3.2.3 OMP_MAX_ACTIVE_LEVELS

The environment variable OMP_MAX_ACTIVE_LEVELS controls the maximum number of nested active parallel regions. A parallel region is active if it is executed by a team consisting of more than one thread. If not set, the default is 4.

Note that setting this environment variable simply controls the maximum number of nested active parallel regions; it does not enable nested parallelism. To enable nested parallelism, OMP_NESTED must be set to TRUE, or omp_set_nested() must be called with an argument that evaluates to true.

The following sample code creates 4 levels of nested parallel regions.

#include <omp.h>
#include <stdio.h>
#define DEPTH 4
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());
    }
}
void nested(int depth)
{
    if (depth > DEPTH)
        return;

    #pragma omp parallel num_threads(2)
    {
        report_num_threads(depth);
        nested(depth+1);
    }
}
int main()
{
    omp_set_dynamic(0);
    omp_set_nested(1);
    nested(1);
    return(0);
}

The following output shows a possible result from compiling and running the sample code when DEPTH is set to 4. Actual results would depend on how the operating system schedules the threads.

% setenv OMP_NESTED TRUE
% setenv OMP_MAX_ACTIVE_LEVELS 4
% 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
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2
Level 4: number of threads in the team = 2

If OMP_MAX_ACTIVE_LEVELS is set to 2, then nested parallel regions at nesting depths of 3 and 4 are executed single-threaded. The following example shows a possible result.

% setenv OMP_NESTED TRUE
% setenv OMP_MAX_ACTIVE_LEVELS 2
% 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 = 1
Level 3: 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
Level 4: number of threads in the team = 1
Level 4: number of threads in the team = 1
Level 4: number of threads in the team = 1
Level 4: number of threads in the team = 1