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

2. Interprocess Communication (IPC) • Mechanism for processes to communicate and to synchronize their actions • Shared memory • Message Passing

3. Communications Models

4. Shared Memory in POSIX • Requires allocating a shared memory region • Attaching region to processes address space • Read/write to memory as usual. • Detach from address space • Delete shared memory.

5. 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.

6. Constraints • Do not over-write an item not yet consumed. • Do not write to a full buffer • Do not read from a previously read item. • Do not read from an empty buffer.

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

8. Buffer is empty when in == out ; • Buffer is full when ((in + 1) % BUFFER_SIZE) == out ;

9. Bounded-Buffer – Producer Process item nextProduced; while (1) { while (((in + 1) % BUFFER_SIZE) == out) ; //BUSY WAIT buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; }

10. Bounded-Buffer – Consumer Process item nextConsumed; while (1) { while (in == out) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; }

11. Producer: while (((in + 1) % BUFFER_SIZE) == out) ;//falls through loop Consumer Blocked: while (in == out) ; out in In = out = 0

12. Producer: while (((in + 1) % BUFFER_SIZE) == out) ;//falls through loop Consumer : while (in == out) ; // falls through loop O out in in = 1, out = 0 //either can fire

13. Producer: while (((in + 1) % BUFFER_SIZE) == out) ;//falls through loop Consumer : while (in == out) ; //falls through loop O O out in in = 2, out = 0 //either can fire

14. Producer: while (((in + 1) % BUFFER_SIZE) == out) ;//falls through loop Consumer : while (in == out) ; //falls through loop O O O in out in = 3, out = 0 //either can fire

15. Producer: while (((in + 1) % BUFFER_SIZE) == out) ; //blocks Consumer : while (in == out) ; // falls through loop O O O O in out in = 4, out = 0 // consumer can fire

16. Producer: while (((in + 1) % BUFFER_SIZE) == out) ; // no longer blocked Consumer : while (in == out) ; // falls through loop X O O O in out in = 4, out = 1 // producer and consumer can fire

17. Producer: while (((in + 1) % BUFFER_SIZE) == out) ; // no longer blocked Consumer : while (in == out) ; // falls through loop X X O O O in out in = 0, out = 2 // producer and consumer can fire

18. Producer: while (((in + 1) % BUFFER_SIZE) == out) ; // no longer blocked Consumer : while (in == out) ; // falls through loop X X X O O in out in = 0, out = 3 // producer and consumer can fire

19. Producer: while (((in + 1) % BUFFER_SIZE) == out) ; // no longer blocked Consumer : while (in == out) ; // falls through loop O X X X O in out in = 1, out = 4 // producer and consumer can fire

20. Message Passing • Requires significantly less programming. • Do not need to share address space (i.e., can be extended to processes executing on a different system when connected via some network).