IO device has its own instructions and control signals OS takes care of these details and provides a simple interface to programmer

d) Controlled access to files

i) Provides Protection mechanisms

ii) Provides common interface to access files stored in different secondary memories (hard diskmagnetic tape) and having different file system

e) System access protects data and resources from unauthorized users Resolves conflicts in case of contention for resources

f) Error detection and response Many internal and external hardware errors software errors occur while a computer is running The OS handles these errors by ending the program retrying the operation that caused error or prints the error message to user

g) Accounting OS collects usage statistics of various resources for improving performance and future enhancements

2) Efficiency OS decides how much processor time is to be given to a process which processes must be in main memory when an IO device can be used by process controls access to use of files etc efficiently OS manages the resources like CPU memory IO devices Secondary memory efficiently

Only part of OS (called Kernel) is loaded into main memory and the rest of OS will be in hard disk

CPU executes OS code which directs the CPU which process to execute Once the process is executed CPU again runs the OS code to know what

to do next

3 Ability to evolve OS will evolve due to following reasons

i) Hardware upgrades and new types of hardware

ii) New Services

iii) Bug Fixes

Q) Evolution of OS

Ans Serial Processing (1940 ndash mid 1950)

There was no OS User directly interacted with computer hardware Single user system

Input and Output Paper tape or punched cards

Software Used Assemblers compilers linkers loaders device drivers libraries of common subroutines

Main Disadvantages Low CPU utilization high setup time

1) Simple batch system

1 User do not have direct access to machine2 Monitor controls job processing Special cards( that start with $ sign) indicate what to do User

program is prevented from performing IO

3 Resident Monitor (is part of monitor that always reside in memory) holds initial control control transfers to job and then back to monitor

4 Automatic job sequencing is done as follows User submits job to computer operator Computer operator batches the job sequentially and

places the entire job on an Input device (Ex card reader) Monitor reads one job at a time from ip device and places current job in User program Area Control is then passed to current job When the job is completed control return back to monitor

5 Monitor handles job setup and reduces setup time by batching jobs with similar requirements Job Control Language (JCL) instructions are given to monitor to execute the job Monitor reads $FTN card and loads the appropriate compiler from tape During execution of user program any input instruction causes one data card to be read After successful or unsuccessful completion of user job monitor will scan all ip cards until it

encounters next JCL card

Hardware features desirable in Batch system

1 Memory protection2 Timer3 Privileged Instruction4 Interrupts

2) Multi-programmed Batch System memory layout is as shown below

If there is enough main memory to hold OS and two programs When one job needs to wait for IO the CPU can switch to other job

This is known as multi programming or multi tasking

Additional hardware desirable are

1 Interrupt driven IO and Direct Memory Access (DMA)2 Memory management Since several jobs must be in main memory

at same time some form of memory management is required

3) Time sharing System are multi-programming systems and can handle OS gives quantum of CPU time to each user program

If lsquonrsquo users are there each user is given 1n of CPU time

Problems to be handled in time sharing system

1 Since multiple jobs are in memory they must be protected from modifying each otherrsquos data

2 File System must be protected by giving access to only authorized users

3 Contention for resources must be handled

Q) Difference between Batch Multi-programming and Time sharing

Batch Multi-programming Time sharingPrincipal Objective Maximize processor use Minimize response timeSource of instructions to OS JCL instructions provided with job Commands entered at terminal

===================================================================

Unit-II

Q) What is process A process is program in execution A process is an active entity and resides in main memory

Q) Explain structure of process in memoryA process contains

1 program code which is sometimes known as text section2 Process stack contains temporary data (Such as function parameters

return addresses and local variables)3 Heap is the memory allocated dynamically during process run time4 Data section contains global variables

Although two processes are associated with the same program they are considered as two separate execution sequences as the data heap and stack sections differ even though text sections are equivalent

Q) Explain Process statesAs a process executes it changes state Each process can be in one of the following states

1 New the process is being created2 Running Instructions are being executed3 Waiting a process is waiting for some

event to occur (such as IO completion etc)

4 Ready process is waiting to be assigned to processor

5 Terminated the process has finished execution

Q) What is PCBEach process is represented in the operating system by a process control block (PCB) or task control block It contains information about

1 Process state the state may be new ready running blocked or terminated2 Program Counter this register stores the address of next instruction to be executed3 CPU registers like accumulators stack pointers index register and general purpose

registers The values of these registers are saved if the process is interrupted4 CPU scheduling information like process priority scheduling parameters etc5 Memory management information include the values of base and limit register segment tables

page tables etc6 Accounting Information include amount of CPU used time limits job or process number etc7 IO status information like list of IO devices allocated to the process list of open files and so on

Q)Operations on Processes

1 Process Creation

Operating System is responsible for creation of new process

Reasons for a new process creation

1 When a batch job is submitted by user2 In interactive environment or in time sharing system a process is created when user logs on3 Operating system creates a process to manage printing So that user need not wait till printing

completes Here OS creates process on behalf of user4 When a process creates another process Creating process is called parent process and new process is

called child process or sub processNew process can in turn create other processes forming a tree of processesOS identifies a process by unique process identifier (or pid) which is an unique integerIn solaris Operating system at the top of the tree is Sched process with pid=0 This process can create several child processes In the below figure it creates three child processes1 Init process is the parent process for all user processes2 Pageout process 3 Fsflush process

Subprocess may obtain resources

1 Directly from OS2 Share some resources among several of its children3 Parent process partitions its resources among its children

Parent process may pass initialization data to child For ex name of the image file and name of the output device may be passed to a display process (child process)

Parent process may execute

1 concurrently with the child2 waits till some or all of the child process have terminated

Address space of child process may be

1 Duplicate copy of parent (Same program and data)2 New program loaded into it

2 Process Termination

A process terminates when it finishes executing its final statement and requests the operating system to delete it (using exit() system call in Unix and Terminate Process() in Win32 API)

All the resources of the process (open files IO buffers and physical memory) are deallocated by OS

A parent may terminate a child process for a variety of reasons

1 Child has exceeded its usage of some of the resources2 Task assigned to child is no longer required3 Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating

Q) What is cascading termination

Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating This is called Cascading Termination initiated by OS

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

e) System access protects data and resources from unauthorized users Resolves conflicts in case of contention for resources

f) Error detection and response Many internal and external hardware errors software errors occur while a computer is running The OS handles these errors by ending the program retrying the operation that caused error or prints the error message to user

g) Accounting OS collects usage statistics of various resources for improving performance and future enhancements

2) Efficiency OS decides how much processor time is to be given to a process which processes must be in main memory when an IO device can be used by process controls access to use of files etc efficiently OS manages the resources like CPU memory IO devices Secondary memory efficiently

Only part of OS (called Kernel) is loaded into main memory and the rest of OS will be in hard disk

CPU executes OS code which directs the CPU which process to execute Once the process is executed CPU again runs the OS code to know what

to do next

3 Ability to evolve OS will evolve due to following reasons

i) Hardware upgrades and new types of hardware

ii) New Services

iii) Bug Fixes

Q) Evolution of OS

Ans Serial Processing (1940 ndash mid 1950)

There was no OS User directly interacted with computer hardware Single user system

Input and Output Paper tape or punched cards

Software Used Assemblers compilers linkers loaders device drivers libraries of common subroutines

Main Disadvantages Low CPU utilization high setup time

1) Simple batch system

1 User do not have direct access to machine2 Monitor controls job processing Special cards( that start with $ sign) indicate what to do User

program is prevented from performing IO

3 Resident Monitor (is part of monitor that always reside in memory) holds initial control control transfers to job and then back to monitor

4 Automatic job sequencing is done as follows User submits job to computer operator Computer operator batches the job sequentially and

places the entire job on an Input device (Ex card reader) Monitor reads one job at a time from ip device and places current job in User program Area Control is then passed to current job When the job is completed control return back to monitor

5 Monitor handles job setup and reduces setup time by batching jobs with similar requirements Job Control Language (JCL) instructions are given to monitor to execute the job Monitor reads $FTN card and loads the appropriate compiler from tape During execution of user program any input instruction causes one data card to be read After successful or unsuccessful completion of user job monitor will scan all ip cards until it

encounters next JCL card

Hardware features desirable in Batch system

1 Memory protection2 Timer3 Privileged Instruction4 Interrupts

2) Multi-programmed Batch System memory layout is as shown below

If there is enough main memory to hold OS and two programs When one job needs to wait for IO the CPU can switch to other job

This is known as multi programming or multi tasking

Additional hardware desirable are

1 Interrupt driven IO and Direct Memory Access (DMA)2 Memory management Since several jobs must be in main memory

at same time some form of memory management is required

3) Time sharing System are multi-programming systems and can handle OS gives quantum of CPU time to each user program

If lsquonrsquo users are there each user is given 1n of CPU time

Problems to be handled in time sharing system

1 Since multiple jobs are in memory they must be protected from modifying each otherrsquos data

2 File System must be protected by giving access to only authorized users

3 Contention for resources must be handled

Q) Difference between Batch Multi-programming and Time sharing

Batch Multi-programming Time sharingPrincipal Objective Maximize processor use Minimize response timeSource of instructions to OS JCL instructions provided with job Commands entered at terminal

===================================================================

Unit-II

Q) What is process A process is program in execution A process is an active entity and resides in main memory

Q) Explain structure of process in memoryA process contains

1 program code which is sometimes known as text section2 Process stack contains temporary data (Such as function parameters

return addresses and local variables)3 Heap is the memory allocated dynamically during process run time4 Data section contains global variables

Although two processes are associated with the same program they are considered as two separate execution sequences as the data heap and stack sections differ even though text sections are equivalent

Q) Explain Process statesAs a process executes it changes state Each process can be in one of the following states

1 New the process is being created2 Running Instructions are being executed3 Waiting a process is waiting for some

event to occur (such as IO completion etc)

4 Ready process is waiting to be assigned to processor

5 Terminated the process has finished execution

Q) What is PCBEach process is represented in the operating system by a process control block (PCB) or task control block It contains information about

1 Process state the state may be new ready running blocked or terminated2 Program Counter this register stores the address of next instruction to be executed3 CPU registers like accumulators stack pointers index register and general purpose

registers The values of these registers are saved if the process is interrupted4 CPU scheduling information like process priority scheduling parameters etc5 Memory management information include the values of base and limit register segment tables

page tables etc6 Accounting Information include amount of CPU used time limits job or process number etc7 IO status information like list of IO devices allocated to the process list of open files and so on

Q)Operations on Processes

1 Process Creation

Operating System is responsible for creation of new process

Reasons for a new process creation

1 When a batch job is submitted by user2 In interactive environment or in time sharing system a process is created when user logs on3 Operating system creates a process to manage printing So that user need not wait till printing

completes Here OS creates process on behalf of user4 When a process creates another process Creating process is called parent process and new process is

called child process or sub processNew process can in turn create other processes forming a tree of processesOS identifies a process by unique process identifier (or pid) which is an unique integerIn solaris Operating system at the top of the tree is Sched process with pid=0 This process can create several child processes In the below figure it creates three child processes1 Init process is the parent process for all user processes2 Pageout process 3 Fsflush process

Subprocess may obtain resources

1 Directly from OS2 Share some resources among several of its children3 Parent process partitions its resources among its children

Parent process may pass initialization data to child For ex name of the image file and name of the output device may be passed to a display process (child process)

Parent process may execute

1 concurrently with the child2 waits till some or all of the child process have terminated

Address space of child process may be

1 Duplicate copy of parent (Same program and data)2 New program loaded into it

2 Process Termination

A process terminates when it finishes executing its final statement and requests the operating system to delete it (using exit() system call in Unix and Terminate Process() in Win32 API)

All the resources of the process (open files IO buffers and physical memory) are deallocated by OS

A parent may terminate a child process for a variety of reasons

1 Child has exceeded its usage of some of the resources2 Task assigned to child is no longer required3 Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating

Q) What is cascading termination

Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating This is called Cascading Termination initiated by OS

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

3 Resident Monitor (is part of monitor that always reside in memory) holds initial control control transfers to job and then back to monitor

4 Automatic job sequencing is done as follows User submits job to computer operator Computer operator batches the job sequentially and

places the entire job on an Input device (Ex card reader) Monitor reads one job at a time from ip device and places current job in User program Area Control is then passed to current job When the job is completed control return back to monitor

5 Monitor handles job setup and reduces setup time by batching jobs with similar requirements Job Control Language (JCL) instructions are given to monitor to execute the job Monitor reads $FTN card and loads the appropriate compiler from tape During execution of user program any input instruction causes one data card to be read After successful or unsuccessful completion of user job monitor will scan all ip cards until it

encounters next JCL card

Hardware features desirable in Batch system

1 Memory protection2 Timer3 Privileged Instruction4 Interrupts

2) Multi-programmed Batch System memory layout is as shown below

If there is enough main memory to hold OS and two programs When one job needs to wait for IO the CPU can switch to other job

This is known as multi programming or multi tasking

Additional hardware desirable are

1 Interrupt driven IO and Direct Memory Access (DMA)2 Memory management Since several jobs must be in main memory

at same time some form of memory management is required

3) Time sharing System are multi-programming systems and can handle OS gives quantum of CPU time to each user program

If lsquonrsquo users are there each user is given 1n of CPU time

Problems to be handled in time sharing system

1 Since multiple jobs are in memory they must be protected from modifying each otherrsquos data

2 File System must be protected by giving access to only authorized users

3 Contention for resources must be handled

Q) Difference between Batch Multi-programming and Time sharing

Batch Multi-programming Time sharingPrincipal Objective Maximize processor use Minimize response timeSource of instructions to OS JCL instructions provided with job Commands entered at terminal

===================================================================

Unit-II

Q) What is process A process is program in execution A process is an active entity and resides in main memory

Q) Explain structure of process in memoryA process contains

1 program code which is sometimes known as text section2 Process stack contains temporary data (Such as function parameters

return addresses and local variables)3 Heap is the memory allocated dynamically during process run time4 Data section contains global variables

Although two processes are associated with the same program they are considered as two separate execution sequences as the data heap and stack sections differ even though text sections are equivalent

Q) Explain Process statesAs a process executes it changes state Each process can be in one of the following states

1 New the process is being created2 Running Instructions are being executed3 Waiting a process is waiting for some

event to occur (such as IO completion etc)

4 Ready process is waiting to be assigned to processor

5 Terminated the process has finished execution

Q) What is PCBEach process is represented in the operating system by a process control block (PCB) or task control block It contains information about

1 Process state the state may be new ready running blocked or terminated2 Program Counter this register stores the address of next instruction to be executed3 CPU registers like accumulators stack pointers index register and general purpose

registers The values of these registers are saved if the process is interrupted4 CPU scheduling information like process priority scheduling parameters etc5 Memory management information include the values of base and limit register segment tables

page tables etc6 Accounting Information include amount of CPU used time limits job or process number etc7 IO status information like list of IO devices allocated to the process list of open files and so on

Q)Operations on Processes

1 Process Creation

Operating System is responsible for creation of new process

Reasons for a new process creation

1 When a batch job is submitted by user2 In interactive environment or in time sharing system a process is created when user logs on3 Operating system creates a process to manage printing So that user need not wait till printing

completes Here OS creates process on behalf of user4 When a process creates another process Creating process is called parent process and new process is

called child process or sub processNew process can in turn create other processes forming a tree of processesOS identifies a process by unique process identifier (or pid) which is an unique integerIn solaris Operating system at the top of the tree is Sched process with pid=0 This process can create several child processes In the below figure it creates three child processes1 Init process is the parent process for all user processes2 Pageout process 3 Fsflush process

Subprocess may obtain resources

1 Directly from OS2 Share some resources among several of its children3 Parent process partitions its resources among its children

Parent process may pass initialization data to child For ex name of the image file and name of the output device may be passed to a display process (child process)

Parent process may execute

1 concurrently with the child2 waits till some or all of the child process have terminated

Address space of child process may be

1 Duplicate copy of parent (Same program and data)2 New program loaded into it

2 Process Termination

A process terminates when it finishes executing its final statement and requests the operating system to delete it (using exit() system call in Unix and Terminate Process() in Win32 API)

All the resources of the process (open files IO buffers and physical memory) are deallocated by OS

A parent may terminate a child process for a variety of reasons

1 Child has exceeded its usage of some of the resources2 Task assigned to child is no longer required3 Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating

Q) What is cascading termination

Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating This is called Cascading Termination initiated by OS

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

===================================================================

Unit-II

Q) What is process A process is program in execution A process is an active entity and resides in main memory

Q) Explain structure of process in memoryA process contains

1 program code which is sometimes known as text section2 Process stack contains temporary data (Such as function parameters

return addresses and local variables)3 Heap is the memory allocated dynamically during process run time4 Data section contains global variables

Although two processes are associated with the same program they are considered as two separate execution sequences as the data heap and stack sections differ even though text sections are equivalent

Q) Explain Process statesAs a process executes it changes state Each process can be in one of the following states

1 New the process is being created2 Running Instructions are being executed3 Waiting a process is waiting for some

event to occur (such as IO completion etc)

4 Ready process is waiting to be assigned to processor

5 Terminated the process has finished execution

Q) What is PCBEach process is represented in the operating system by a process control block (PCB) or task control block It contains information about

1 Process state the state may be new ready running blocked or terminated2 Program Counter this register stores the address of next instruction to be executed3 CPU registers like accumulators stack pointers index register and general purpose

registers The values of these registers are saved if the process is interrupted4 CPU scheduling information like process priority scheduling parameters etc5 Memory management information include the values of base and limit register segment tables

page tables etc6 Accounting Information include amount of CPU used time limits job or process number etc7 IO status information like list of IO devices allocated to the process list of open files and so on

Q)Operations on Processes

1 Process Creation

Operating System is responsible for creation of new process

Reasons for a new process creation

1 When a batch job is submitted by user2 In interactive environment or in time sharing system a process is created when user logs on3 Operating system creates a process to manage printing So that user need not wait till printing

completes Here OS creates process on behalf of user4 When a process creates another process Creating process is called parent process and new process is

called child process or sub processNew process can in turn create other processes forming a tree of processesOS identifies a process by unique process identifier (or pid) which is an unique integerIn solaris Operating system at the top of the tree is Sched process with pid=0 This process can create several child processes In the below figure it creates three child processes1 Init process is the parent process for all user processes2 Pageout process 3 Fsflush process

Subprocess may obtain resources

1 Directly from OS2 Share some resources among several of its children3 Parent process partitions its resources among its children

Parent process may pass initialization data to child For ex name of the image file and name of the output device may be passed to a display process (child process)

Parent process may execute

1 concurrently with the child2 waits till some or all of the child process have terminated

Address space of child process may be

1 Duplicate copy of parent (Same program and data)2 New program loaded into it

2 Process Termination

A process terminates when it finishes executing its final statement and requests the operating system to delete it (using exit() system call in Unix and Terminate Process() in Win32 API)

All the resources of the process (open files IO buffers and physical memory) are deallocated by OS

A parent may terminate a child process for a variety of reasons

1 Child has exceeded its usage of some of the resources2 Task assigned to child is no longer required3 Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating

Q) What is cascading termination

Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating This is called Cascading Termination initiated by OS

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Operating System is responsible for creation of new process

Reasons for a new process creation

1 When a batch job is submitted by user2 In interactive environment or in time sharing system a process is created when user logs on3 Operating system creates a process to manage printing So that user need not wait till printing

completes Here OS creates process on behalf of user4 When a process creates another process Creating process is called parent process and new process is

called child process or sub processNew process can in turn create other processes forming a tree of processesOS identifies a process by unique process identifier (or pid) which is an unique integerIn solaris Operating system at the top of the tree is Sched process with pid=0 This process can create several child processes In the below figure it creates three child processes1 Init process is the parent process for all user processes2 Pageout process 3 Fsflush process

Subprocess may obtain resources

1 Directly from OS2 Share some resources among several of its children3 Parent process partitions its resources among its children

Parent process may pass initialization data to child For ex name of the image file and name of the output device may be passed to a display process (child process)

Parent process may execute

1 concurrently with the child2 waits till some or all of the child process have terminated

Address space of child process may be

1 Duplicate copy of parent (Same program and data)2 New program loaded into it

2 Process Termination

A process terminates when it finishes executing its final statement and requests the operating system to delete it (using exit() system call in Unix and Terminate Process() in Win32 API)

All the resources of the process (open files IO buffers and physical memory) are deallocated by OS

A parent may terminate a child process for a variety of reasons

1 Child has exceeded its usage of some of the resources2 Task assigned to child is no longer required3 Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating

Q) What is cascading termination

Parent is terminating and OS (Ex VMS) does not allow a child to continue if parent is terminating This is called Cascading Termination initiated by OS

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Q) What happens to child when parent terminates in Unix

Init process becomes the parent of all its children

Q)What is context switchWhen the PCB of the currently executing process is saved the operating system loads the PCB of the next process that has to be run on CPU This is a heavy task and it takes a lot of time

Q) Basic Concepts of threads

A thread consists of a program counter a stack and a set of registers and a thread ID Threads are also called light weight processesA process with multiple threads make a great serverThe threads share a lot of resources with other threads belonging to the same process So a context switch among threads for the same process is easy It involves switch of register set the program counter and the stack

Q) Explain two modes of CPU executionProtection of memory IO can be provided via two modes of CPU execution user mode and kernel mode

In kernel privileged supervisor mode OS has access to privileged instructions Privileged instructions can access IO devices control interrupts manipulate memory (pagetable TLB etc)

Privileged instructions are instruction that can only be executed in kernel mode

All the user level processes run in user mode Some critical operations are not allowed to be done by user processes The user processes must use system calls to perform these operations When a system call occurs OS will enter into kernel mode and accesses privileged instructions to perform the desired service to user-level process

For example for Input or Output process makes a system call telling the operating system to read or write particular area and this request is satisfied by the operating system

Q) Explain Inter process communicationAns Cooperating processes require inter process communication (IPC) mechanism to exchange data and information There are two communication models (a) Message passing (b) shared memory as shown below

(a) Shared ndash Memory Systems

1) Communicating processes must establish a region of shared memory2) Shared memory region resides in address space of the creating process

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

3) Other processes that wish to communicate using shared memory segment must attach the shared memory to their address space4) Processes can exchange information by reading and writing data in shared areas5) Shared memory systems are convenient to communicate6) Shared memory systems are faster and provides maximum speed as

i) System calls are required only to establish shared memory regionsii) Once shared memory is established all accesses are treated as routine memory access No

assistance of kernel is required

(b) Message passing systems

1 Are useful for exchanging smaller amounts of data2 Easy to implement for inter computer communication3 More time consuming than shared memory system as implemented using system calls

and need kernel intervention

To send messages communication link must exist between them Communication link can be physically or logically implemented

Different methods to logically implementing a link are

1 Direct or indirect communication2 Synchronous or asynchronous communication3 Automatic or explicit buffering

1a Direct communication

A link is established automatically between every pair of processes that want to communicate

A link is associated with exactly two processes

Addressing

i) Symmetry in addressing Sender process and receiver process must name each other to communicate Send() and Receive() primitives are as follows Send(P message) ndashsends a message to process P

Receive (Q message) ndash receive a message from process Qii) Asymmetry in addressing Only sender names the receiver process Send() and Receive()

primitives are as follows Send(P message) ndashsends a message to process P

Receive (id message) ndash receive a message from any process

Disadvantage of both types of addressing is limited modularity as changing the id of a process requires to find all the references to old id and then modify them

1b Indirect communication

1 Messages are sent received tofrom Mailboxes or ports2 Each mailbox has unique id (integer value) 3 Two processes can communicate only if the process have a shared mailbox4 A link is established between two processes if they have a shared mailbox

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

P1 P2

R1

5 A link may be associated with more than two processes6 Mailbox may owned by process or OS

a If mailbox is owned by process we can distinguish between the owner( can receive messages only) and user(can send messages only) When the processes that owns a mailbox terminates the mailbox disappears

b If mailbox is owned by OS OS must provide mechanisms to i Create a new mailbox

ii Send and receive messages through mailboxesiii Delete the mailboxiv Pass ownership to other processes

2 Synchronous or asynchronous communicationMessage passing may be either blocking(synchronous) or non-blocking(Asynchronous)

1 Blocking Send The sending process is blocked until the message is received by the receiving process or mailbox

2 Nonblocking Send The sending process sends the message and resumes operation3 Blocking Receive The receiver blocks until message is available4 Nonblocking receive The receiver retrieves either a valid message or a null

3 Automatic or explicit bufferingMessages exchanged by communicating processes reside in temporary queue Such queues can be implemented in 3 ways

i Zero Capacity Queue length =0 the link cannot have messages waiting in itsender must block until receiver receives the message

ii Bounded Capacity Queue length = finite (say n) When the queue is full the sender must block until space is available in queue

iii Unbounded Capacity Queue length = infinitehellipthe sender never blocks===============================================================================Q) what is deadlock

A set of processes is deadlocked when every process in the set is waiting for a resource that is currently allocated to another process in the set Here the process P1 is allocated resource R2 and P2 is allocated R1

P1 requires R1 and P2 requires R2

Process P1 and P2 will wait forever This situation is called deadlock

Q) What are the four conditions that are necessary for deadlock to occur

1 Mutual Exclusion - At least one resource must be held in a non-sharable mode If any other process requests this resource then that process must wait for the resource to be released

2 Hold and Wait - A process must be simultaneously holding at least one resource and waiting for at least one resource that is currently being held by some other process

3 No preemption - Once a process is holding a resource then that resource cannot be taken away from that process until the process releases it

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

4 Circular Wait - A set of processes P0 P1 P2 PN must exist such that every P[ i ] is waiting for P[ ( i + 1 ) ( N + 1 ) ]

Q)Methods for handling deadlocks

1 By using deadlock prevention and avoidance protocols system will never enter a deadlocked state1 Allow the system to enter a deadlocked state detect it and recover it2 Ignore the problem and pretend that deadlock never occurs

To make sure that the system must not enter a deadlocked state the system can use

1 Deadlock prevention 2 Deadlock avoidance

==============================================================================

Deadlock Prevention

1 Mutual Exclusion We cannot prevent deadlocks by denying the mutual exclusion condition because some resources are nonsharable (Ex Printer)

2 Hold and Wait

To make sure that the hold-and-wait condition never occurs in the system two protocols that can be used are

Protocol 1 All the resources requested must be allocated before process begins execution

Protocol2 A process can request resources only when it has none If a process requires additional resources it must release all the resources that are currently allocated

Example Consider a process that copies data from DVD drive to a file on disk sorts the file and then prints the results to a printer

If Protocol1 is used it must request the DVD drive disk file and printer at the beginning and must hold them till the end

Disadvantages

1 Starvation A process may wait forever because at least one resource that is need is always allocated to some other process Hence Starvation is possible

2 Resource Utilization is low Process will hold the printer from beginning till end even though it is used at the end

If Protocol 2 is used the process will initially request DVD drive and disk file It copies from the DVD drive to disk and then releases both the DVD drive and disk file It then requests the disk file and printer

Disadvantage There may be a chance that our data may not remain on the disk file

3 No pre-emption

To make sure that this condition does not hold the following protocol is used

Protocol If a process (say A) requests some resources

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Case 1 If resources are available then Allocate them

Case 2 if resources are allocated to some other process(say B) that is waiting for additional resources

then Preempt the desired resources from the waiting process (B) and allocate them to requesting process(A)

The process B can be restarted only when it is allocated additional resources it is requesting and takes away the resources that were given to process A

Case 3 if resources are neither available nor held by a waiting process then Process A waits

This protocol is applied to resources like CPU register and memory space as the state of the resources can be saved

4 Circular Wait

To make sure Circular Wait condition never occurs

1 Each Resource is assigned a unique integer number

2 Each Process must request resources in an increasing order of enumeration

We define a one-to-one function F R rarr N where R is the set of resource types and N is the set of natural numbers

A process has requested a resource type say Ri at the beginning

Protocol 1 After that the process can request resource type R j if and only if F (Riquestiquest j)gtF(Riquestiquest i)iquestiquest

Protocol 2 If a process requests a resource type R j it must release all the resources (say (Riquestiquest i )iquest ) whose F (Riquestiquest i)ge F (Riquestiquest j)iquestiquest

Example Let F (tape drive) =1 F(disk drive) = 5 and F(printer)=12

A process can request any number of tape drives disk drives and printers

Protocol 1If a process A has already requested disk drive now A can request only printer and cannot request tape drive

Protocol2 In order to request tape drive the process A must release the disk drive and then can request tape drive

If the above two protocols are used then the circular wait condition never occurs We can prove this by contradiction

Proof Assume circular wait exists Let the set of processes involved in the circular wait be P0 P1 helliphellip Pn where P0 is waiting for resource R0which is held by P1

P1 is allocated R0 and P1 is waiting for resource R1which is held by P2 so F (Riquestiquest0)ltF(R iquestiquest1)iquestiquest

P2 is allocated R1 and P2 is waiting for resource R2which is held by P3 so F (Riquestiquest1)ltF (Riquestiquest2)iquestiquest

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

P1 P2

R2R2

R1

Pn is allocated Rnminus1 and Pn is waiting for resource Rnwhich is held by P0 so F (Riquestiquestn)ltF(R iquestiquest0)iquestiquest

Hence by transitivity F (Riquestiquest0)ltF(R iquestiquest0)iquestiquest Hence our assumption that circular wait exists is FALSE

===============================================================================Q) Resource Allocation graphDeadlocks can be understood more clearly through the use of Resource-Allocation Graphs having the following properties

1 Resource Types are represented as square nodes on the graph Dots inside the square nodes indicate number of resources

( Ex two dots might represent two laser printers )2 Processes are represented as circles3 Request Edges - If P1 has requested R1 a directed edge from P1 to R1 is request

edge4 Assignment Edges - A directed edge from R2 to P1 indicating that resource R2 has

been allocated to process P1 and that P1 is currently holding resource R2 Note that a request edge can be converted into an assignment edge when request is granted

If a resource-allocation graph does contain cycles AND each resource contains only a single instance then a deadlock exists If a resource category contains more than one instance then cycle in the resource-allocation graph indicates the possibility of a deadlock but does not guarantee one==================================================================Q) Deadlock AvoidanceFor each resource request system can decide whether the request should be granted or not To make this decision the system must have information like

1 resources currently available2 resources currently allocated to each process3 Future requests and releases of each process4 Maximum number of resources it may need

Given this information it is possible to construct an algorithm that makes sure that the system will never enter a deadlocked stateThere are Two deadlock-avoidance algorithms They are

1 Resource-Allocation Graph Algorithm2 Bankers Algorithm

Safe State A system is in safe state if there exists a safe sequence of processes P0 P1 P2 PN such that Resource Requests for Pi lt= Resources allocated to Pi + resources held by all processes Pj where j lt i All safe states are deadlock free

Unsafe state If a safe sequence does not exist then the system is in an unsafe state which MAY lead to deadlock

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

1 Resource-Allocation Graph Algorithm Resource-allocation graphs can detect deadlocks only if number of resources of each type are one In this case unsafe states can be recognized and avoided by adding claim edges denoted by dashed

lines which point from a process to a resource that it may request in the future All the claim edges are added only at the beginning of process When a process makes a request the claim edge Pi-gtRj is converted to a request edge When a resource is released the assignment edge changes back to a claim edge This approach works by denying requests that would produce cycles in the resource-allocation graph

taking claim edges into effectConsider for example the resource allocation graph as shown

If P2 requests resource R2 then the claim edge from P1-gtR2 will be made request edge as follows

The resulting resource-allocation graph would have a cycle in it and so the request cannot be granted Q) Bankerrsquos Algorithm or Deadlock avoidance algorithm with exampleThere are 12 tape driveslet the current state of the system is as shown in the below figureProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2 9 7

Available = 12-(5+2+2) = 3Resource- Request AlgorithmNow when a request for 1 tape drive by process P2 is made we run resource-request algorithm to check whether the request must be granted or not The request is granted if the after granting the request all the processes in the system can complete For thatWe check 1 Is the request of P2 lt= need of P2

1 lt= 7 therefore TRUE2 Is the request of P2 lt= Available

1 lt= 3 therefore TRUE3 Pretend the request is granted for P2

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Now the current state is as shown belowProcess Allocated Max Need Need= MaxNeed ndash AllocatedP0 5 10 5P1 2 4 2P2 2+1=3 9 7-1=6

Available = 3-1= 2Now Run the safety algorithm to check whether the system is in safe state

Safety Algorithm1 Let WORK = Available = 22 Find unfinished process such that Need of unfinished process lt= WORKCheck P0

Need of P0 = 5Work = 2

Is 5 lt= 2 FALSE

Check P1Need of P1 = 2Work = 2

Is 2 lt= 2TRUETherefore P1 can finishIf P1 finishes Work = Work+Allocated to P1

Work=2+2= 4Now again check if P0 can completeNeed of P0 = 5Work =4Is 5 lt=4 FALSE

Check if P3 completesNeed of P3= 6Work =4Is 6lt= 4FALSESo neither P0 can complete and P3 can completeSo the system is in unsafe stateThe request for 1 tape drive by P3 is not granted

2 Bankers Algorithm For resources that contain more than one instance the resource-allocation graph method does not

work So we use Bankerrsquos algorithm When a process starts up it must state in advance the maximum allocation of resources it may

request When a request is made the scheduler determines whether granting the request would leave the

system in a safe state If not then the process must wait until the request can be granted safely The bankers algorithm relies on several key data structures ( where n is the number of processes and

m is the number of resource categories ) o Available[ m ] indicates how many resources are currently available o Max[ n ][ m ] indicates the maximum demand of each process of each resource

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

o Allocation[ n ][ m ] indicates the number of each resources allocated to each processo Need[ n ][ m ] indicates the remaining resources needed of each type for each process ( Note

that Need[ i ][ j ] = Max[ i ][ j ] - Allocation[ i ][ j ] for all i j ) For simplification of discussions we make the following notations observations

o One row of the Need vector Need[ i ] can be treated as a vector corresponding to the needs of process i and similarly for Allocation and Max

Safety Algorithm In order to apply the Bankers algorithm we first need an algorithm for determining whether or not a

particular state is safe This algorithm determines if the current state of a system is safe according to the following steps

1 Let Work and Finish be vectors of length m and n respectively Work is a working copy of the available resources Finish is a vector of booleans indicating whether a particular process can finish Initialize Work = Available and Finish to false for all elements

2 Find an i such that both (A) Finish[ i ] == false and (B) Need[ i ] lt Work This process has not finished but could with the given available working set If no such i exists go to step 4

3 Set Work = Work + Allocation[ i ] and set Finish[ i ] to true This corresponds to process i finishing up and releasing its resources back into the work pool Then loop back to step 2

4 If finish[ i ] == true for all i then the state is a safe state because a safe sequence has been found

Resource-Request Algorithm ( The Bankers Algorithm ) Now we have a tool for determining if a particular state is safe or not This algorithm determines if a new request is safe and grants it only if it is safe to do so When a request is made ( that does not exceed currently available resources )

pretend it has been granted and then see if the resulting state is a safe one If so grant the request and if not deny the request as follows

1 Let Request[ n ][ m ] indicate the number of resources of each type currently requested by processes If Request[ i ] gt Need[ i ] for any process i raise an error condition

2 If Request[ i ] gt Available for any process i then that process must wait for resources to become available

else the process can continue to step 3

3 Check to see if the request can be granted safely by pretending it has been granted and then seeing if the resulting state is safe If resulting state is safe

grant the requestelse

then the process must wait until its request can be granted safelyThe procedure for granting a request ( or pretending to for testing purposes ) is

Available = Available - Request

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Allocation = Allocation + Request Need = Need - Request

Unit III Memory management

Just as processes share the CPU they also share physical memory Memory management unit of OS takes care of memory allocation deallocation and other issues Program must be brought into memory for it to be run Addresses are two types

i) relocatable or relative address wrt to the beginning of program ii) Absolute addresses

Q)Address BindingAns Binding means mapping logical address space to physical address space Address binding can happen at three different stagesCompile time If you know at compile time only

where in memory the program is going to be allocated then compiler generates absolute addresses

Otherwise compiler generates relocatable addresses

Load time loader binds the relocatable addresses generated by compiler to absolute address Hence binding is done at load timeIf Binding is done at compile and load time physical and logical addresses are same

Execution time If address binding is done at run time the process can be moved during its execution from one memory segment to another Here we call logical addresses as virtual addressRun time mapping of virtual to physical address is done by a hardware device called Memory Management Unit (MMU)

Q)Logical vs Physical Address SpaceAnsLogical address ndash generated by the CPU also referred to as virtual addressPhysical address ndash address seen by the memory unitSet of all logical addresses is called logical address space Set of all physical addresses is called physical address space

Q) Memory-Management Unit (MMU)Ans MMU is hardware device that maps virtual to physical addressIn MMU scheme the value in the relocation register is added to every address generated by CPU to read the contents of memoryThe user program only knows logical addresses it never sees the real physical addresses

Q)Dynamic loadingAns Since physical memory is small it may not be possible for the entire program to be in main memory so we can use dynamic loadingIt is responsibility of users to design their programs to take advantage of dynamic loadingOnly the main function is loaded into main memory and when main() calls another function the main() will check whether the function is in main memory IF not the loader will load the desired function to main memory and updates the programrsquos address table

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Routine is not loaded until it is called We achieve better memory-space utilization as unused routine is never loaded

Q) Dynamic Linking+ Linking postponed until execution time+ Small piece of code stub used to locate the appropriate memory-resident library routine+ Stub replaces itself with the address of the routine and executes the routine+ Operating system needed to check if routine is in processesrsquo memory address+ Dynamic linking is particularly useful for libraries

Q) OverlaysAns Overlays are needed when process is larger than amount of main memory allocated to it Instructions and data that are needed at any given time is kept in main memory Overlays can be implemented by user Programming design of overlay structure is complex Overlays for a two-pass assembler is as shown in the figure

Q) SwappingAns A process can be swapped temporarily out of memory to a backing store and then brought back into memory for continued executionBacking store ndash fast disk large enough to store copies of all memory images of all usersPriority based scheduling uses a variant of swapping policy called roll out roll in If a higher priority process arrives the memory manager swaps out lower priority process and then swaps in higher priority process When the higher priority process finishes the lower priority process can be swapped in again to main memory A process that is swapped out will be swapped back into the same memory space it occupied previouslyMajor part of the swap time is transfer time We can swap idle process only and cannot swap a process that is waiting for IO

Q) Contiguous memory allocation Ans Each process is contained in a single contiguous section of memory1 Fixed Size Partition (or) Single-partition allocation

Divide the main memory into Fixed sized partitions Each partition may contain exactly one process Relocation register contains value of starting physical address Limit register contains range of logical addressesEvery address generated by a CPU is checked as follows

If logical address lt Limit register then logical address is added with Relocation register to get the corresponding memory address

else a trap to OS is generatedSince every address is checked we can protect the OS and other user programs from being modified by other running process

2 Multiple-partition allocation

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

1) Fixed Size Partitions Divide the main memory into Fixed sized blocks Here memory allocated to a process may be larger than required The difference between allocated memory and requested memory is called internal fragmentation Internal fragmentation is unused memory inside the partition

0 P1 Block0(0th address to 3rd address)4 P1 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 Block3(11th address to 12th address)

if P1 requires just 5 addresses still it is allocated 2 blocks ie 8 addresses So remaining 3 addresses are left unused

2) Variable Size partitions Here main memory is divided into partitions of variable sizesOperating system maintains information about

a) allocated partitions b) free partitions (hole)

In the beginning all main memory is empty and is considered one large block of available memory a hole

Exact memory required by process is only given When a process terminates it releases memory which can be allocated to another process

Memory is allocated to processes until finally no available block of memory (or hole) is large enough to hold the next processExternal fragmentation exists when there is enough memory to satisfy a request but the available memory are not contiguous Wastage of memory outside the partitionOne Solution to problem of external fragmentation is

a Compaction shuffle the memory contents so as to place all free memory together into one large block

===============================================================================Q) Dynamic Storage-Allocation Problem (or) Most commonly used strategies to select a free hole from the set of available holesAns To satisfy a request of size n from a list of free holes below 3 policies can be used

1 First-fit Allocate the first hole that is big enough

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

2 Best-fit Allocate the smallest hole that is big enough must search entire list if it is not ordered by size Produces the smallest leftover hole

3 Worst-fit Allocate the largest hole must also search entire list Produces the largest leftover holeFirst-fit and best-fit better than worst-fit in terms of speed and storage utilization

===============================================================================Q) Non-Contiguous Memory allocation

1 Paging Paging is a memory management scheme that provides non-contiguous memory allocation0 P1 Block0(0th address to 3rd address)4 P2 Block1(4th address to 7th address)8 Block2(8th address to 11th address)

12 P1 Block3(11th address to 12th address)

Logical address space of a process can be noncontiguous Ex P1 is allocated Block0 and Block3

1 Divide physical memory into fixed-sized blocks called frames (size is power of 2)2 Divide logical memory into blocks of same size called pages3 Logical address is divided into 2 parts

Page number (p) ndash Page table is indexed by page numberPage offset (d) ndash

4 A page table is allocated to each process (A pointer to page table is stored in PCB of process )Page table translates logical to physical addresses Page 0 is in frame1 Page 1 is in frame 4 etc5 Internal fragmentation may occur due to paging

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

6 If the size of logical address space= 2m and page size = 2n Then the higher order mminusn bits of logical address correspond to page number and lsquonrsquolower order bits for displacement within the page

Ex size of logical address space = 8=23 so m=3

page size =4=22 So n=2

So mminusn = 1 bit for page number to represent 01

remaining n=2 bits for displacement within the page

Q) Implementation of Page Table

Page table can be kept as

1) Set of dedicated registers efficient if the page table is small

2) Page table can be kept in memory page table base register (PTBR) points to page table Page-table length register (PTLR) indicates size of the page table Changing page table requires changing only the value in these registers Advantages of keeping page table in memorya Less context switch time

b Two memory accesses are required One memory access to access the page table and another memory access to access the required memory address

This problem can be solved by using a special fast-lookup hardware cache called associative memory or translation look-aside buffers(TLBs)

i Each entry in TLB consists of 2 parts

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

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Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

I) page number field

II) Value field or frame number field

ii When a logical address is generated by CPU its page number is presented to TLB The page number is compared with all the entries of TLB simultaneously

If page number is found in TLB Itrsquos frame number is immediately available -gt TLB hitIf page number not found check in page table ndashTLB miss

By using TLB Search is fast But TLB hardware is expensive So TLB size is kept small So TLB contains only frequently used few page table entries

Q) Protection in Paging Validinvalid bit attached to each entry in the page table OS sets this validinvalid bit for each page to allow or disallow access to the page When the bit is set to invalid the page is not in processrsquos logical address space Hence it generates Trap to OS

Q) Shared pages in Paging

Reentrant code is the code that never changes during execution Reentrant code can be shared

1 One copy of read-only (reentrant) code is shared among processes (ie text editors compilers window systems) Ex ed1ed2ed3 is shared among Process P1 and P2

2 Shared code must appear in same location in the logical address space of all processes

Each process keeps a separate copy of the private code and data EX data1 and data2

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

The pages for the private code and data can appear anywhere in the logical address space

==============================================================================Q) Segmentation

Segmentation is a memory management scheme that support userrsquos view of memory

When the user program is compiled the compiler generates segments like

1) The code segment

2) Global variables segment

3) Heap memory segment

4) Stack segment etc

Each entry in segment table has

segment base (Starting physical address of segment) and limit register (specifies the length of the segment)

Logical address is divided into 2 parts

segment-number offset

Segment table is indexed by segment number

Segment-table base register (STBR) stores the location of segment table in main memory

Segment-table length register (STLR) stores number of segments used by a program

The segment number (say lsquosrsquo) is used to find the entry in segment table The required entry is lsquosrsquo locations away from the beginning of the segment table Once the required entry in the segment table is found the offset (lsquodrsquo) is compared with limit

If( offset lt limit) then offset is added with the base entry to generate the physical address

Q) Shared Segments Code sharing occurs at

the segment level Shared segments must

have same segment number

Allocation - dynamic storage allocation problem

use best fitfirst fit may cause external fragmentation

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Protection protection bits associated with segments

readwriteexecute privileges array in a separate segment - hardware can check for illegal array indexes

===============================================================================Q) Virtual memory ndash

Ans In paging and segmentation a program will execute only if the entire process is in main memory But here in virtual memory only part of the program needs to be in memory for execution

1 A program that is large enough than the available main memory can still run as only part of program is only loaded into main memory Logical address space can therefore be much larger than physical address space

2 Allows address spaces to be shared by several processes

Virtual memory can be implemented via Demand paging Demand segmentation

The large blank space between heap and stack is part of virtual address space Virtual memory allows files and memory to be shared by 2 or more processes through page sharing as shown in below figure (b) Figure (a) depicts that virtual memory that is larger than physical memory

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

==============================================================================

Q)Demand pagingDemand paging is technique of loading pages from disk to main memory only when the page is needed Hence using less amount of physical memory we get faster response

Demand paging is similar to PAGING + SWAPPING

Instead of swapping a whole process the pager brings only those pages needed into memory

To distinguish between pages that are in memory and pages that are on the disk validinvalid bit is used

Validinvalid bit is attached to each entry in page table When this bit is set to valid the page is legal and in memory When this bit is set to invalid either the page is illegal or the page is in disk

When CPU generates a logical address whose page table entry is set to invalid page fault occurs Hence the required page is in disk Some section of hard disk called swap space is used to hold pages that are not present in memory

Q) Procedure for Handling a Page Fault

1) CPU generates a logical address and if the validinvalid bit for the page that has this logical address is set to invalid page fault occurs

2) Page fault causes a trap to operating system

3)Check if the logical address is within the logical address space of process ie PTBR and PTLR is checked if (required logical address is not within the logical address space of process)

Terminate the processelse

page is not in memory and page is in disk

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

4) To bring the required page into memory we need to find a free main memory frame map main memory address to disk block and fetch disk block and load the block into free frame5) When the required page is brought into memory update the page table to indicate the page is in memory6) Restart instruction interrupted by illegal address trap The process will continue as if page had always been in memory

Q) What is pure demand pagingAns If no page belonging to the executing process is in main memory the process will fault for every page it needs Page faults will occur until every page that is needed is in memory This scheme is known as pure demand paging NEVER BRING A PAGE UNTIL IT IS REQUIRED

Q) Page Replacement1 Find the location of the desired page on the disk2 Find a free

i) If there is a free frame use itii) If there is no free frame use Page replacement Algorithms to find some page in memory that is

not really in use and swap itiii) If the modify bit associated with the victim frame is set to 1 the page is written to disk Make the

changes to validinvalid bit of victim frames page table entry to invalid to indicate the victim frame is no longer in memory

else no need to write to disk3 Read the desired page into victim frame and change the frame and page tables4 Restart the user process

=================================================================================================Q) Page Replacement AlgorithmsGoal Produce a low page-fault rateThe algorithms are evaluated by running it on a particular string of memory references (reference string) and by computing the number of page faults on that string

1 First in First out (FIFO) Page replacement algorithm Uses the time when a page was brought into memory The page that was brought first into memory is replaced ie oldest page is chosen for replacement We create a FIFO queue to hold all pages in memory

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

When a page is brought into memory we insert it at the tail of the queue and we replace the page at the head of the queue

FIFO 15 page faultsProblems due to FIFO is Beladyrsquos anamolyFor a refrence string shown below123412512345When number of frames allocated to a process is 1 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 2 number of page faults=12 due to FIFO page replacementWhen number of frames allocated to a process is 3 number of page faults=9 due to FIFO page replacementWhen number of frames allocated to a process is 1 number of page faults=10 due to FIFO page replacementAs the number of frames allocated to a process increases page faults must decrease But in FIFO page replacement page faults increase2 Optimal Page replacement AlgorithmReplace the page that will not be used for longest period of time Used for measuring how well your algorithm performs It is not practical to implement OPT algorithm as it is not possible to know what the future references will beex

Optimal page replacement 9 page faults

3 Least Recently used (LRU) Page replacement algorithm Replace the page that has not been used for longest period of time LRU makes use of time of last use of the page Ex

Number of page faults = 12

LRU requires hardware assistance to determine the time of last use of the page We can use 1) Stack or 2) Counter to implement LRU page replacement algorithm

1) Stack implementation ndash keep a stack of page numbers in a double link formwhen a Page is referenced it is moved to the top of stackThis implementation requires 6 pointers to be changed

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

2) counter implementation Time-of-use field is allocated to each frame allocatedCPU maintains a counter Counter is incremented for every page reference Counter value is copied to the time-of-use field of the referenced page

Disadvantages of LRU page replacement algorithm Updating counter or stack must be done for every memory reference HENCE LRU IS SLOW and implementation requires hardware assistance ==============================================================================Q) LRU approximation page replacement1) Using reference bit2) Using additional reference bits algorithm3) Second chance algorithm4) Enhanced Second chance algorithm5) Counting based Page replacement

Has two schemes1) Least Frequently Used (LFU)2) Most Frequently Used (MFU)

6) Page Buffering AlgorithmsQ) What is ThrashingAns A process is thrashing if it is spending more time paging than executingThrashing may occur when global (or) local page replacement is used as followsSolution to Thrashing is Working Set strategyIf we provide a process with as many frames as it needs thrashing can be avoided Since it is not possible to know how many frames a process needs we use locality model of process execution

here Locality is set of pages that are actively used together Locality model states that as the process executes it moves from locality to locality If we allocate enough frames to a process to accommodate the size of current locality it will not fault again until it changes current localityWorking Set model uses working-set and a parameter ∆ = working-set windowThe set of pages in the most recent ∆ page references is the working setLet WS S i be the working-set size for process Pi ie Pi needs WS S i framesLet D be the total demand for frames then D=sum WS Si

and mbe the total number of available frames

If iquestm thrashing occurs because some processes will not have enough framesSoIF (Dgtm iquest

OS selects a process to suspendelse OS may initiate another processThe working set strategy prevents thrashing while keeping the degree of multi programming as high as possibleThe main difficulty is to keep track of moving working set window ========================================================================Q) How does thrashing occur in global and local page replacementAns In Global page replacement when a page fault occurs for a process any frame in main memory can be replaced causing page fault to some other process that needs the replaced frame Page fault processes must use paging device to swap pages in and out As more number of processes queue up for the paging device

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

the READY queue empties And CPU becomes idle So OS introduces new processes as CPU is free This further increases number of page faults

In Local page replacement Each process is allocated certain frames in main memory and when a page fault occurs only the frames allocated to it will be replacedWhen a process starts thrashing other processes are not affected But since the processes will be in queue for paging device most of the time average service time for page fault increases Hence effective access time will increase even for a process that is not thrashingQ) Page Table Structure Or Structure of page table in memory

Hierarchical Paging Hashed Page Tables Inverted Page Tables

Hierarchical Paging If Page table is large then we break up the page table into multiple page tables ie page table is paged A simple technique is a two-level page tableTwo-Level Paging Example A logical address (on 32-bit machine with 4K page size) is divided into 1 a page number consisting of 20 bits 2 a page offset consisting of 12 bitsSince the page table is paged the page number is further divided into 1 a 10-bit page number 2 a 10-bit page offset Thus a logical address is as follows page number page offset

pi p2 d

10 10 12 where p1 is an index into the outer page table and p2 is the displacement within the page of the outer page table Two-Level Page-Table Scheme

Address-Translation Scheme Address-translation scheme for a two-level 32-bit paging architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Hashed Page Tables (or) Hash table Hashed page tables are common in address spaces gt 32 bitsEach entry in hash table contains a linked list of elements that hash to same locationHash table is indexed by hash valueEach element consists of 3 fields

1 Page number2 Frame number3 Pointer to next element in linked list

The algorithm works as follows Page number is passed to hash function to get hash value Locating the hash value in hash table is easy as Hash table is indexed by hash value Page number is then compared with field 1 in the first element in linked list If there is a match the corresponding frame number is used to get physical address If there is no match next entries in linked list are searched

Inverted Page Table

Invertible page table has one entry for each frame of physical memory Each entry in invertible page table has

Process-id Page number

Logical address also has 3 parts Process-id Page number

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

OffsetProcess-id and page number of the logical address is compared with each entry in the invertible page tableIf a match is found at ith entry i is added to offset to get logical address If there is no match it means it is illegal addressAdvantages and disadvantages

Decreases memory needed to store each page table Increases time needed to search the table when a page reference occurs Solution Use hash table to

limit the search to one mdash or at most a few mdash page-table entries Inverted Page Table Architecture

Unit-IV CPU Scheduling

Basic conceptsTo maximize CPU utilization some process must be running at all times

In multi programming several processes will be in main memory at a given time If running process has to wait for IO the OS takes the CPU away from that process and assigns CPU to other process in ready queue So CPU must be scheduled and CPU scheduling is fundamental OS function

CPU- IO burst cycleProcess execution begins with a CPU burst that is followed by an IO burst which is followed by another CPU burst then another IO burst and so on as shown in the figure

An IO bound program has many short CPU bursts A CPU-bound program has few long CPU bursts

Schedulers a module in OS for scheduling decisions1048708 Long-term scheduler (or job scheduler) ndash selects which processes should be brought into the ready queue1048708 Medium-term scheduler ndash selects which processes should be swapped inout the memory1048708 Short-term scheduler (or CPU scheduler) ndash Whenever the CPU becomes idle the operating system must select one of the processes in ready queue to be executed This selection is done by short term scheduler (or) CPU scheduler

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Dispatcher gives CPU control to the process selected by short term scheduler This function involves following

1 switching context2 switching to user mode3 jumping to proper location in the user program to restart that program

Time taken to stop one process and start another process is known as dispatch latency This must be kept smallReady QueueReady queue is implemented as a FIFO queue a priority queue a tree or simply an unordered linked listThe records in ready queue are PCBs of processes

Pre-emptive Scheduling

CPU scheduling decisions take place under one of four conditions

1 When a process switches from the running state to the waiting state2 When a process switches from the running state to the ready state for example in response to an

interrupt3 When a process switches from the waiting state to the ready state say at completion of IO 4 When a process terminates

For conditions 1 and 4 there is no choice - A new process must be selected For conditions 2 and 3 there is a choice - continue running the current process (or) select a different

one If CPU scheduling takes place only under conditions 1 and 4 the system is said to be non-

preemptive or cooperative In non-preemptive a process runs until it itself gives up the CPU ie when it is waiting for IO (or) when process completes Otherwise the system is said to be pre-emptive

Windows used non-preemptive scheduling up to Windows 3x and started using pre-emptive scheduling with Win95 Macs used non-preemptive prior to OSX and pre-emptive since then Note that pre-emptive scheduling is only possible on hardware that supports a timer interrupt

Note that pre-emptive scheduling can cause problems when two processes share data because one process may get interrupted in the middle of updating shared data structures

Preemption can also be a problem if the kernel is busy implementing a system call ( eg updating critical kernel data structures ) when the preemption occurs Solution wait until the system call has either completed or blocked before allowing the pre-emption But this solution is problematic for real-time systems

Some critical sections of code protect themselves from concurrency problems by disabling interrupts before entering the critical section and re-enabling interrupts on exiting the section But this should only be done only on very short pieces of code that will take less CPU time

Scheduling Criteria to decide CPU scheduling algorithm1 CPU utilization in real system cpu utilization will range from 40 to 902 Throughput Number of processes that are completed per unit time3 Turnaround time Time when the process has completed ndash Time when the process was submittedie it includes waiting time and burst time4 Waiting time amount of time that a process spends waiting for CPU5 Response time Time when the first response came - Time when the process was submitted

Optimization Criteria Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Min turnaround timeMin waiting timeMin response time CPU Scheduling Algorithms1) First-Come First-Served (FCFS) Scheduling First come first served CPU scheduling algorithm is non pre-emptive Processes are scheduled in the order they have arrivedFCFS is implemented by Queue When the CPU is free it is allocated to the process at the head of the queueAdvantages Easy to implementDisadvantages 1 Average waiting time is quite long

2 Convoy effect (it occurs as small processes wait in queue for big process to leave CPU)Example Process Burst Time P1 24 P2 3 P3 3

Suppose that the processes arrive in the order P1 P2 P3

The Gantt Chart for the schedule is P1 P2 P3

0 24 27 30 Waiting time for P1 = 0 P2 = 24 P3 = 27 Average waiting time (0 + 24 + 27)3 = 17 2) Shortest-Job-First (SJF) SchedulingSchedule the Process with shortest burst time If CPU burst time of two processe are same then FCFS is usedAdvantages Average waiting time decreasesDisadvantages It is difficult to know the length of next CPU burstSJF can be either pre-emptive (or) non pre-emptive1 Non pre-emptive SJF ndash once CPU is given to the process it cannot be preempted until completes its CPU burst2 Pre-emptive SJF (or) Shortest-Remaining-Time-First (SRTF) If the newly arrived process is shorter than what is remaining of currently executing process then pre-empt the currently executing process

Example Process Arrival Time Burst Time P1 00 7

P2 20 4 P3 40 1

P4 50 4

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

SJF (non-preemptive) Gantt Chart P1 P3 P2 P4

0 7 8 12 16 Average waiting time = [0 +(8-2)+(7-4) +(12-5)] 4 =4

Example of Preemptive SJF

Process Arrival Time Burst TimeP1 00 7P2 20 4P3 40 1P4 50 4

SJF (preemptive) P1 P2 P3 P2 P4 P1

0 2 4 5 7 11 16 Average waiting time = (9 + 1 + 0 +2)4 =3 3) Priority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer equiv highest priority)If two processes have equal priority then FCFS is usedSJF is a priority algorithm in which larger the CPU burst lower the priorityThere are two schemes 1 Preemptive 2 nonpreemptive

Problem equiv Starvation ndash low priority processes may never executeSolution equiv Aging ndash as time progresses increase the priority of the process

4) Round Robin (RR) Each process gets a small unit of CPU time (time quantum) usually 10-100 milliseconds After this time has elapsed the process is preempted and added to the end of the ready queueIf there are n processes in the ready queue and the time quantum is q then each process gets 1n of the CPU time in chunks of at most q time units at once No process waits more than (n-1)q time unitsPerformance 1 q large _ FIFO 2 q small _ q must be large with respect to context switch otherwise overhead is too highExample of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

P3 3 The Gantt chart isP1 P2 P3 P1 P1 P1 P1 P1

0 4 7 10 14 18 22 26 30 Average waiting time = [(30-24)+4+7]3 = 173 =566 5)Multilevel Queue scheduling Ready queue is partitioned into separate queuesFor ex ready queue is divided into 2 queues

1 foreground (interactive) queue2 background (batch) queue

Each queue has its own scheduling algorithmFor ex Round Robin scheduling algorithm can be used in foreground queue

FCFS scheduling algorithm can be used in background queueScheduling must be done between the queues This can be done in 2 ways1 Fixed priority scheduling

Foreground queue has highest priority All the processes in foreground queue must be completed and queue must be empty Then only the processes in background queue will be given CPU As shown in below figure Disadvantage starvation

2 Time slice ndash each queue gets a certain amount of CPU timewhich it can schedule amongst its processes ie 80 to foreground in RR and 20 to background in FCFS

6) Multilevel Feedback Queue Scheduling A process can move between queues The idea is to separate processes according to their CPU bursts IO ndash bound and interactive processes will be in highest priority queueIf a process takes more CPU time it is moved to lower priority queue If a process waits too long in lower priority queue it is moved to higher priority queue to prevent starvationAs shown in the below figure let there be 3 queues Q0 Q1 Q2

1 Q0 ndash time quantum 8 milliseconds 2 Q1 ndash time quantum 16 milliseconds 3 Q2 ndash FCFS

Scheduling

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

1 A process entering ready queue is put in queue Q0 When it gains CPU the process receives 8 msec If it does not finish in 8 milliseconds process is moved to queue Q1 2 At Q1 process again receives 16 additional milliseconds If it still does not complete it is moved to queue Q2

Q) SynchronizationSeveral processes run in an operating system Some process share resources due to which problems like data inconsistency may arise

Example of process synchronization is Producer Consumer problem (or) Bounded-Buffer problemProducer Consumer problem To make sure that producer process should not add data when the buffer is full andConsumer process should not take data when the buffer is empty

The code for Producer process can be modified as followswhile (true) Produce an item in next producedwhile (counter == BUFFER_SIZE) do nothingbuffer[in]=nextProducedin = (in+1) BUFFER_SIZEcounter++

here an integer variable counter is initialized to zeroCounter is incremented every time a new item is added to the buffer and counter is decremented every time an item is removed from bufferin is an index that always points to an empty slot after the last filled slotout is an index that always points to the first filled slot after at the head of the circular queue

The code for consumer process is as shown belowwhile(true)

while (counter == 0) do nothingnextConsumed = buffer[out]out=(out+1)BUFFER_SIZEcounter--

The above producer and consumer code produces RACE CONDITIONOutcome of execution of several co-operating processes depends on the order in which accesses takes place is called Race ConditionFor Example if counter =5

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Given counter =5 and if producer produces one item and consumer consumes one item then correct value of counter= 5 only But here the counter = 4 ie inconsistent resultIf T5 is executed before T4 then the counter =6 ie inconsistent result

Solution to avoid race condition make sure only one process at a time must update the shared variable (here it is counter)

Q) Critical section problemAns Each process has a segment of code called critical section Critical section code is the code that contains common variables of co-operating processesTo avoid data inconsistencyTwo processes must not execute critical section code at same timeFor this to happen1 Each process must request permission to enter its critical sectionmdashEntry section2 End of critical section ndashExit section3 Rest of the code after critical section ndash Remainder section

Solution to critical section must satisfy the following three requirements1 Mutual exclusion if one process is in critical section no other process can be in critical section2 Progress Processes not in remainder section must decide which process will enter critical section next3 Bounded Waiting All the processes must be allowed to enter critical section No process must wait indefinitely

Q) Petersonrsquos solution for critical section problem (Software based solution)AnsTime Process 1 Process2T0 do

flag[1] = TRUE turn =2

do flag[2] = TRUE

T1 turn =1

T2 While( flag[2] ampamp turn == 2)Since turn = 2 while loop will not run

While( flag[1] ampamp turn == 1)Loops continuously until flag[1]

becomes falseT3 Enter critical section

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

T4 flag[1] = FALSE Enter Critical section

T5 Enters remainder section flag[2]=FALSE

while (TRUE) Enters remainder section

while (TRUE)

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type

File Attributes

1 Name ndash Name of the file for user understanding2 Identifier used by OS to identify a file ( eg inode number )3 Type ndash Text files executable files binary files etc 4 Location ndash location of the file on the hard drive 5 Size current size of the file6 Protection Controls who can read write or execute the file7 Time Date and user identification gives the information when the file was created last modified

and last used etc This data is used for protection security and usage monitoring

All information about files is kept in directory structure The directory entry consists of the filersquos name and its unique identifier The identifier locates the other file attributes Directory structure is kept on disk

File Operations

The file Abstract Data Type supports many common operations 1 Creating a file find free space on disk add entry to directory 2 Writing a file write data at current file position pointer location and update pointer3 Reading a file read data at current file position pointer location and update pointer4 Repositioning within a file or file seek change pointer location to a given value5 Deleting a file free the space allocated to file and remove entry from directory6 Truncating a file delete the data and update file size

Open File Table

Most OSes require that files be opened before access and closed after access Information about all files that are open currently in the system is stored in an open file table This

system wide open file table is maintained by OS And information about files opened by the process is stored by the processrsquos open file table Open file table has the following information

1 File pointer - records the current position in the file for the next read or write2 File-open count - How many times has the current file been opened ( simultaneously by

different processes ) and not yet closedWhen this counter reaches zero the file can be removed from the table

3 Disk location of the file4 Access rights

Some systems provide support for file locking 1 A shared lock is for reading only2 A exclusive lock is for writing as well as reading3 An advisory lock OS makes sure the locks are acquired and released appropriately4 A mandatory lock software developers makes sure the locks are acquired and released

appropriately5 UNIX used advisory locks and Windows uses mandatory locks

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

File Types

Implementing file types A common technique is to include the type as part of the filename The name is split into 2 parts

1 File Name 2 Extension

File name and extension are separated by a period The system uses the extension to indicate the type of file and the type of operations that can be done on that fileUser opens Microsoft word and clicks on File menu and Open option User will only specify the filename without extension application will look for a file with the given name and extension it expects Hence extensions are not supported by the operating systemUnix system uses a crude magic number stored at the beginning of the file to indicate the type of fileIn Mac OS X operating system file produced by word processor has the word processorrsquos name as its creator When user double clicks on the file word will open the required fileSome common file types are as shown below

File Structure Some files contain an internal structure which may or may not be known to the OS for ex executable files have a specific structure so that it can determine

where in memory to load the file and what is the location of the first instruction

If OS supports many file formats it will increase the size and complexity of the OS UNIX treats all files as sequences of bytes with no internal structure (With the exception of

executable binary programs which it must know how to load and find the first executable statement etc)

Macintosh files have two forks - a resource fork and a data fork The resource fork contains information relating to the UI such as icons and button images and can be modified independently Data fork contains the code or data

Internal File structure

Disk files are accessed in units of physical blocks typically 512 bytes

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Internally files are organized in units of logical units like single byte (or)

data record size (or) structure size

The number of logical units which fit into one physical block determines its packingIf a student file is organized internally as records Each record takes 64 bytes of memoryAnd main memory is divided into blocks of 512 bytes Then packing = 51264=8 8 records can be stored per block

Internal fragmentation may occur due to this

102 Access Methods

The information in file can be accessed in the following ways1 Sequential access2 Direct access (or) Relative access

1 Sequential access we cannot randomly access any record

Records are read and written sequentially one record after another

A sequential access file emulates magnetic tape operation and generally supports the following operations read next - read a record and advance the tape to the next position write next - write a record and advance the tape to the next position rewind skip n records - May or may not be supported N may be limited to positive numbers or may be

limited to +- 1

2 Direct access (or) Relative accessA file is made up of fixed length records so that we can randomly access any record

No restriction on order of reading or writing

Direct access files are useful when we need to access large amounts of data randomly

Ex database

Operations supported include n is relative block number ie relative to beginning of file

1 read n - read record number n 2 write n - write record number n 3 jump to record n - could be 0 or the end of file4 Query current record - used to return back to this record later5 Sequential access can be easily emulated using direct access The inverse is complicated and

inefficient

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

3 Other access methods can be built on top of direct access method We construct an index for the file which contains pointers to various blocks To search a file we first access index and then use pointer to access file

If a student file has 120000 records of students sorted by student id We can create an index consisting of 1st student id in each blockif 64 records can be stored in each block then number of blocks needed to store 120000 records = 12000064 asymp 2000 blocks So index has 2000 entries Index is kept in main memory so that searching becomes faster

If the index itself becomes large we create an index for index file (Ex B-tree)

Q) Allocation Methods

To allocate space to files on disk is such a way that disk space is utilized effectively and files can be accessed quickly

Three major methods of allocating disk space are

1 Contiguous 2 Linked

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

3 Indexed

1 Contiguous allocation Each file occupies set of contiguous blocks on disk

If the file is n blocks long and starts at location b then it occupies blocks b b+1 b+2hellipb+n-1

The directory entry for each file indicates the address of starting block(ie b) and length (n)

Advantages

1 For direct access to block i we can directly access by finding the block as b+i Since the file is stored sequentially sequential access is easy Hence access is easy

2 Supports both direct and sequential access3 Number of disk seeks required is minimum 4 Seek time is minimum

Disadvantages

1 Difficult to find space to a new file (or) add more data to existing fileFirst fit and Best fit strategies are used to find a free hole( set of free contiguous blocks) from list of available holes But both are inefficient wrt storage utilization

2 External fragmentation even though there are enough free blocks required for a new file they are not contiguous and hence cannot be allocated to new fileSolution copy all the files onto another disk or tape We then copy the files back onto original disk contiguously This is known as compaction But compaction is time consuming

3 Size of the file must be known when the file is created Even though final size is known entire size may not be used by the file at the beginning Hence leads to wastage of space

If size of the file is not known in advance then (i) If we allocate too little space to file file cannot be extended Solution find a larger hole copy the contents of the file to the new space and release the previous space This may be possible as long as space exists Also this is time consuming(ii) If we overestimate the amount of space needed much of the space is unused This will lead to internal fragmentation

To minimize these drawbacks some os uses modified contiguous allocation scheme Here contiguous blocks of memory is allocated at firstIf the file needs more blocks another set of contiguous space known as extent is added Location and a block count plus a link to the first block of next extentIf extents are too large and of fixed size internal fragmentation occursIf extents are of variable sizes external fragmentation occurs

Linked AllocationEach file is a linked list of disk blocks The disk blocks can be scattered anywhere on the diskThe directory contains a pointer to first and last blocks of the file For example a file of 5 blocks might start at a block 9 and continue at block 16 then block 1 and then block 10 and finally block 25 Each block contains a pointer to next block If each block is 512 bytes and disk address (pointer ) requires 4 bytes then the user sees blocks of 512-4=508 bytes To create a new file we simply create a new entry in the directory The pointer will have nil for empty file Also the size field of empty file is 0

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

To add more data to existing file free block is found and data is written to it and is linked to end of file To read a file we read by following the pointers block to block

Advantages 1 Easy to find space to a new file (or) add more data to existing file2 Size of the file need not be declared when the file is created3 There is no external fragmentation

Disadvantages1 Seek time is more2 Number of disk seeks required is more3 Pointers use larger percentage of filersquos disk space4 Inefficient for direct access files To access i th record i disk reads are required5 Not Reliable if pointers were lost or damaged

Solution to disadvantage 3 is to use to group multiple blocks as clusters(for ex 4 blocks = 1cluster) We allocate clusters instead of blocks to file Makes logical to physical block mapping simpleImproves disk throughput by reducing disk access timeReduces wastage of space due to pointers as only few pointers may be neededFree ndashlist management also becomes simpleDisadvantage internal fragmentation more space is wasted when the cluster is partially full

Solution to disadvantage 5 is to use doubly linked list (or) store the file name and relative block number in each block

An variation of linked allocation is the use of file allocation table (FAT)A section of disk at the beginning of each volume contains FAT FAT has one entry for each disk block and is indexed by block numberThe block number contains the block number of next block that contains the file The last block will have special end of the file value as table entry An unused block is indicated by table value 0

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Random access time is improved

Disadvantage number of disk seeks is more The disk head must move to read the FAT and find the location of block then move to location of the block

3 Indexed allocation

Each file has its own index block Index block is an array of disk block addresses The ith entry in index block points to ith block of the file

The directory contains the address of the index block

When the file is created all pointers in the index block are set to nil

To add more data to existing file free block is found and data is written to it The address of block is put in the index block

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Advantages

1 Easy to find space to a new file (or) add more data to existing file2 Supports direct (or) random access efficiently3 No external fragmentation

Disadvantage

1 Seek time is more2 Number of disk seeks required is more3 Not Reliable4 Index file must be kept in memory If this memory is not available then we have the read the index

block and then desired data block ie 2 disk accesses are required To access a block near the end of the file we need to read all the index blocks to read the needed data block

5 Pointer overhead of index block is gt than that of linked allocation For example if we have file of only one or two blocks Entire index block is allocated to store these one or two pointers

Various mechanisms followed for size of index block

1 linked scheme size of index block is one disk block Index block contains a small header containing name of the file and set of the first 100 disk block addresses The next address contains the address of another index block for large files

2 multilevel index to access a block the OS uses first-level index to find second-level index block which points to file block This approach can be continued to third or fourth level

3 combined scheme

For example Say there are 15 pointers of index block in the filersquos inode The first 12 of these pointers have address of blocks that contains file data The next 3 pointers point to indirect blocks The first points to single indirect block which is an index block that contain addresses of filersquos data blocks

The second points to double indirect blocks and third points to triple indirect blocks as shown below

Q) Free space management

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

To keep track of free disk space the system maintains free space list To create a file we search free space list and allocate the required space to new file

The free space list can be implemented as

1 Bit vector (or) Bit map2 Linked list3 Grouping4 Counting 5 Space maps

1 Bit vector (or) Bit map Each block is represented by 1 bit If the block is free the bit is 1 and if the block is allocated the bit is 0 For example consider a disk where blocks 23458 blocks are free The free-space bit map would be 001111001000

Advantages simple and easy to get contiguous files

Disadvantages

1 Bit map requires extra space2 Bit map is inefficient if kept in disk Bit maps of smaller disk can be kept in main memory but may

not be possible to keep the bit map of larger disk in main memory3 Bit map must be modified both when blocks are allocated and freed Freeing 1 GB of data on a 1-TB

disk could cause thousands of blocks of bit maps to be updated because those data blocks are scattered all over the disk

2 Linked List Free-space list is implemented by linking all the free disk blocks together A pointer to the first free block is stored in special location on the disk For example consider a disk where blocks 23458 blocks are freeWe keep a pointer to block 2 as the first free block Block 2 would contain a pointer to block 3 Block 3 would point to block 4 and block 4 would point to block 5and so onAdvantage No waste of spaceDisadvantage Cannot get contiguous space easily ie To traverse the free-space list we must read each block which requires more IO time

3 Grouping First free block stores the addresses of n free blocks The first n-1 free blocks are free and the last block contains the addresses of another n free blocks and so onAdvantage large number of free blocks can be found quickly4 Counting free-space list can be maintained by keeping the address of first free block and the number (n)of free contiguous blocks that follow the first block

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Each entry in the free-space list consists of disk address and a countEach entry requires more space andlist is shorter only if count is gt1These entries can be stored in a B-tree for efficient insertion deletion and search

5 Space maps ZFS divides the disk space into chunks of manageable sizes called metaslabs Each metaslab has a space mapZFS uses counting algorithm to store information about free blocks Space map is a log of all block activities in time order in counting formatWhen ZFS allocates or frees space from a metaslab it loads the associated space map into memory and updates it Finally free-space list is updated on diskSynchronization Hardware Hardware solution to synchronization is to provide atomic operations These operations operate as a single instruction without interruption Two such operations are 1 Test and Set() hardware instruction2 Swap() instruction

1 Test and Set instruction is as shown

Solution using Test and SetThe shared variable lock is initialized to false

2 Swap() instruction definition is as follows

Solution using Swap1 Shared Boolean variable lock is initialized to false and each process has a local boolean variable key

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Both Test and Set Swap instructions satisfy the mutual exclusion requirement but unfortunately do not guarantee bounded waiting If there are multiple processes trying to get into their critical sections there is no guarantee of what order they will enter and any one process could have the bad luck to wait forever until they got their turn in the critical section (Since there is no guarantee as to the relative rates of the processes a very fast process could theoretically release the lock whip through their remainder section and re-lock the lock before a slower process got a chance As more and more processes are involved vying for the same resource the odds of a slow process getting locked out completely increase )

Below figure illustrates a solution using test-and-set that satisfies bounded waiting using two shared data structures boolean lock and boolean waiting[ N ] where N is the number of processes in contention for critical sections

Bounded-waiting mutual exclusion with TestAndSet( )

It first looks in an order ( starting with the next process on the list ) for a process that has been waiting and if it finds one then it releases that particular process from its waiting state without unlocking the critical section thereby allowing a specific process into the critical section while continuing to block all the others

Q) What is semaphoreAns Semaphore is used to solve synchronization problems A semaphore S is an integer variable accessed only through 2 standard atomic operations 1 wait() 2 signal()Wait primitive Signal primitivewait(S)

while(S lt= 0) do nothingS--

signal (S)

S++

Each process that wishes to use a resource performs wait() operation When a process releases a resource it performs signal() operation

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Semaphores Usage1 binary semaphore or mutex locks can take values 0 (or) 1

2 Counting Semaphore can take any integer value Counting semaphore is used to count the number remaining resources The counter is initialized to the number of resources available whenever the counting semaphore gt 0 then a process can enter a critical section and use one of the resources When the counter = 0 ( or negative in some implementations ) then the process blocks until another process frees a resource and increments the counting semaphore with a signal callFor example if 3 resources are there and 4 processes Value of counting semaphore =3if process P1 requires a resource it perform wait() operationNow counting semaphore = 2if process P2 requires a resource it perform wait() operationNow counting semaphore = 1if process P3 requires a resource it perform wait() operationNow counting semaphore = 0if process P4 requires a resource it perform wait() operation P4 waits until a resource is available

3 Semaphores can also be used to synchronize certain operations

Semaphore ImplementationWhen a process is in critical section and any other process that tries to enter critical section must loop continuously wasting CPU time (busy waiting) This type of semaphore is called spin lock as the process spins while waiting For example suppose it is important that process P1 execute statement S1 before process P2 executes statement S2

First we create a semaphore named synch that is shared by the two processes and initialize it to zero Then in process P1 we insert the code

S1signal( synch )

and in process P2 we insert the code wait( synch )

S2 Because synch was initialized to 0 process P2 will block on the wait until after P1 executes the call

to signal Each semaphore has a integer value and maintains a waiting queue of waiting processes

Semaphore implementation with no busy waiting To overcome busy waiting we block the waiting process and waiting process is restarted by wakeup() operation(ie the process is removed from waiting queue to ready state)

Problems due to semaphore1 Deadlocks Deadlocks occur when multiple processes are blocked each waiting for a resource that can only be freed by one of the other (blocked) processes shown below

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

2 Starvation In Starvation one or more processes gets blocked forever and never get a chance to enter critical section For example if we do not specify the algorithms for adding processes to the waiting queue or selecting one to be removed from the queue in the signal( ) call If a LIFO queue is chosen then the first process that starts waiting will never get a chance

3 Priority inversionLet Process LMH have priorities as shown belowLltMltHprocess H is waiting for a resource that has been held by process L so Process L is running When process M enters it pre-empts process L and M is running nowHere M runs before H even though M has lower priority than HTo avoid this L is given high priority so that M does not pre-empt L=====================================================================Q) Classical problems on synchronization

1 The Bounded Buffer Problem (also called the The Producer-Consumer Problem)2 The Readers-Writers Problem3 The Dining Philosophers Problem

These problems are used to test newly proposed synchronization scheme

1 The Bounded Buffer ProblemConsider

n buffers and each buffer holds one item a producer process which creates the items (1 at a time) a consumer process which processes them (1 at a time)

Producer process cannot produce an item if all the buffers are full and Consumer process cannot consume if all the buffers are empty As both processes modify the contents of buffer synchronization must be there For that we use 3 Semaphores

1 empty = n (empty semaphore counts the number of empty buffers)2 full=0 (full semaphore counts the number of filled buffers)3 mutex=1 (mutex provides mutual exclusion for access to the buffer)

Producer process Consumer processdo

hellip produce an item wait(empty) wait(mutex) hellip

do wait(full) wait(mutex) hellip remove an item from buffer hellip

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

add item to buffer hellip signal(mutex) signal(full)

while (1)

signal(mutex) signal(empty) hellip consume the item hellip while (1)

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

2 The Readers-Writers ProblemA data item such as a file is shared among several processesEach process is classified as reader or writerMultiple readers can read the file at same timeA writer must have exclusive access (ie cannot share with either a reader or another writer)Two versions of the readers-writers problem

readers priority new reader need not wait because a writer is waiting writers priority if a writer is waiting to access the database no new readers can start reading

A solution to both version may cause starvation in the readers priority version writers may starve in the writers priority version readers may starve

A semaphore solution to the readers priority version uses 3 semaphores1 readcount = 0 (readcount semaphore counts the number of reading shared data2 mutex =1 (mutex semaphore is used for mutually exclusion when readcount is updated)3 wrt = 1 (wrt semaphore is common to both writer and reader)

wrt provides mutually exclusive access to shared data Reader process Writer process

wait(mutex) readcount++ if (readcount == 1) wait(rt) signal(mutex) hellip reading is performed hellip wait(mutex) readcount-- if (readcount == 0) signal(wrt) signal(mutex)

do wait(wrt) hellip writing is performed hellip signal(wrt)

while (TRUE)

3 The Dining Philosophers Problemn philosophers sit around a table thinking and eatingAs time passes a philosopher gets hungry and tries to pick up the chopstick on his left and on his right A philosopher may only pick up one chopstick at a time and cannot pick up a chopstick already in the hand of neighbour philosopherThe dining philosophers problems is an example of a large class or concurrency control problems it is a simple representation of the need to allocate several resources among several processes in a deadlock-free and starvation-free mannerA semaphore solution represent each chopstick with a semaphore

semaphore chopstick[5] Initially all values are 1

Philosopher ido

51

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

wait(chopstick[i])wait(chopstick[(i+1) 5])hellipeathellipsignal(chopstick[i])signal(chopstick[(i+1) 5])hellipthinkhellip

while (1)This solution guarantees no two neighbouring philosophers eat simultaneously but has the possibility of creating a deadlockQ) Types of operating systems

MULTIPROCESSING SYSTEMS has multiple hardware CPUs

In symmetric multiprocessing (SMP) systems The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

52

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

ASYMMETRIC MULTIPROCESSING SYSTEMS In asymmetric multiprocessing (ASMP) all CPUs are not equal (Master and slave CPUs) ASMP have dedicated processors to specific tasks For example one processor may be dedicated to disk operations another to video operations etc These systems dont have the flexibility to assign processes to the least-loaded CPU unlike an SMP system

REAL TIME SYSTEM is used to implement a computer application that must complete its execution within its time constraint

Two kinds of real-time systems have evolved

1 A hard real-time system can guarantee that response requirement will be met under all conditions 2 A soft real-time system cannot guarantee that response requirement meets under all conditions Ex

Digital audio or multimedia systems Digital telephones

DISTRIBUTED SYSTEMS execute parts of a computation in different systems at the same time It uses distributed control ie it spreads its decision-making actions across different computers in the system so that failures of individual computers or the network does not cripple its operation

A distributed operating system appears as a uniprocessor system even though it has multiple processors The users may not know where their programs are being run or where their files are located that should all be handled automatically by the operating system

Distributed systems allow applications to run on several processors at the same time thus requiring more complex processor scheduling algorithms

DESKTOP SYSTEMS A desktop system is a personal computer (PC) system used at a single location Modern desktop systems support multiprogramming Common examples are Linux FreeBSD Windows 8 and the Macintosh operating system

HANDHELD SYSTEMS (palmtop computers) Two of the most popular operating systems for handhelds are Symbian OS and Android OS They have very less memory

53

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

CLUSTERED SYSTEMS A computer cluster consists of a set of loosely connected computers that work together so that in many respects they can be viewed as a single system The components of a cluster are usually connected to each other through fast local area networks each node (computer used as a server) running its own instance of an operating system

Q) Modern operating system

New developments in hardware applications and security threats lead to the development of modern operating system

New Hardware developments like increased machine speed high speed network increased size and variety of memory devices

New Applications developments like multimedia applications internet and web access and client server computing

Developments in modern operating system can be categorized as

1 Microkernel architecture assigns only few functions to kernel ( IPC and CPU scheduling etc) Other OS services are provided by processes called servers that run in user mode This approach decouples server and kernel development This approach is well suited to distributed environment

2 Multithreading a process is divided into threads that can run concurrently Useful for applications that perform a number of independent tasks Example database server that listens and processes many client requests

3 Symmetric multiprocessing As now-a-days there are multiple microprocessors in a single system SMP operating system provides greater efficiency when there are multiple processors The processes share the same main memory and io facilities All processors can perform the same functions (hence term symmetric)

SMP schedules processes or threads across all of the processors

Advantages

1 Increased performance if some portion of the program can be run in parallel the performance of SMP OS increases

2 increased availability as all processes can perform the same task if one processor fails others will work 3 incremental growth one can add a new processor to increase the performance4 Scaling cost can be dependent on the number of processors used

4 Distributed operating systems Distributed systems allow applications to run on several processors at the same time

5 Object oriented design adds modularity to kernel Os can be customized without effecting system integrity

Q) Linux Operating systems It is open source as its source code is freely available It is free to use Linux was designed considering UNIX compatibility Linux is a multi-user multitasking system Main design goals are speed efficiency and standardization Components of Linux System

54

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Linux Operating System has primarily three components as shown in the below diagram

Kernel - Kernel is the core part of Linux It is responsible for all major activities of this operating system It interacts directly with hardware Kernel hides low level hardware details to system or application programs

System Library - System libraries are special functions or programs These are used by application programs to use Kernelrsquos features

System Utility - System Utility programs are responsible to do specialized individual level tasks like updating log file accepting login requests from terminals etc

Kernel Modules Sections of kernel code that can be compiled loaded and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver a file system or a networking protocol

Three components to Linux module support 1 module management Supports loading modules into memory and allows to talk to the rest of the kernel 2driver registration Allows modules to inform rest of the kernel that a new driver has become available 3 conflict resolution protect reserved resources of one driver from accidental use by another driver

Kernel Mode vs User ModeKernel component code executes in a special privileged mode called kernel mode with full access to all resources of the computer This code represents a single process executes in single address space and do not require any context switch and hence is very efficient and fast Kernel runs each processes and provides system services to processes provides protected access to hardwares to processesCode which is not required to run in kernel mode is in System Library User programs and other system programs works in User Mode which has no access to system hardwares and kernel code User programs utilities use System libraries to access Kernel functions to get systems low level tasksBasic FeaturesFollowing are some of the important features of Linux Operating System

Portable - Portability means softwares can works on different types of hardwares in same way Linux kernel and application programs supports their installation on any kind of hardware platform

Open Source - Linux source code is freely available and it is community based development project Multiple teams works in collaboration to enhance the capability of Linux operating system and it is continuously evolving

Multi-User - Linux is a multiuser system means multiple users can access system resources like memory ram application programs at same time

Multiprogramming - Linux is a multiprogramming system means multiple applications can run at same time

Hierarchical File System - Linux provides a standard file structure in which system files user files are arranged

Shell - Linux provides a special interpreter program which can be used to execute commands of the operating system It can be used to do various types of operations call application programs etc

55

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Security - Linux provides user security using authentication features like password protection controlled access to specific files encryption of data

Architecture Linux System Architecture is consists of following layers

Hardware layer - Hardware consists of all peripheral devices (RAM HDD CPU etc)

Kernel - Core component of Operating System interacts directly with hardware provides low level services to upper layer components

Shell - An interface to kernel Takes commands from user and executes kernels functions

Utilities - Utility programs giving user most of the functionalities of an operating systems

Q) windows XP operating System1Extensibility mdash layered architecture

Executive which runs in protected mode provides the basic system services On top of the executive several server subsystems operate in user mode Modular structure allows additional environmental subsystems to be added without affecting the

executive 2Portability mdash XP can be moved from on hardware architecture to another with relatively few changes

Written in C and C++ Processor-dependent code is isolated in a dynamic link library (DLL) called the ldquohardware abstraction

layerrdquo (HAL)3 Reliability mdashXP uses hardware protection for virtual memory and software protection mechanisms for operating system resources 4 Compatibility mdash applications that follow the IEEE 10031 (POSIX) standard can be complied to run on 2000 without changing the source code 5 Performance mdash XP subsystems can communicate with one another via high-performance message passing

Preemption of low priority threads enables the system to respond quickly to external events Designed for symmetrical multiprocessing

6 International support mdash supports different locales via the national language support (NLS) APIXP Architecture 1 Layered system of modules 2 Protected mode mdash hardware abstraction layer (HAL) kernel executive 3 User mode mdash collection of subsystems

a Environmental subsystems emulate different operating systems b Protection subsystems provide security functions

XP architecture

56

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

Q) Windows network Operating systemExample Windows 2000 operating system

All the pictures are taken from Silberschatz Abraham et al Peter Baer Galvin and Greg Gagne Operating system concepts Reading Addison-Wesley

Visit my blog enthusiaststudentblogspotin57

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture

mtechmessengerblogspotin

Keywords OS exam notes OS notesOperating system notes Opearing system galvin notesSimple batch system Multi- programmed Batch System Time sharing System Difference between Batch Multi-programming and Time sharing What is process structure of process in memory Process states PCB Operations on Processes Process Creation Process Termination cascading terminationcontext switch Basic Concepts of threads two modes of CPU execution Inter process communication Shared ndash Memory Systems Message passing systemswhat is deadlock four conditions that are necessary for deadlock to occur Methods for handling deadlocks Deadlock prevention Deadlock avoidanceResource Allocation graphBankers AlgorithmSafe State Unsafe state Safety Algorithm Resource-Request Algorithm Memory management Address Binding Logical vs Physical Address Space MMU Memory-Management Unit Dynamic loading Dynamic Linking Overlays Swapping Contiguous memory allocation Single-partition allocation Multiple-partition allocation Internal fragmentation Variable Size partitions External fragmentation First-fit Best-fit Worst-fit Non-Contiguous Memory allocation Paging Implementation of Page Table PTLR PTBR page table base register Page-table length register translation look-aside buffers TLB Protection in Paging Shared pages in Paging Segmentation Shared Segments Virtual memory Demand paging Procedure for Handling a Page Fault pure demand paging Page Replacement Page Replacement Algorithms FIFO Optimal Page replacement AlgorithmLeast Recently used (LRU) Page replacement algorithm ThrashingWorking Set model global and local page replacement

58

• 101 File Concept Data is stored in secondary memory in the form of files A file is a sequence of bits bytes lines or records A file has structure which depends on its type
• File Attributes
• File Operations
• Kernel Mode vs User Mode
• Basic Features
• Architecture