The OMP_NESTED=TRUE|FALSE environment variable enables or disables nested parallelism. Its setting can be overridden by calling the omp_set_nested runtime library function.
If nested parallelism is disabled, nested parallel regions are serialized and run in the current thread.
In the current implementation, nested parallel regions are always serialized. As a result, OMP_SET_NESTED does not have any effect, and omp_get_nested always returns 0. If -qsmp=nested_par option is on (only in non-strict OMP mode), nested parallel regions might employ additional threads as available. However, no new team is created to run nested parallel regions.
The default value for OMP_NESTED is FALSE.