1.8 Semaphore Statement

1. 1.8 Semaphore Statement • One of the main problems in coordinating parallel processes is the management of access to shared resources. • Mechanisms • Grouped statements (1.7) • Semaphore statement (1.8) • Region statement (1.9)

2. An example for a critical section, which shall be executed atomically: Fig. 1.8, P.57 (splitting one statement to two, to make it possible to be interleaved with others)

3. To prevent unwanted interferences, we can use • grouped statements • It is a locking device. Disallows any action by a parallel process until the process executing the sequence completes. • We want as much parallel processing as possible. • General constructs that achieve locking and coordination between processes are referred to as synchronization constructs.

4. Semaphore statements • request(r), release(r) • request(r): • <await r>0; r:=r-1> • release(r): • r:=r+1

5. Use of semaphores for mutual exclusion • Mutual exclusion problem: devise a protocol, that contains critical, non-critical, and coordination statements; where coordination statements guarantee exclusive execution of the critical sections. • Example: Fig. 1.9, p. 60

6. A location containing a request(r) statement: is a checkpoint, synchronizing the process with other processes containing request(r) and release(r) statements, on the same semaphore variable r. • An important assumption: variables used in the protocol for coordination between the processes (semaphore variables) are not modified in the critical sections. • Problem 1.4

7. Mutual exclusion of multiple processes by semaphores. • Fig. 1.10, p. 62

8. Other uses of Semaphores • To protect critical sections: request-release pairs appear within the same process. • Semaphores can be used in other signaling and synchronizations. • Fig. 1.12, p. 64

9. Example: producer-consumer • Fig.1.12, p.64 • Producer computes x, then append x to the end of buffer b, cycle back. • Consumer removes elements from the top of the buffer, deposits them in y, cycle back.

10. Semaphores • r: protect the critical section, buffer • ne: “number of empties”, ensuring that producer cannot deposit more than N items (that have not been removed by consumer). • nf: “ number of fulls”, ensuring that consumer does not take from an empty buffer. • nf and ne: unidirectional signaling devices. • nf, a signal generated by producer’s release and sensed by consumer’s request. • ne, a signal generated by consumer’s release and sensed by producer’s request.

11. 1.9 Region statement • Semaphores provide powerful synchronization constructs. • But they are unstructured and do not enforce a disciplined methodology. • Region statement: protect critical sections, and provide additional testing capabilities.

12. Example, Fig. 1.13, p. 68 • Problem 1.5 • Resource declaration • r: resource protecting(y1,y2, …,yn) • Region statement • region r when c do S • S does not contain any cooperation statement or other region statement.

13. Shared variables, must belong to precisely one resource. • c is the guard condition, and S is the body of the region statement. • The only shared variables that may be referenced within c or S must belong to r. • The region statement will be successfully executed, if the condition c be true and no other statement protected by r is currently executing. • See page 67

14. Comparing Region statements to Semaphores • Region statement comparing to Semaphores: • More structured • More powerful • More expensive implementation

15. implementation • For a semaphore: • keep track of the current value of the semaphore and • the identity of the processes waiting for it (request). • For a region statement: • keep track of the current status of the resource and • The identities of the waiting processes and • The status of their individual guard conditions.

16. Synchronization within Selection Statements • In the previous examples: • synchronization statements were essential, • and they appear in the context of concatenation. • It may not be essential, or several synchronizations may be possible • See Fig. 1.14, p. 69 • Producer-consumer with l producers, n consumers and m buffers

17. 1.10 Model 3: Message-Passing Text • We saw • Diagrams • Shared-variables text • Now • Message-passing text (like CSP)