Nested parallellism in OpenMP -

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i want map tasks 3 threads follows:

  1. each of taska, taskb, , taskc must executed separate threads.

  2. taska has subtasks task(1), task(2), , task(3).

  3. taskb has subtasks task(11), task(12), , task(13).

  4. taskc has subtasks task(21), task(22), , task(23).

  5. if 1 of taska, taskb, , taskc finishes , there @ least 1 unstarted subtask of task, thread associated finished task should steal unstarted subtask.

i not able achieve setting. able following mwe. in mwe, threads not obey rules 2, 3, 4.

here mwe:

double task(int taskid) {     int tid = omp_get_thread_num();     int nthreads = omp_get_num_threads();        printf("%d/%d: taskid=%d\n", tid, nthreads, taskid);      int i;     double t = 1.1;     for(i = 0; < 10000000*taskid; i++) {         t *= t/i;     }     return t; }  double taska() {     int tid = omp_get_thread_num();     int nthreads = omp_get_num_threads();        printf("%s %d/%d\n", __function__, tid, nthreads);     double a, b, c;     //#pragma omp parallel     //#pragma omp single     {     #pragma omp task untied shared(a)     a=task(1);     #pragma omp task  untied shared(b)     b=task(2);     #pragma omp task  untied shared(c)     c=task(3);     }     return a+b+c; }  double taskb() {     int tid = omp_get_thread_num();     int nthreads = omp_get_num_threads();        printf("%s %d/%d\n", __function__, tid, nthreads);       double a, b, c;     //#pragma omp parallel     //#pragma omp single     {     #pragma omp task  untied  shared(a)     a=task(11);     #pragma omp task  untied  shared(b)     b=task(12);     #pragma omp task  untied  shared(c)     c=task(13);     }     return a+b+c; }  double taskc() {     int tid = omp_get_thread_num();     int nthreads = omp_get_num_threads();        printf("%s %d/%d\n", __function__, tid, nthreads);       double a, b, c;     //#pragma omp parallel     //#pragma omp single     {     #pragma omp task  untied  shared(a)     a=task(21);     #pragma omp task  untied  shared(b)     b=task(22);     #pragma omp task  untied  shared(c)     c=task(23);     }     return a+b+c; } int main() {     omp_set_num_threads(3);     double a,b,c;      #pragma omp parallel     #pragma omp single     {         #pragma omp task untied         a=taska();         #pragma omp task untied         b=taskb();         #pragma omp task untied         c=taskc();     }     #pragma omp taskwait     printf("%g %g %g\n", a, b, c);     return 0; }

compiled as:

icpc -wall -fopenmp -o2 -o nestedomp nestedomp.c

output:

taskc 1/3 1/3: taskid=21 taska 2/3 taskb 0/3 0/3: taskid=23 2/3: taskid=22 1/3: taskid=1 1/3: taskid=2 2/3: taskid=3 0/3: taskid=11 1/3: taskid=12 2/3: taskid=13

here, thread 0 starts processing task 23, must start processing 1 or 11.

you use thread id structure work distribution:

#pragma omp parallel num_threads(3) {  int tid = omp_get_thread_num();   if (tid == 0)     //  task 0  } else if (tid == 1) {    //  task 1  } else    //  task 2    }

you can set number of threads according needs , introduce nesting @ task level.


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