Cooperating Processes

1. 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 • Robustness Operating System Concepts

2. Producer-Consumer Problem • Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process. • unbounded-buffer places no practical limit on the size of the buffer. • bounded-buffer assumes that there is a fixed buffer size. Operating System Concepts

3. Mechanisms for IPC Operating System Concepts

4. Bounded-Buffer – Shared Memory Solution • Shared data #define BUFFER_SIZE 10 typedef struct { . . . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; Operating System Concepts

5. Bounded-Buffer – Shared Memory Solution • Producer while (1) { while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; } • Consumer while (1) { while (in == out) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; • Can only use BUFFER_SIZE-1 elements Operating System Concepts

6. Message Passing • Processes communicate without shared variables • If P and Q wish to communicate, they need to: • establish a communicationlink between them • send(message) • receive(message) • Producer - Consumer while (1) { send(consumer,produceNext()); } while (1) { nextToConsume = receive(producer); } Operating System Concepts

7. Message Passing - Link Properties • How are links established? • Can a link be associated with more than two processes? • How many links can there be between processes? • What is the capacity of a link? (buffers?) • Fixed or variable size messages? • Is the link simplex or duplex or full duplex? Operating System Concepts

8. Message Passing - Logical Properties • Direct or indirect communication (process or mailbox) • Symmetric or asymmetric (names both ways or one way) • Automatic or explicit or no buffering • Send by copy or send by reference • Fixed or variable size messages • Synchronous (blocking) or asynchronous (non-blocking) Operating System Concepts

9. Symmetry of naming Operating System Concepts

10. Direct Communication • Symmetric: Processes name each explicitly • send (P, message) – send a message to process P • receive(Q, message) – receive a message from process Q • Links are established automatically, on demand. • A link is associated with exactly two processes. • Between each pair there exists exactly one link. • Asymmetric: Only sender names the receiver • send (P, message) – send message to process P • receive(?, message) – receive message from anyone • Asymmetric: Only receiver names the sender • send (?, message) – send message to anyone (copies?) • receive(Q, message) – receive message from process Q Operating System Concepts

11. 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. • Operations • Create a new mailbox (Ownership? Who receives?) • send(A,message) – send a message to mailbox A • message = receive(A) – receive a message from A • Destroy a mailbox (? If owner terminates) • 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 links. Operating System Concepts

12. Indirect Communication • Mailbox sharing • P1, P2, and P3 share mailbox A. • P1, sends; P2and 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 • Allow the system to select arbitrarily the receiver. • Sender is notified who the receiver was. • Atomicity, regardless Operating System Concepts

13. Synchronization • Send and receive can be blocking or non-blocking • Blocking send waits until message is received • Non-blocking send allows sender to continue • Sender is not assured of reception - must ack • Sender send(receiver,message); receive(receiver,ack_message); • Receiver receive(sender,message); send(sender,”ack”); • Blocking receive waits until message is available • Non-blocking receive returns null if no message • Blocking send and receives forces a rendezvous Operating System Concepts

14. Buffering • Queue of messages attached to the link; implemented in one of three ways. • Zero capacity – 0 messagesSender must wait for receiver (must rendezvous). • Bounded capacity – finite length of n messagesSender must wait if link full, or overwrite • Unbounded capacity – infinite length (right!). Operating System Concepts

15. Exception Conditions • Process terminates • Receiver waiting for a message from terminated sender • Notify or terminate receiver • Sending to a terminated receiver • Delete message • Notify sender? • Lost messages • Detect with timeouts - expect an ack within some time • Resend (duplicates) • Scrambled messages • Error checking codes Operating System Concepts