chapter03

Chapter 3: ProcessesChapter 3: Processes

3.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts

Chapter 3: ProcessesChapter 3: Processes

Process Concept

Process Scheduling

Operations on Processes

Cooperating Processes

Interprocess Communication

Communication in Client-Server Systems


Process ConceptProcess Concept

An operating system executes a variety of programs:

Batch system – jobs

Time-shared systems – user programs or tasks

Textbook uses the terms job and process almost interchangeably

Process – a program in execution; process execution must progress in sequential fashion

A process includes:

program counter which represents current activity

Stack which holds temporary data such as function parameters, local variables and return addresses.

data section which contains global variables

Heap which contains memory that is dynamically allocated during process runtime.

Text includes the application's code, might be shared by a number of processes?

The process's address space.


Process in MemoryProcess in Memory


Process StateProcess State

As a process executes, it changes state

new: The process is being created

running: Instructions are being executed

waiting: The process is waiting for some event to occur

ready: The process is waiting to be assigned to a

processor

terminated: The process has finished execution


Diagram of Process StateDiagram of Process State


Process Control Block (PCB)Process Control Block (PCB)

Information associated with each process

Process state

Program counter indicates the address of next instruction.

CPU registers

CPU scheduling information includes process priority and pointers to scheduling queues.

Memory-management information includes value of limit and base registers

Accounting information includes amount of CPU time used, time limits etc.

I/O status information includes the list of I/O devices allocated to processes, list of open files


Process Control Block (PCB)Process Control Block (PCB)


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PCBPCB

Kernel User

text

data

heap

stack

PSW

IR

PC

SP

General Purpose Registers

.

RAM CPU

StateMemoryFilesAccountingPriorityUserCPU register storage

OS Data Structures

Process Address Space


CPU Switch From Process to ProcessCPU Switch From Process to Process


Process Scheduling QueuesProcess 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.

It is stored as a linked list. It contains pointers to the first and final PCB’s.

Each PCB includes a pointer field that points to the next PCB in the

ready queue.

Device queues – set of processes waiting for an I/O device

Processes migrate among the various queues


Ready Queue And Various I/O Device QueuesReady Queue And Various I/O Device Queues


Representation of Process SchedulingRepresentation of Process Scheduling


SchedulersSchedulers

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU

Medium term Scheduler (Swaper)


Addition of Medium Term SchedulingAddition of Medium Term Scheduling


Schedulers (Cont.)Schedulers (Cont.)

Short-term scheduler is invoked very frequently (milliseconds) (must be fast)

Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow)

The long-term scheduler controls the degree of multiprogramming

Processes can be described as either:

I/O-bound process – spends more time doing I/O than computations, many short CPU bursts

CPU-bound process – spends more time doing computations; few very long CPU bursts

If all processes are I/O bound, The ready queue will almost always be empty

If all processes are CPU bound, The I/O waiting queue will almost always be empty, devices will

go unused System will again be unbalanced


Context SwitchContext Switch

When CPU switches to another process, the system must save the state of the

old process and load the saved state for the new process

Context-switch time is overhead; the system does no useful work while switching

Time dependent on hardware support

Part of OS responsible for switching the processor among the processes is

called Dispatcher


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Two-state process modelTwo-state process model

EnterQueue

Dispatch

Pause

ExitProcessor

Dispatcher is now redefined:• Moves processes to the waiting queue• Remove completed/aborted processes• Select the next process to run


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a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0

Ready Ready Ready

How process state changes1How process state changes1


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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0


Running Ready Ready



TimeoutTimeout



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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0


Ready Running Ready


I/OI/O



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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

Ready Blocked Running

TimeoutTimeout

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0



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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0



Running Blocked Ready

I/OI/O



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Blocked Blocked Running

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0


The Next Process to Run cannot be simply selected from the front


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Blocked Blocked Running

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

Suppose the Green process finishes I/OSuppose the Green process finishes I/O



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Blocked Ready Running

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0

TimeoutTimeout



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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0


Blocked Running Ready




TimeoutTimeout



Process CreationProcess Creation

Reasons to create a process Submit a new batch job/Start program

User logs on to the system

OS creates on behalf of a user (printing)

Spawned by existing process

Parent process create children processes, which, in turn create other processes, forming a tree of processes

Resource sharing

Parent and children share all resources

Children share subset of parent’s resources

Parent and child share no resources

Execution

Parent and children execute concurrently

Parent waits until children terminate


Process Creation (Cont.)Process Creation (Cont.)

Address space

Child duplicate of 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


Process CreationProcess Creation


C Program Forking Separate ProcessC 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);}else { /* parent process */

/* parent will wait for the child to complete */

wait (NULL);printf ("Child Complete");exit(0);

}}


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The fork() system callThe fork() system call

At the end of the system call there is a new process waiting to run once the scheduler chooses it

A new data structure is allocated

The new process is called the child process.

The existing process is called the parent process.

The parent gets the child’s pid returned to it.

The child gets 0 returned to it.

Both parent and child execute at the same point after fork() returns


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Unix Process ControlUnix Process Control

int pid;int status = 0;

if (pid = fork()) {/* parent */…… pid = wait(&status);

} else {/* child */……exit(status);

}

Parent uses wait to sleep until the child exits; wait returns child pid and status.

Wait variants allow wait on a specific child, or notification of stops and other signals

Parent uses wait to sleep until the child exits; wait returns child pid and status.

Wait variants allow wait on a specific child, or notification of stops and other signals

Child process passes status back to parent on exit, to report success/failure

Child process passes status back to parent on exit, to report success/failure

The fork syscall returns a zero to the child and the child process ID to the parent

The fork syscall returns a zero to the child and the child process ID to the parent

Fork creates an exact copy of the parent process

Fork creates an exact copy of the parent process


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Process TerminationProcess Termination

Batch job issues Halt instruction User logs off Process executes a service request to terminate Parent terminates so child processes terminate Operating system intervention

such as when deadlock occurs Error and fault conditions

E.g. memory unavailable, protection error, arithmetic error, I/O failure, invalid instruction


Process TerminationProcess 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 operating system

Parent may terminate execution of children processes (abort)

Child has exceeded allocated resources

Task assigned to child is no longer required

If parent is exiting

Some operating system do not allow child to continue if its parent terminates

– All children terminated - cascading termination


Inter process CommunicationInter process Communication

Processes executing concurrently in the operating system may be either independent processes or 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


Advantages of Process CooperationAdvantages of Process Cooperation

Information sharing: Allow concurrent access to same piece of information that several users may be interested in it.

Computation speed-up: break a task to subtasks, each of which will be executing in parallel with the others. (Should be multiple processing elements such as CPUs)

Modularity: divided the system functions into separate processes or threads.

Convenience: for user, user may work on many tasks at the same time.


MODELS OF IPCMODELS OF IPC

Cooperating processes require an inter process communication mechanism that will allow them to exchange data and information.

1. Shared Memory

2. Message Passing


SHARED MEMORYSHARED MEMORY

1. Region of memory that is shared by cooperating processes is established.

2. Processes can then exchange information by reading and writing data to shared region


MESSAGE PASSINGMESSAGE PASSING

Communication takes place by means of messages exchanged between the cooperating processes.


Communications Models Communications Models


Advantages & Disadvantages of Message Advantages & Disadvantages of Message Passing & Shared MemoryPassing & Shared Memory

1. Message passing is useful for exchanging smaller amounts of data

2. Message Passing is easier to implement than shared memory for IPC

3. Shared Memory allows maximum speed and convenience as it can be done at memory speeds within a computer

4. Shared memory is faster than message passing as message passing is implemented using system calls.


Contd….Contd….

1. Message passing requires the more time consuming task of kernel intervention.

2. Shared memory system calls are required only to establish shared memory regions, no assistance from the kernel is required.


Producer-Consumer ProblemProducer-Consumer Problem

Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process

A compiler may produce assembly code, which is consumed by an assembler. The assembler, in turn, may produce object modules, which are consumed by the loader

A buffer of items that can be filled by the producer and emptied by the consumer. should be available

A producer can produce one item while the consumer is consuming another item.

The producer and consumer must be synchronized

the consumer does not try to consume an item that has not yet been produced.

the consumer must wait until an item is produced


Producer-Consumer ProblemProducer-Consumer Problem

unbounded-buffer places no practical limit on the size of the

buffer

the consumer may have to wait for new items, but

the producer can always produce new items

bounded-buffer assumes that there is a fixed buffer size

the consumer must wait if the buffer is empty, and

the producer must wait if the buffer is full.


Bounded-Buffer – Shared-Memory SolutionBounded-Buffer – Shared-Memory Solution

Shared data

#define BUFFER_SIZE 10

Typedef struct {

. . .

} item;

item buffer[BUFFER_SIZE];

int in = 0; % in: the next free position in the buffer

int out = 0; % out: the first full position in the buffer The buffer is empty when in == out ;

The buffer is full when ((in + 1) % BUFFERSIZE) == out

Solution is correct, but can only use BUFFER_SIZE-1 elements


Bounded-Buffer – Insert() MethodBounded-Buffer – Insert() Method

The producer process has a local variable nextproduced in which the new item to be produced is stored:

while (true) { /* Produce an item */

while (((in = (in + 1) % BUFFER SIZE count) == out)

; /* do nothing -- no free buffers */

buffer[in] = item;

in = (in + 1) % BUFFER SIZE;

{


Bounded Buffer – Remove() MethodBounded 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;

{

The consumer process has a local variable nextconsumed in which the item to be consumed is stored:


Message PassingMessage Passing

Mechanism for processes to communicate and to synchronize their actions

Message system – processes communicate with each other without resorting to shared variables

IPC facility provides two operations: send(message) – message size fixed or variable receive(message)

If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive

Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties) e.g. send(), receive()


Implementation QuestionsImplementation Questions

How are links established?

Can a link be associated with more than two processes?

How many links can there be between every pair 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?


Methods for logical implementation of linkMethods for logical implementation of link

1. Direct or Indirect Communication

2. Synchronous and Asynchronous Communication

3. Automatic or explicit Buffering


Direct CommunicationDirect Communication

Processes must name each other explicitly:

send (P, message) – send a message to process P

receive(Q, message) – receive a message from process Q

Properties of communication link

Links are established automatically because the processes need to know only the identities to communicate.

A link is associated with exactly one pair of communicating processes

Between each pair there exists exactly one link

The link may be unidirectional, but is usually bi-directional


Indirect CommunicationIndirect Communication

Messages are directed and received from mailboxes (also referred to as ports)

Each mailbox has a unique id

Processes can communicate only if they share a mailbox

Properties of communication link

Link established only if processes share a common mailbox

A link may be associated with many processes

Each pair of processes may share several communication links

Link may be unidirectional or bi-directional



Operations

create a new mailbox

send and receive messages through mailbox

destroy a mailbox

Primitives are defined as:

send(A, message) – send a message to mailbox A

receive(A, message) – receive a message from mailbox A



Mailbox sharing

P1, P2, and P3 share mailbox A

P1, sends; P2 and P3 receive

Who gets the message?

Solutions

Allow a link to be associated with at most two processes

Allow only one process at a time to execute a receive operation

Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.



If the mailbox is owned by a process (that is, the mailbox is

part of the address space of the process), then we distinguish between the owner (who can only receive

messages through this mailbox) and the user (who can only send messages to the mailbox).

Since each mailbox has a unique owner, there can be

no confusion about who should receive a message sent to this mailbox.

When a process that owns a mailbox terminates, the mailbox disappears.

Any process that subsequently sends a message to this mailbox

must be notified that the mailbox no longer exists.



If a mailbox owned by the operating system is independent and is not attached to any particular process: The operating system then must provide a mechanism

that allows a process to do the following:

Create a new mailbox.

Send and receive messages through the mailbox.

Delete a mailbox

Note: the ownership and receive privilege may be passed to other processes through appropriate system calls.


SynchronizationSynchronization

Message passing may be either blocking or non-blocking

Blocking is considered synchronous

Blocking send has the sender block until the message is received

Blocking receive has the receiver block until 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


BufferingBuffering

Queue of messages attached to the link; implemented in one of three ways

1.Zero capacity – 0 messagesSender must wait for receiver (rendezvous)

2.Bounded capacity – finite length of n messagesSender must wait if link full

3.Unbounded capacity – infinite length Sender never waits


Client-Server CommunicationClient-Server Communication

Sockets

Remote Procedure Calls

Remote Method Invocation (Java)


SocketsSockets

A socket is defined as an endpoint for communication

Concatenation of IP address and port

The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8

Communication consists between a pair of sockets


Socket CommunicationSocket Communication


Remote Procedure CallsRemote Procedure Calls

Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.

Stubs – client-side proxy for the actual procedure on the server.

The client-side stub locates the server and marshalls the parameters.

The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server.

A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely. For example, a program that uses Remote Procedure Calls ( RPC ) is compiled with stubs that substitute for the program that provides a requested procedure. The stub accepts the request and then forwards it (through another program) to the remote procedure. When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.

http://searchsoa.techtarget.com/sDefinition/0,,sid26_gci214272,00.html


Execution of RPCExecution of RPC

End of Chapter 3End of Chapter 3

chapter03

Technology

process priority

q iobound process

execution process execution

run18operating system

old process andload

cpu time

io devices processes

completedaborted processes