openMP nested parallel for loops vs inner parallel for

Question

If I use nested parallel for loops like this:

#pragma omp parallel for schedule(dynamic,1)
for (int x = 0; x < x_max; ++x) {
    #pragma omp parallel for          


        

Answer 1

NO.

The first #pragma omp parallel will create a team of parallel threads and the second will then try to create for each of the original threads another team, i.e. a team of teams. However, on almost all existing implementations the second team has just only one thread: the second parallel region is essentially not used. Thus, your code is more like equivalent to

#pragma omp parallel for schedule(dynamic,1)
for (int x = 0; x < x_max; ++x) {
    // only one x per thread
    for (int y = 0; y < y_max; ++y) { 
        // code here: each thread loops all y
    }
}

If you don't want that, but only parallelise the inner loop, you can do this:

#pragma omp parallel
for (int x = 0; x < x_max; ++x) {
    // each thread loops over all x
#pragma omp for schedule(dynamic,1)
    for (int y = 0; y < y_max; ++y) { 
        // code here, only one y per thread
    }
}