Computer Architecture and Operating Systems CS 3230: Operating System Section Lecture OS-4 Process Communication Department of Computer Science and Software Engineering University of Wisconsin-Platteville

Outlines  Cooperating Process  Inter-Process Communication  Communication by Memory Sharing

Concurrency  Multiple applications  Multiprogramming  Structured application  Application can be a set of concurrent processes  Operating-system structure  Operating system is a set of processes or threads

Cooperating Processes  Process Types:  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  Computation speed-up Improve performance by overlapping activities or performing work in parallel  Enable an application to have a better program structure as a set of cooperating processes  Information sharing

Cooperating Processes  Cooperating issues:  How do the processes communicate?  How do the processes share data?  Operating System Concerns:  Keep track of active processes  Allocate and de-allocate resources  Protect data and resources  Result of process must be independent of the speed of execution of other concurrent processes

Process Communications  Communication types:  Message passing : Inter-Process Communication (IPC)  Shared Memory

Inter-Process Communication  Mechanism for processes to communicate  IPC facility provides two operations:  send (message) – message size fixed or variable  receive (message)  The communicating processes can be (peer to peer) or (client-server)  If P and Q wish to communicate, they need to:  establish a communication link between them  exchange messages via send/receive

IPC Addressing: Direct  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

IPC Addressing: Indirect  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

IPC Addressing: Indirect  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

IPC Addressing: Indirect  Mailbox sharing problem  P 1, P 2, and P 3 share mailbox A  P 1, sends; P 2 and P 3 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

IPC Synchronization  Process can:  block until the message is sent/received (blocking) - safer, easier to code, slower  proceed immediately (non-blocking) - faster, harder to code, riskier, needs OS support  Process can:  block until the message it sent is received (synchronous) - easier to code, deadlock, slower  proceed without receipt confirmation (asynchronous) - faster, needs message acknowledgement

IPC Link Buffering  Link Capacity : the number of message that can be temporarily queued in a given link  Zero capacity: (queue of length 0)  No messages wait  Sender must block until receiver receives the message  Limited capacity: (queue of length n)  If receiver’s queue is not full, new message is put on queue, and sender can continue executing immediately  If queue is full, sender must block until space is available in the queue  Unlimited capacity: (infinite queue)  Sender can always continue

Cooperation Among Processes by Memory Sharing  One process is a producer of information; another is a consumer of that information  Processes communicate through shared variables  Producer writes data to a shared buffer  Consumer reads data from the shared buffer  Shared buffer:  unbounded-buffer no practical limit on the size of the buffer  bounded-buffer there is a fixed buffer size  Synchronization problems  Critical section problem (next lecture)

Example: Bounded Buffer Shared data #define BUFFER_SIZE 10 typedef struct {... } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; int counter = 0;

Example: Bounded Buffer Producer process item nextProduced; while (1) { while (counter == BUFFER_SIZE) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; counter++; }

Example: Bounded Buffer Consumer process item nextConsumed; while (1) { while (counter == 0) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--; }