3.3 Calling OpenMP Runtime Routines Within Nested Parallel Regions

This section discusses calls to the following OpenMP runtime routines within nested parallel regions:

• omp_set_num_threads()

• omp_get_max_threads()

• omp_set_dynamic()

• omp_get_dynamic()

• omp_set_nested()

• omp_get_nested()

• omp_set_schedule()

• omp_get_schedule()

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 for 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 2  Calls to OpenMP Runtime 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);
}

The following example shows a possible result from running the above program:

% 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.