Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Operating Systems Lecture 10 Processes II Read Ch

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts 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 sharing  Computational speedup  Modularity  Convenience

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

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

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

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Bounded-Buffer – Producer Process item nextProduced; while (1) { while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; }

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Bounded-Buffer – Consumer Process item nextConsumed; while (1) { while (in == out) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; }

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

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

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

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

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

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

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Indirect Communication Mailbox sharing  P 1, P 2, and P 3 share mailbox A.  P 1, sends; P 2 and P 3 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.

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

Silberschatz, Galvin and Gagne  2002 Modified for CSCI 399, Royden, Operating System Concepts Buffering Queue of messages attached to the link; implemented in one of three ways. 1.Zero capacity – 0 messages Sender must wait for receiver (rendezvous). 2.Bounded capacity – finite length of n messages Sender must wait if link full. 3.Unbounded capacity – infinite length Sender never waits.