OMP_MAX_ACTIVE_LEVELS – Set the maximum number of nested parallel regions ¶Specifies the initial value for the maximum number of nested parallel
regions. The value of this variable shall be a positive integer.
If undefined, then if OMP_NESTED is defined and set to true, or
if OMP_NUM_THREADS or OMP_PROC_BIND are defined and set to
a list with more than one item, the maximum number of nested parallel
regions will be initialized to the largest number supported, otherwise
it will be set to one.
omp_set_max_active_levels – Limits the number of active parallel regions, OMP_NESTED – Nested parallel regions, OMP_PROC_BIND – Whether threads may be moved between CPUs,
OMP_NUM_THREADS – Specifies the number of threads to use
OpenMP specification v4.5, Section 4.9