Sun Studio 12: OpenMP API User's Guide

4.3 Using OpenMP Library Routines Within Nested Parallel Regions

Calls to the following OpenMP routines within nested parallel regions deserve some discussion.

- omp_set_num_threads()
- omp_get_max_threads()
- omp_set_dynamic()
- omp_get_dynamic()
- omp_set_nested()
- omp_get_nested()

The 'set' calls affect future parallel regions at the same or inner nesting levels encountered by the calling thread only. They do not affect parallel regions encountered by other threads.

The 'get' calls return the values set by the calling thread. When a thread becomes the master of a team executing a parallel region, all other members of the team inherit the values of the master thread. When the master thread exits a nested parallel region and continues executing the enclosing parallel region, the values for that thread revert to their values in the enclosing parallel region just before executing the nested parallel region.

Example 4–2 Calls to OpenMP Routines Within Parallel Regions

#include <stdio.h>
#include <omp.h>
int main()
{
    omp_set_nested(1);
    omp_set_dynamic(0);
    #pragma omp parallel num_threads(2)
    {
        if (omp_get_thread_num() == 0)
            omp_set_num_threads(4);       /* line A */
        else
            omp_set_num_threads(6);       /* line B */

        /* The following statement will print out
         *
         * 0: 2 4
         * 1: 2 6
         *
         * omp_get_num_threads() returns the number
         * of the threads in the team, so it is
         * the same for the two threads in the team.
         */
        printf("%d: %d %d\n", omp_get_thread_num(),
               omp_get_num_threads(),
               omp_get_max_threads());

        /* Two inner parallel regions will be created
         * one with a team of 4 threads, and the other
         * with a team of 6 threads.
         */
        #pragma omp parallel
        {
            #pragma omp master
            {
                /* The following statement will print out
                 *
                 * Inner: 4
                 * Inner: 6
                 */
                printf("Inner: %d\n", omp_get_num_threads());
            }
            omp_set_num_threads(7);      /* line C */
        }

        /* Again two inner parallel regions will be created,
         * one with a team of 4 threads, and the other
         * with a team of 6 threads.
         *
         * The omp_set_num_threads(7) call at line C
         * has no effect here, since it affects only
         * parallel regions at the same or inner nesting
         * level as line C.
         */

        #pragma omp parallel
        {
            printf("count me.\n");
        }
    }
    return(0);
}

Compiling and running this program gives the following as one possible result:

% a.out
0: 2 4
Inner: 4
1: 2 6
Inner: 6
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.