ECE1747 Parallel Programming课件.ppt

Parallel SOR with Barriers (1 of 2) void* sor (void* arg) { int slice = (int)arg; int from = (slice * (n-1))/p + 1; int to = ((slice+1) * (n-1))/p + 1; for some number of iterations { … } } 精品文档 Parallel SOR with Barriers (2 of 2) for (i=from; ito; i++) for (j=1; jn; j++) temp[i][j] = 0.25 * (grid[i-1][j] + grid[i+1][j] + grid[i][j-1] + grid[i][j+1]); barrier(); for (i=from; ito; i++) for (j=1; jn; j++) grid[i][j]=temp[i][j]; barrier(); 精品文档 Parallel SOR with Barriers: main int main(int argc, char *argv[]) { pthread_t *thrd[p]; /* Initialize mutex and condition variables */ for (i=0; ip; i++) pthread_create (thrd[i], attr, sor, (void*)i); for (i=0; ip; i++) pthread_join (thrd[i], NULL); /* Destroy mutex and condition variables */ } 精品文档 Note again Many shared memory programming systems (other than Pthreads) have barriers as basic primitive. If they do, you should use it, not construct it yourself. Implementation may be more efficient than what you can do yourself. 精品文档 Busy Waiting Not an explicit part of the API. Available in a general shared memory programming environment. 精品文档 Busy Waiting initially: flag = 0; P1: produce data; flag = 1; P2: while( !flag ) ; consume data; 精品文档 Use of Busy Waiting On the surface, simple and efficient. In general, not a recommended practice. Often leads to messy and unreadable code (blurs data/synchronization distinction). May be inefficient 精品文档 Private Data in Pthreads To make a variable private in Pthreads, you need to make an array out of it. Index the array by thread identifier, which you should keep track of . Not very elegant or efficient. 精品文档 Other Primitives in Pthreads Set the attributes of a thread. Set the attributes of a mutex lock. Set scheduling parameters. 精品文档 ECE 1747 Parallel Programming Machine-independent Performance Optimization Techniques 精品文档 Returning to Sequential vs. Parallel Sequential execution time: t seconds. Startup overhead of parallel execution: t_st seconds