OpenMP: Get total number of running threads

Question

I need to know the total number of threads that my application has spawned via OpenMP. Unfortunately, the omp_get_num_threads() function does not w

Answer 1

Having in mind you know the exact amount of threads being created, the simplest solution I come up with is keeping your own thread counter.

Be aware I'm completely in the dark about OpenMP as I've never really used it.

Answer 2

I think there isn't any such routine in at least OpenMP 3; and if there was, I'm not sure it would help, as there's obviously a huge race condition in between the counting of the number of threads and the forking. You could end up overshooting your target number of threads by almost a factor of 2 if everyone sees that there's room for one thread left and then everyone spawns a thread.

If this really is the structure of your program, though, and you just want to limit the total number of threads, there are options (all of these are OpenMP 3.0):

1. Use the OMP_THREAD_LIMIT environment variable to limit the total number of OpenMP threads
2. Use OMP_MAX_ACTIVE_LEVELS, or omp_set_max_active_levels(), or test against omp_get_level(), to limit how deeply nested your threads are; if you only want 16 threads, limit to 4 levels of nesting
3. If you want finer control than powers of two, you can use omp_get_level() to find your level, and call omp_get_ancestor_thread_num(int level) at various levels to find out which thread was your parent, grandparent, etc and from that (using this simple left-right forking) determine a global thread ID. (I think in this case it would go something like ∑_l=0..L-1 a_l 2^L-l where l is the level number starting at 0 and a is the ancestor thread number at that level). This would let you (say) allow threads 0-3 to fork but not 4-7, so that you'd end up with 12 rather than 16 threads. I think this only works in such a regular situation; if each parent thread forked a different number of child threads, I don't think you could determine a unique global thread ID because it looks like you can only query your direct ancestors.

Answer 3

The code you have shown has a problem in that an "omp section" has to be within the lexical scope of an "omp sections". I am assuming that you meant the "omp parallel" to be an "omp parallel sections". The other way to do this, is to use "omp task" and then you don't have to keep count of the number of threads. You would just assign the threads to the parallel region and allow the OpenMP implementation to assign the tasks to the threads.