silberschatz, galvin and gagne 2002 modified for csci 399, royden, 2005 4.1 operating system...

Silberschatz, Galvin and Gagne 2002Modified for CSCI 399, Royden, 20054.1Operating System Concepts

Operating Systems

Lecture 10Processes II

Read Ch 4.4 - 4.6


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

Question: Why would you want to have cooperating processes? (we will discuss this in class) Information sharingComputational speedupModularityConvenience


Producer-Consumer Problem

Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process.E.g. Compiler and assembler

Use a buffer that is filled by the producer and emptied by the consumer.

These must be synchronized (so consumer does not try to consume an item that has not yet been produced).

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

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


Bounded-Buffer – Shared-Memory Solution

A buffer may be provided by the OS through an IPC (interprocess communication) facility, or it may be coded by the application through shared memory.

Shared data

#define BUFFER_SIZE 10

typedef struct {

. . .

} item;

item buffer[BUFFER_SIZE];

int in = 0;

int out = 0;


Buffer is a Circular Array

The buffer is implemented as a circular array.

Two logical pointers: in--Next free position in buffer out--First full position in buffer

Buffer is empty when in == out Buffer is full when out == ((in + 1) % BUFFER_SIZE); Question: How many items can actually be held in

the buffer?


Bounded-Buffer – Producer Process

item nextProduced;

while (1) {

while (((in + 1) % BUFFER_SIZE) == out)

; /* do nothing */

buffer[in] = nextProduced;

in = (in + 1) % BUFFER_SIZE;

}


Bounded-Buffer – Consumer Process

item nextConsumed;

while (1) {while (in == out)

; /* do nothing */nextConsumed = buffer[out];out = (out + 1) % BUFFER_SIZE;

}


Interprocess Communication (IPC)

OS provides a means of communication between processes.

OS provides a mechanism for processes to communicate and to synchronize their actions.

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

This is particularly useful in distributed environments. Best accomplished by a message passing system.


The IPC concept

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)


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.

Disadvantage of direct communication: (we will discuss in class) Limited modularity. If change the name of one process, may

have to change multiple other processes that communicate with it.


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.



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 sharingP1, P2, and P3 share mailbox A.

P1, sends; P2 and P3 receive.

Who gets the message? Solutions (we will discuss these in class).

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 (synchronous) or non-blocking (asynchronous).

Blocking send: Sending process is blocked until the message is received by the receiving process or by the mailbox.

Non-blocking send: Sending process sends message and resumes execution without waiting.

Blocking receive: Receiver blocks until a message is available.

Non-blocking receive: Receiver retrieves either a valid message or a null.

When both send and receive are blocking, there is a rendezvous of the sender and receiver.


Buffering

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.

silberschatz, galvin and gagne 2002 modified for csci 399, royden, 2005 4.1 operating system...

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