Multi-dimensional nested OpenMP loop

Question

What is the proper way to parallelize a multi-dimensional embarrassingly parallel loop in OpenMP? The number of dimensions is known at compile-time, but which dimensions will be large is not. Any of them may be one, two, or a million. Surely I don't want N omp parallel's for an N-dimensional loop...

Thoughts:

• The problem is conceptually simple. Only the outermost 'large' loop needs to be parallelized, but the loop dimensions are unknown at compile-time and may change.

• Will dynamically setting omp_set_num_threads(1) and #pragma omp for schedule(static, huge_number) make certain loop parallelizations a no-op? Will this have undesired side-effects/overhead? Feels like a kludge.

• The OpenMP Specification (2.10, A.38, A.39) tells the difference between conforming and non-conforming nested parallelism, but doesn't suggest the best approach to this problem.

• Re-ordering the loops is possible but may result in a lot of cache-misses. Unrolling is possible but non-trivial. Is there another way?

Here's what I'd like to parallelize:

for(i0=0; i0<n[0]; i0++) {
  for(i1=0; i1<n[1]; i1++) {
    ...
       for(iN=0; iN<n[N]; iN++) {
         <embarrasingly parallel operations>
       }
    ...
  }
}

Thanks!

Answer 1

The collapse directive is probably what you're looking for, as described here. This will essentially form a single loop, which is then parallized, and is designed for exactly these sorts of situations. So you'd do:

#pragma omp parallel for collapse(N)
for(int i0=0; i0<n[0]; i0++) {
  for(int i1=0; i1<n[1]; i1++) {
    ...
       for(int iN=0; iN<n[N]; iN++) {
         <embarrasingly parallel operations>
       }
    ...
  }
}

and be all set.