Chapter 8 Operating System Support - Yonsei

Chapter 8Chapter 8

Operating SystemOperating SystemSupportSupport

YonseiYonsei UniversityUniversity

YonseiYonsei UniversityUniversity8-2

• Operating System Overview• Scheduling• Memory Management• Pentium II and PowerPC Memory Management

Contents Contents


OS Objectives & FunctionsOS Objectives & Functions Operation System Overview

• OS is a program that– Manages the computer’s resources, provides

services for programmers – Schedules the execution of other programs

• Objective– Convenience

• Makes a computer more convenient– Efficiency

• Allows the computer system resources to be used in a efficiency manner


OS : User/Computer InterfaceOS : User/Computer Interface Operation System Overview

• To ease a complex task, a set of systems program is provided

• Some of programs are referred to as utilities• These implement use functions that assist

in program creation, the management of files, the control of I/O devices

• Programmers make use of these facilities in developing an application

• OS provides a programmer with a convenient interface for using the system


Layers of a Computer SystemLayers of a Computer System Operation System Overview


OS ServicesOS Services Operation System Overview

• Program Creation– Assists the programmer in creating programs

• Program execution– Handles program execution for user

• Access to I/O devices• Controlled access to files• System access

– Access to the system as a whole and to specific system resource

• Error detection and response• Accounting


OS : Resource ManagerOS : Resource Manager Operation System Overview

• OS functions in the same way ordinary computer software; it is a program executed by the processor

• OS frequently relinquishes control and must depend on the processor to allow it to regain control



• The OS is nothing more than a computer program

• The OS directs the processor in the use of the other system resources and in timing of its execution of other programs

• Kernel, nucleus – Contains the most frequently used functions in

operating system and at a given time, other portions of the operating system currently in use

• The OS decides when an I/O device can be used by a program in execution, and controls access to and use of files




Types of Operating SystemsTypes of Operating Systems Operation System Overview

• Interactive system– User/programmer interacts directly with the computer,

usually through a keyboard/display terminal, the request the execution of a job or to perform a transaction

• Batch system(opposite of interactive)– User’s program is batched together with programs

from other users and submitted by a computer operator

• Multiprogramming • Uniprogramming

– only one program at a time


Early SystemsEarly Systems Operation System Overview

• Early computer(late 1940 to the mid-1950)– No operating system– The processor were run from a console– The program in processor code were loaded via input

device• Two main problems

– Scheduling– Setup time


Simple Batch SystemsSimple Batch Systems Operation System Overview

• Early processors were very expensive – It was important to maximize processor utilization

• Monitor : Simple batch operating systems• Batch : Job together sequentially• Resident monitor

• Users submit jobs to operator• Operator batches jobs• Monitor controls sequence of events to process batch• When the job is completed, it returns control to the

monitor, which immediately reads in the next job• Monitor handles scheduling


Memory LayoutMemory Layout Operation System Overview


Simple Batch SystemsSimple Batch Systems Operation System Overview

• At a certain point in time, the processor is executing instructions from the portion of main memory containing the monitor

• These instructions cause the next job to be read in to another portion of main memory

• Monitor handles the scheduling problem• How about the job setup time?

– The monitor handles this as well• Job control language(JCL)

– Used to provide instructions to the monitor


Overall Format of JobOverall Format of Job Operation System Overview

$JOB$FTN...$LOAD$RUN...$END

InstructionsInstructions

DataData


Hardware Features of Simple Batch Hardware Features of Simple Batch Operation System Overview

• Memory protection– While the user program is executing, it must not

alter the memory area containing the monitor• Timer

– Prevent a single job from monopolizing the system• Privileged instructions

– Executed only by the monitor• Interrupts

– Flexibility in relinquishing control to and regaining control from user programs


MultiprogrammedMultiprogrammed Batch SystemBatch System Operation System Overview

• Simple batch system– The processor is often idle– The problem is that I/O devices are slow

• Suppose that there is room for the operating system and two user programs

• When one job needs to wait for I/O, the processor can switch to the other job, which likely is not waiting for I/O

Read one record 0.0015 secondsExecute 100 instructions 0.0001 secondsWrite one record 0.0015 secondsTOTAL 0.0031 secondsPercent CPU util = 0.0001/0.0031 = 0.032 = 3.2 %

System Utilization Example


Multiprogramming ExampleMultiprogramming Example Operation System Overview

• Uniprogramming



• Multiprogramming with two programs



• Multiprogramming with three programs


Program Execution AttributesProgram Execution Attributes Operation System Overview

JOB1 JOB2 JOB3

Type of jog Heavy compute Heavy I/O Heavy I/O

Duration 5 min 15 min 10 min

Memory required 50K 100K 80K

Need disk? No No Yes

Need terminal? No Yes No

Need printer? No No Yes


Effects of MultiprogrammingEffects of Multiprogramming Operation System Overview

Uniprogramming Multiprogramming

Processor use 17% 33%

Memory use 30% 67%

Disk use 33% 67%

Printer use 33% 67%

Elapsed time 30 min 15 min

Throughput rate 6 jobs/h 12 jobs/h

Mean response time 18 min 10 min


Utilization HistogramsUtilization Histograms Operation System Overview


TimeTime--Sharing SystemsSharing Systems Operation System Overview

• User interface acts directly with the computer• Such as transaction processing, an interactive

mode is essential• Multiprogramming can be used handle multiple

interactive jobs

Batch Multiprogramming Time SharingPrincipal objective Maximize processor use Minimize response timeSource of instructions to operating system

Job control language instructions provided with the job

Command entered at the terminal


SchedulingScheduling Scheduling

• Key to Multiprogramming• Process

– A program in execution– The “animated spirit” of a program– That entity to which a processor is assigned

Long-term scheduling

The decision to add to the pool of processes to be executed

Medium-term scheduling

The decision to add to the number of processes that are partially or fully in main memory

Short-term scheduling

The decision as to which available process will be executed by the processor

I/O scheduling The decision as to which process’s pending I/O request shall be handled by an available I/O device


Long Term SchedulingLong Term Scheduling Scheduling

• Determines which programs are admitted to the system for processing

• Controls the degree of multiprogramming• Once admitted, a job or user program

becomes a process and is added to the queue for the short-term scheduler

• Newly created process begins in a swapped-out condition, in which case it is added to a queue for the medium-term scheduler

• Decides that the operating system can take on one or more additional processes

• Decides which job or jobs to accept and turn into processes


MediumMedium--Term SchedulingTerm Scheduling Scheduling

• Part of the swapping function• Swapping-in decision is based on the need

to manage the degree of multiprogramming• On a system that does not use virtual

memory, memory management is also an issue


ShortShort--Term SchedulingTerm Scheduling Scheduling

• Dispatcher• Fine-grained decision of which job to execute

next• Which job actually gets to use the processor

in the next time slot


Process States of Short Term SchedulerProcess States of Short Term Scheduler• State

– Status at any point in time– Used because it connotes that certain information

exists that defines the status at that point– New

• Program is admitted by the high-level scheduler but is not yet ready to execute

– Ready• Process is ready to execute

– Running• Process is being executed

– Waiting• Process is suspended

– Halted


FiveFive--State Process ModelState Process Model Scheduling


Process Control BlockProcess Control Blockidentifieridentifier

StateState

PriorityPriority

Program counterProgram counter

Memory PointerMemory Pointer

Context DataContext Data

I/O StatusI/O StatusInformationInformation

AccountingAccountingInformationInformation

..

..

• Identifier• State• Priority• Program Counter• Memory pointers• Context data

– Present in registers in the processor

• I/O status information• Accounting information


Scheduling TechniquesScheduling Techniques Scheduling

• Begin at a point in time process A is running• The processor is executing instructions from

the program contained in A’s memory• The processor ceases to execute instructions

in A and begins executing instructions• Three reasons

– Process A issues a service call to the OS– Process A causes an interrupt– Some event unrelated to process A that requires

attention causes an interrupt


Scheduling ExampleScheduling Example Scheduling


OS QueueOS Queue Scheduling

• Long-term Queue– List of jobs waiting to use the system– High-level scheduler will allocate memory and create a

process for one of the waiting items• Short-term Queue

– All processes in the ready state– One of these process could use the processor next– Priority levels may also be used

• I/O Queue– I/O queue for each I/O device– More than one process may request the use of the

same I/O device


Key Elements for MultiprogrammingKey Elements for Multiprogramming Scheduling


Queuing Diagram RepresentationQueuing Diagram Representation Scheduling


Memory ManagementMemory Management Memory management

• In a uniprogramming system,– Main memory is divided into two parts:

• The operating system(resident monitor)• The program currently being executed

• In a multi programming system,– The “user” part of memory is subdivided to

accommodate multiple processes• The task of subdivision is carried out

dynamically by the OS and is known as memory management


SwappingSwapping Memory management

• Problem : I/O activities are much slower than computation and therefore the processor in a uniprogramming system is idle most of the time

• Solutions– Main memory could be expanded and so be able to

accommodate more processes• 2 Flaws• First, main memory is expensive• Second, the appetite of programs for memory has grown as

fast as the cost of memory has dropped

– Swapping



• Long-term queue of process requests, typically stored on disk

• These are bought in, one at a time, as space becomes available

• Rather than remain idle, the processor swaps one of these processes back out to disk into an intermediate queue

• This is a queue of existing processes that have been temporarily kicked out of memory

• OS brings in another process form the interface another process form the intermediate queue



• Swapping is an I/O operation, therefore there is the potential for making the problem worse, not better

• Because disk I/O is generally the fastest I/O on a system(compared with tape or printer I/O), swapping will usually enhance performance


What is Swapping?What is Swapping? Memory management

• Long term queue of processes stored on disk

• Processes “swapped” in as space becomes available

• As a process completes it is moved out of main memory

• If none of the processes in memory are ready (i.e. all I/O blocked)– Swap out a blocked process to intermediate

queue– Swap in a ready process or a new process– But swapping is an I/O process...


Use of SwappingUse of Swapping Memory management

(a)Sample Job Scheduling(a)Sample Job Scheduling


Use of SwappingUse of Swapping Memory management

(b)Swapping(b)Swapping


PartitioningPartitioning Memory management

• Simplest scheme for partitioning available memory is to use fixed-size partitions

• Fixed size partitions– Although the partitions are of fixed size, they need not

be of equal size– When a process is brought into memory, it is placed in

the smallest available partition that will hold it– Even with the use of unequal fixed-size partitions,

there will be wasted memory– Leads to variable sized partitions


Fixed PartitionsFixed Partitions Memory management


Variable Sized Partitions Variable Sized Partitions Memory management

• More efficient approach • Allocate exactly the required memory to a

process• This leads to a hole at the end of memory,

too small to use– Only one small hole - less waste

• When all processes are blocked, swap out a process and bring in another

• New process may be smaller than swapped out process


Variable Sized Partitions Variable Sized Partitions Memory management

• Memory becomes more and more fragmented, and memory utilization declines– Eventually have lots of holes : fragmentation

• Solutions:– Coalesce - Join adjacent holes into one large hole– Compaction

• From time to time go through memory and move all hole into one free block (c.f. disk de-fragmentation)

• Operating system shifts the processes in memory to place all the free memory together in one block

• This is a time-consuming procedure, wasteful of processor time


Effect of Dynamic PartitioningEffect of Dynamic Partitioning Memory management


Effect of Dynamic PartitioningEffect of Dynamic Partitioning Memory management


RelocationRelocation Memory management

• No guarantee that process will load into the same place in memory

• Instructions contain addresses– Locations of data– Addresses for instructions (branching)

• Logical address - relative to beginning of program

• Physical address - actual location in memory (this time)

• Automatic conversion using base address



• The process is not likely to be loaded into the same place in main memory each time it is swapped in

• If compaction is used, a process may be shifted while in main memory

• The process in memory consists of instruction plus data

• The instructions will contain addresses for memory locations of two types– Addresses of data items– Addresses of instructions used for branching

instruction



• Addresses are not fixed• They will change each time a process is

swapped in• To solve this problem, a distinction is made

between logical addresses and physical addresses– Logical address

• A location relative to the beginning of the program

– Physical address• Actual location in memory

• Automatically converts from logical to physical address


PagingPaging Memory management

• Memory is partitioned into equal fixed-size chunks

• Chunk of a program, known as pages, could be assigned to available chunks of memory, known as frames, or page frames

• Wasted space in memory for that process is a fraction of the last page

• The list of free frames is maintained by the operating system

• A process does not require contiguous page frames

• Use page table to keep track


Allocation of Free FramesAllocation of Free Frames Memory management


Paging Paging Memory management

• The OS maintains a page table for each process

• The page table show the frame location for each page of the process

• Main memory is divided into many small equal-size frames

• Each process is divided into frame-size pages• Smaller processes require fewer pages, larger

processes require more


Logical and Physical AddressesLogical and Physical Addresses Memory management


Virtual MemoryVirtual Memory Memory management

• Demand Paging• Page Table Structure


Demand PagingDemand Paging Memory management

• Each page of a process is brought in only when it is needed(on demand)

• If the program branch to an instruction on a page not in main memory, if the program references data on a page not in memory, a page fault is triggered



• Only a few pages of any given process are in memory

• When it brings one page in, it must throw another page out

• Page fault– Required page is not in memory– Operating System must swap in required page– May need to swap out a page to make space– Select page to throw out based on recent history



• Thrashing– Processor spends most of its time swapping pages

rather than executing instructions– Too many processes in too little memory– Operating System spends all its time swapping– Little or no real work is done– Disk light is on all the time

• Solutions– Good page replacement algorithms– Reduce number of processes running– Fit more memory



• Operating system tries to guess, based on recent history, which pages are least likely to be used in the near future

• It is possible for a process to be larger than all of main memory– We do not need all of a process in memory for it to run– We can swap in pages as required

• Because a process executes only in main memory, that memory is referred to as real memory

• Programmer or user perceives a much larger memory – that which is allocated on the disk– Virtual Memory

• So we can now run processes that are bigger than total memory available!


Page Table StructurePage Table Structure Memory management

• The basic mechanism for reading a word from memory involves the translation of a virtual, or logical, address, consisting of frame number and offset, into a physical address, consisting of frame number and offset, using a page table

• The page number of a virtual address is used to index that table and look up the corresponding frame number

• This is combined with the offset portion of the virtual address to produce the desired real address



• Most Virtual memory schemes store page tables in virtual memory– Page tables are subject to paging– When a process is running, at least a part of its page table

must be in main memory, including the page table entry of the currently executing page

• Two level scheme – Page directory

• Each entry points to a page table

– Inverted page table



• Inverted Page Table– The page number portion is mapped into a hash table

using a simple hashing function– The hash table contains a pointer to inverted page table,

which contains the page table entries– There is one entry in the hash table and inverted page

table for each real memory page– A fixed portion of real memory is required for the tables

regardless of the number of processes or virtual pages supported


Inverted Page Table StructureInverted Page Table Structure Memory management


Translation Translation LookasideLookaside Buffer Buffer Memory management

• Every virtual memory reference causes two physical memory accesses– Fetch the appropriate page table entry– Fetch the desired data

• The straightforward virtual memory scheme would have the effect of doubling the memory access time

• Cache functions as a memory cache and contain page table entries that have been most recently used

• Use special cache for page table : TLB


Translation Translation LookasideLookaside Buffer Buffer Memory management

• A virtual address is in the form of a page number, offset

• The memory system consults the TLB to see if the matching page table entry is present

• If it is, the real address is generated by combining the frame number with the offset

• If not, the entry is accessed from a page table


Operation of Paging and TLBOperation of Paging and TLB Memory management


LookasideLookaside Buffer and CacheBuffer and Cache Memory management


SegmentationSegmentation Memory management

• Visible to the programmer – Paging is invisible

• Provided as a convenience for organizing programs and data

• Associating privilege and protection attributes with instructions and data

• Usually different segments allocated to program and data


Advantages of SegmentationAdvantages of Segmentation Memory management

• Simplify the handling of growing data structure

• Allow programs to be altered and recompiled independently

• Lend itself to sharing among processes• Lend itself to protection• Some systems combine segmentation

with paging


SegmentationSegmentation Memory management

• Simplifies handling of growing data structures

• Allows programs to be altered and recompiled independently, without re-linking and re-loading

• Lends itself to sharing among processes• Lends itself to protection• Some systems combine segmentation

with paging


Pentium II Memory ManagementPentium II Memory Management PentiumII&PowerPCMemory Management

• Address Spaces– Unsegmented unpaged memory– Unsegmented paged memory– Segmented unpaged memory– Segmented paged memory

• Segmentation– Table Indicator– Segment number– Request privilege level(RPL)

• Paging












Pentium II Memory ParameterPentium II Memory Parameter PentiumII&PowerPCMemory Management


Pentium II Memory ParameterPentium II Memory Parameter PentiumII&PowerPCMemory Management


Memory Address TranslationMemory Address Translation PentiumII&PowerPCMemory Management


Pentium II SegmentationPentium II Segmentation PentiumII&PowerPCMemory Management

• Each virtual address is 16-bit segment and 32-bit offset

• 2 bits of segment are protection mechanism• 14 bits specify segment• Unsegmented virtual memory 232 = 4Gbytes• Segmented 246=64 terabytes

– Can be larger – depends on which process is active– Half (8K segments of 4Gbytes) is global– Half is local and distinct for each process


Pentium II ProtectionPentium II Protection PentiumII&PowerPCMemory Management

• Protection bits give 4 levels of privilege– 0 most protected, 3 least– Use of levels software dependent– Usually level 3 for applications, level 1 for O/S and

level 0 for kernel (level 2 not used)– Level 2 may be used for apps that have internal se

curity e.g. database– Some instructions only work in level 0


Pentium II PagingPentium II Paging PentiumII&PowerPCMemory Management

• Segmentation may be disabled– In which case linear address space is used

• Two level page table lookup– First, page directory

• 1024 entries max• Splits 4G linear memory into 1024 page groups of 4Mb• Each page table has 1024 entries corresponding to 4Kb pages• Can use one page directory for all processes, one per process

or mixture• Page directory for current process always in memory

– Use TLB holding 32 page table entries– Two page sizes available 4k or 4M


PowerPC MemoryPowerPC Memory--ManagementManagement PentiumII&PowerPCMemory Management

• Comprehensive set of addressing mechanism• Block address translation• Address one drawback of paging mechanism• Block addressing enable the map four large

blocks of instruction memory and four large block of data memory in a way that bypass the paging mechanism


PowerPC Memory Management PowerPC Memory Management PentiumII&PowerPCMemory Management

• 32 bit – paging with simple segmentation– 64 bit paging with more powerful segmentation

• Or, both do block address translation– Map 4 large blocks of instructions & 4 of memory to bypass

paging– e.g. OS tables or graphics frame buffers

• 32 bit effective address– 12 bit byte selector

• =4kbyte pages– 16 bit page id

• 64k pages per segment– 4 bits indicate one of 16 segment registers

• Segment registers under OS control


Memory Management(PPC32bit) Memory Management(PPC32bit) PentiumII&PowerPCMemory Management


PowerPC Address TranslationPowerPC Address Translation PentiumII&PowerPCMemory Management


PowerPC Memory ManagementPowerPC Memory Management PentiumII&PowerPCMemory Management


PowerPC Memory ManagementPowerPC Memory Management PentiumII&PowerPCMemory Management

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Chapter 8 Operating System Support - Yonsei