operating system

CMET 331 Operating Systems

3. Processes

• Process Concept • Process Scheduling • Operations on Processes • Cooperating Processes • Interprocess Communication

Outline

Process 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 —stack —data section

Process vs. Program • Process – dynamic, Program – static • Process – temporary, Program – permanent

Process in Memory

A Heap Memory is a area in main memory that is used to assign dynamically allocated memory to a application.

Process 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 State

Process Control Block (PCB) Information associated with each process • Process state • Program counter • CPU registers • CPU scheduling information • Memory-management information • Accounting information • I/O status information

Process Control Block (PCB)

CPU Switch From Process to Process

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

Schedulers • The OS must select, for scheduling purposes,

processes for these queues in some fashion. The selection process is carried out by the appropriate scheduler.

• 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

Addition of Medium Term Scheduling

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 (the number of processes in memory)

• 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

Context 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

Process Creation • 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.) • Address space

—Child duplicate of parent —Child has a program loaded into it

A tree of processes on a typical Solaris

Process Creation - 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 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

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 cooperation

—Information sharing —Computation speed-up —Modularity —Convenience

• Interprocess Communication (IPC)

Interprocess Communication Model

(a) Message passing (b) Memory sharing

Three Issues of IPC • how one process can pass information to

another • making sure two or more processes do not get

into each other's way when engaging in critical activities (suppose two processes each try to grab the last 1 MB of memory)

• proper sequencing when dependencies are present: if process A produces data and process B prints it, B has to wait until A has produced some data before starting to print.

The Producer-Consumer Problem • Paradigm for cooperating processes –

Producer process produces information that is consumed by a Consumer process. —Example 1: a print program produces

characters that are consumed by a printer.

—Example 2: an assembler produces object modules that are consumed by a loader.

The Producer-Consumer Problem • We need a buffer to

hold items that are produced and later consumed: —unbounded-buffer

places no practical limit on the size of the buffer.

—bounded-buffer assumes that there is a fixed buffer size.

The Producer-Consumer Problem • Producer-Consumer Problem also known as the

bounded buffer problem —Two processes share a common, fixed-size buffer. —one of them, the producer, puts information into the

buffer —the other one, the consumer, takes it out —How to handle when producer wants to put a new

item in the buffer, but it is already full? • Solution is for the producer to go to sleep, to be awakened

when the consumer has removed one or more items; Similarly, if the consumer wants to remove an item from the buffer and sees that the buffer is empty, it goes to sleep until the producer puts something in the buffer and wakes it up

Idea for Producer-Consumer Solution • The bounded buffer is implemented as a circular array

with 2 logical pointers: in and out. • The variable in points to the next free position in the

buffer. • The variable out points to the first full position in the

buffer.

• Shared data #define BUFFER_SIZE 10 typedef struct {

. . . } item;

item buffer[BUFFER_SIZE]; int in = 0; int out = 0;

• The shared buffer is implemented as a circular array with pointers; in and out — in points to next free position in the buffer — out points to the first full position in the buffer — buffer is empty when in==out — buffer is full when ((in+1)%BUFFER_SIZE)==out

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

Bounded-Buffer – Shared-Memory Solution

Bounded-Buffer – Insert() Method

while (true) { /* Produce an item */ while (( (in + 1)% BUFFER_SIZE)

== out); /* do nothing -- no free buffers */

buffer[in] = item; in = (in + 1) % BUFFER SIZE;

}

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;

Message 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 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?

Direct 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 —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 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

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

Indirect Communication • Mailbox sharing

—P1, P2 and P3 share mailbox A —P1 sends; P2 and P3 receive —Who gets the message?

• Answers depend on which of the following methods we chosen: —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.

Synchronization • Message passing may be either blocking or non-

blocking • Blocking is considered synchronous

—Blocking send has the sender block until the message is delivered

—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

End of Chapter 3