4.2.2 SUNW_MP_MAX_POOL_THREADS

The OpenMP runtime library maintains a pool of threads that can be used as slave threads in parallel regions. Setting the SUNW_MP_MAX_POOL_THREADS environment variable controls the number of threads in the pool. The default value is 1023.

The thread pool consists of only non-user threads that the runtime library creates. It does not include the initial thread or any thread created explicitly by the user’s program. If this environment variable is set to zero, the thread pool will be empty and all parallel regions will be executed by one thread.

The following example shows that a parallel region can get fewer threads if there are not sufficient threads in the pool.The code is the same as above. The number of threads needed for all the parallel regions to be active at the same time is 8. The pool needs to contain at least 7 threads. If we set SUNW_MP_MAX_POOL_THREADS to 5, two of the four inner-most parallel regions may not be able to get all the slave threads they ask for. One possible result is shown below.

% setenv OMP_NESTED TRUE
% setenv SUNW_MP_MAX_POOL_THREADS 5
% a.out
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 - 1
Level 3: number of threads in the team - 1