3.2.1 OMP_NESTED

Nested parallelism can be enabled or disabled by setting the OMP_NESTED environment variable. By default, nested parallelism is disabled.

The following example has three levels of nested parallel constructs.

#include <omp.h>
#include <stdio.h>
void report_num_threads(int level)
{
    #pragma omp single
    {
        printf("Level %d: number of threads in the team = %d\n",
                  level, omp_get_num_threads());
    }
 }
int main()
{
    omp_set_dynamic(0);
    #pragma omp parallel num_threads(2)
    {
        report_num_threads(1);
        #pragma omp parallel num_threads(2)
        {
            report_num_threads(2);
            #pragma omp parallel num_threads(2)
            {
                report_num_threads(3);
            }
        }
    }
    return(0);
}

Compiling and running this program with nested parallelism enabled produces the following (sorted) output:

% setenv OMP_NESTED TRUE
% a.out | sort
Level 1: number of threads in the team = 2
Level 2: number of threads in the team = 2
Level 2: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2

Running the program with nested parallelism disabled produces the following output:

% setenv OMP_NESTED FALSE
% a.out | sort
Level 1: number of threads in the team = 2
Level 2: number of threads in the team = 1
Level 2: number of threads in the team = 1
Level 3: number of threads in the team = 1
Level 3: number of threads in the team = 1