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1
Operating Systems
404452 Section 1
9 October 2004
Chapters 4
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Chapter 4 Operating Systems 2
Chapter 4: Processes
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems
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Chapter 4 Operating Systems 3
Process Concept
An operating system executes a
variety of programs: Batch system jobs
Time-shared systems userprograms or tasks
Textbook uses the termsjob and
process almost interchangeably Process a program in execution;
process execution must progressin sequential fashion
A process includes:
program counter
stack
data section
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Chapter 4 Operating Systems 4
Process State
As a process executes, it
changes state new: The process is
being created
running: Instructionsare being executed
waiting: The processis waiting for someevent to occur
ready: The process iswaiting to be assigned
to a process terminated: The
process has finishedexecution
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Chapter 4 Operating Systems 5
Process Control Block (PCB)
Information associated with
each process Process state
Program counter
CPU registers
CPU scheduling information
Memory-managementinformation
Accounting information
I/O status information
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Chapter 4 Operating Systems 6
CPU Switch From Process to Process
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Chapter 4 Operating Systems 7
Process Scheduling Queues
Job queue set of all processes in the system Ready queue set of all processes residing in
main memory, ready and waiting to execute
Device queues set of processes waiting for
an I/O device Processes migrate among the various queues
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Chapter 4 Operating Systems 8
Ready Queue And Various I/O DeviceQueues
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Chapter 4 Operating Systems 9
Representation of Process Scheduling
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Chapter 4 Operating Systems 10
Schedulers
Long-term scheduler (or job scheduler) selects which processes should be broughtinto the ready queue
Short-term scheduler (or CPU scheduler) selects which process should be executed nextand allocates CPU
Medium-term scheduler removes partiallyexecuted (incomplete) processes in and out ofmemory
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Chapter 4 Operating Systems 11
Addition of Medium Term Scheduling
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Chapter 4 Operating Systems 12
Schedulers (Cont.)
Short-term scheduler is invoked very
frequently (milliseconds) (must be fast) Long-term scheduler is invoked very
infrequently (seconds, minutes) (may beslow)
The long-term scheduler controls the degree ofmultiprogramming
Processes can be described as either:
I/O-bound process spends more time
doing I/O than computations, many shortCPU bursts
CPU-bound process spends more timedoing computations; few very long CPU
bursts
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Chapter 4 Operating Systems 13
Context Switch
When CPU switches to another process, the
system must save the state of the old processand load the saved state for the new process
Context-switch time is overhead; the systemdoes no useful work while switching
Time dependent on hardware support
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Chapter 4 Operating Systems 14
Process Creation
Parent process create children processes,
which, in turn create other processes, forminga tree of processes
Resource sharing
Parent and children share all resources
Children share subset of parents resources Parent and child share no resources
Execution
Parent and children execute concurrently
Parent waits until children terminate
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Chapter 4 Operating Systems 15
Process Creation (Cont.)
Address space
Child duplicate of parent (child has the sameprogram and data with parent).
Child has a program loaded into it
UNIX examples
fork system call creates new process exec system call used after a fork to replace the
process memory space with a new program
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Chapter 4 Operating Systems 16
C Program Forking Separate Process
int main()
{Pid_t pid;
/* fork another process */pid = fork();if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");exit(-1);
}else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL); //replaceaddress space with new program}
else { /* parent process *//* parent will wait for the child to
complete */wait (NULL);printf ("Child Complete");exit(0);
}
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Chapter 4 Operating Systems 17
A tree of processes on a typical Solaris
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Chapter 4 Operating Systems 18
Process Termination
Process executes last statement and asks the
operating system to delete it (exit) Output data from child to parent (via wait)
Process resources are deallocated by operatingsystem
Parent may terminate execution of childrenprocesses (abort)
Case 1: Child has exceeded allocated resources
Case 2: Task assigned to child is no longer required
Case 3: parent is exiting
Some operating system do not allow child tocontinue if its parent terminates
All children terminated - cascading termination
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Chapter 4 Operating Systems 19
Cooperating Processes
Independent process cannot affect or be
affected by the execution of another process Cooperating process can affect or be affected
by the execution of another process (parallelprogramming)
Advantages of process cooperation Information sharing (concurrent users interested in
the same information).
Computation speed-up
Modularity
Convenience (single user can issues several task inthe same time)
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Chapter 4 Operating Systems 20
InterProcess Communication (IPC)
Cooperating processes requires means of managing the
exchange of information and data.
This means (ways) are know as IPC which is a mechanismfor processes to communicate and to synchronize theiractions
Two main ways to establish IPC:
1) Shared memory
Part of the memory is shared between the cooperatingprocesses. The cooperating processes can read/write tothis shared space.
2) Message passing
The cooperating processes communicate throughoutexchanging messages.
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Chapter 4 Operating Systems 21
Shared memory : Producer-Consumer Problem
Paradigm for cooperating processes,producerprocess produces informationthat is consumed by a consumerprocess
unbounded-bufferplaces nopractical limit on the size of thebuffer
bounded-bufferassumes that there
is a fixed buffer size
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Chapter 4 Operating Systems 22
Bounded-Buffer Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
Typedef struct {
. . .
} item;item buffer[BUFFER_SIZE];
int in = 0;// points to next free location
int out = 0;// points to the first full location
Buffer is FULL if (in+1 % BUFFER_SIZE ==out)
Buffer is Empty if in == out
Solution is correct, but can only useBUFFER_SIZE-1 elements
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Chapter 4 Operating Systems 23
Bounded-Buffer Insert() Method
while (true) {/* Produce an item */
while ( (in + 1) % BUFFER_SIZE_count) == out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
{
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Chapter 4 Operating Systems 24
Bounded Buffer Remove() Method
while (true) {
while (in == out)
; // do nothing -- nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER_SIZE;
return item;{
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Chapter 4 Operating Systems 25
Message passing
Message system processes communicate witheach other without resorting to shared variables
Useful in distributed environments.
IPC facility provides two operations:
send(message) message size fixed or variable
receive(message)
IfPand Q wish to communicate, they need to: establish a communicationlinkbetween them
exchange messages via send/receive
Implementation of communication link
physical (e.g., shared memory, hardware bus) logical (e.g., logical features: Direct/Indirect
synchronous/asynchronous )
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Chapter 4 Operating Systems 26
Implementation Questions
How are links established?
Can a link be associated with more than twoprocesses?
How many links can there be between everypair of communicating processes?
What is the capacity of a link? Is the size of a message that the link can
accommodate fixed or variable?
Is a link unidirectional or bi-directional?
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Chapter 4 Operating Systems 27
Communications Models
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Chapter 4 Operating Systems 28
Direct Communication
Processes must name each other explicitly:
send (P, message) send a message toprocess P
receive(Q, message) receive a messagefrom process Q
Properties of communication link Links are established automatically
A link is associated with exactly one pair ofcommunicating processes
Between each pair there exists exactly one link
The link may be unidirectional, but is usually bi-directional
Disadvantage changing process identifierrequires modifying all references to it.
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Chapter 4 Operating Systems 30
Indirect Communication
Operations
create a new mailbox send and receive messages through
mailbox
destroy a mailbox
Primitives are defined as:
send(A, message) send a message tomailbox A
receive(A, message) receive a message
from mailbox A
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Chapter 4 Operating Systems 31
Indirect Communication
Mailbox sharing
P1, P2, and P3 share mailbox A P1, sends; P2and P3 receive
Who gets the message?
Solutions
Allow a link to be associated with at mosttwo processes
Allow only one process at a time to executea receive operation
Allow the system to select arbitrarily thereceiver. Sender is notified who the receiverwas.
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Chapter 4 Operating Systems 32
Indirect communication (2)
Mailbox owned by a process.
- The process that owns the mailbox canexecutes receive only. This elevates theproblem of who should receive the message.
- If the process who owns the mailbox
terminates, the mailbox should be destroyed Mailbox owned by the OS
- Created and controlled by the OS.
- Only the OS can destroy the mailbox.
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Chapter 4 Operating Systems 33
Synchronization
Message passing may be either blocking or
non-blocking Blocking is considered synchronous
Blocking send has the sender block untilthe message is received
Blocking receive has the receiver blockuntil a message is available
Non-blocking is considered asynchronous
Non-blocking send has the sender send the
message and continue Non-blocking receive has the receiver
receive a valid message or null
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Chapter 4 Operating Systems 34
Buffering
Regardless whether the communication is direct or
indirect the messages must reside in temporaryqueue.
Queue of messages attached to the link;implemented in one of three ways
1. Zero capacity the queue has 0 lengthSender must block for receiver
2. Bounded capacity finite length ofn messages-If queue is not full sender send and continue
If queue is full
Sender must block3. Unbounded capacity infinite length
Sender never waits
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Chapter 4 Operating Systems 35
Client-Server Communication
Sockets
Remote Procedure Calls Remote Method Invocation (Java)
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Chapter 4 Operating Systems 36
Sockets
A socket is defined as an endpoint for
communication Any pair of processes wish to communicates
must employ a pair of sockets. One on eachside.
Concatenation of IP address and port
The socket 161.25.19.8:1625 refers to port1625 on host 161.25.19.8
Communication consists between a pair ofsockets
Sockets are client-server architecture, whereserver listen to well-known ports. Oncemessage is received on these ports it acceptsthe connection from client socket.
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Chapter 4 Operating Systems 37
Socket Communication
* How a process in different computer can establish a connection with the web server ?
* How could another process in the samer computer establish a connection with the webserver ?
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Chapter 4 Operating Systems 38
Remote Procedure Calls
Remote procedure call (RPC) abstracts procedure callsbetween processes on networked systems.
Stubs (Proxy for the remote object). It is a softwarethat acts as an interface with the actual RPC and hidesthe details of real RPC communication.
Each RPC on the client has a specific stub.
Marshalling: the process of converting the parametersinto a format the could be transmitted over the networkand understood by the server.
The client-side stub locates the port on the server andmarshalls the parameters.
The server-side stub receives this message, unpacks themarshalled parameters, and performs the procedure onthe server.
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Chapter 4 Operating Systems 39
Execution of RPC
RPC steps:1-Client invokes RPC.
2-The suitable stub on the clientis called with the procedureparameters.
3-The stub locates procedureport on the server, marshalls theparameters and sends therequest.
4-The server stub receives the
request, unmarshalls theparameters, performs therequired procedure and returnsthe result.
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Chapter 4 Operating Systems 40
Remote Method Invocation
Remote Method Invocation (RMI) is a Java
mechanism similar to RPCs. RMI allows a Java program on one machine to
invoke a method on a remote object.
Objects are considered remote if they are
located in different (JVM).
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Ch 4 O i S 41
Marshalling Parameters
Stub: Client side proxy that is called when the client invokes a
remote object.Skeleton: server side proxy that is responsible for unmarshallingthe parameters and calling the desired remote method.