test

Sun Studio 12: OpenMP API User's Guide

4.2.3 SUNW_MP_MAX_NESTED_LEVELS

The environment variable SUNW_MP_MAX_NESTED_LEVELS controls the maximum depth of nested active parallel regions that require more than one thread.

Any active parallel region that has an active nested depth greater than the value of this environment variable will be executed by only one thread. A parallel region is considered active if it it has no IF clause, or if it has an IF clause that evaluates to true. The default maximum number of active nesting levels is 4.

The following code will create 4 levels of nested parallel regions. If SUNW_MP_MAX_NESTED_LEVELS is set to 2, then nested parallel regions at nested depth of 3 and 4 are executed single-threaded.


#include <omp.h>
#include <stdio.h>
#define DEPTH 5
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);
}

Compiling and running this program with a maximum nesting level of 4 gives the following possible output. (Actual results will depend on how the OS schedules threads.)


% setenv SUNW_MP_MAX_NESTED_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

Running with the nesting level set at 2 gives the following as a possible result:


% setenv SUNW_MP_MAX_NESTED_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

Again, these examples only show some possible results. Actual results will depend on how the OS schedules threads.