Parallel Algorithms & Parallel Programming
ď ƒ Check Your Progress 3 1) (a) syntax for parallel directive : #pragma omp parallel [set of clauses] where clause is one of the following: structured-block if(scalar-expression) private(list) firstprivate(list) default(shared | none) shared(list) copyin(list) (b) syntax for sections directive : #pragma omp sections [set of clauses.] { #pragma omp section structured-bloc #pragma omp section structured-block . . . } The clause is one of the following: private(list) firstprivate(list) lastprivate(list) reduction(operator: list) nowait
(c) syntax for master directive : #pragma omp master structured_block 2) In parallel programming, shared memory programming in particular, processes very often compete with each other for common resources. To ensure the consistency, we require some mechanisms called process synchronization. There are numerous techniques to enforce synchronization among processes. We discussed one of them in unit 3.2.2 using lock and unlock process. Now we are introducing semaphores. Semaphores were devised by Dijkstra in 1968. It consists of two operations P and V operating on a positive integer s (including zero).P waits until s is greater than zero and then decrements s by one and allows the process to continue. V increments s by one and releases one of the waiting processes (if any). P and V operations are performed atomically. Mechanism for activating waiting processes is also implicit in P and V operations. Processes delayed by P(s) are kept in abeyance until released by a V(s) on the same semaphore. Mutual exclusion of critical sections can be achieved with one semaphore having the value 0 or 1 (a binary semaphore), which acts as a lock variable.
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