openMP nested parallel for loops vs inner parallel for

c++, openmp, parallel-processing

Solution

If your compiler supports OpenMP 3.0, you can use the `collapse` clause:

#pragma omp parallel for schedule(dynamic,1) collapse(2)
for (int x = 0; x < x_max; ++x) {
    for (int y = 0; y < y_max; ++y) { 
    //parallelize this code here
    }
//IMPORTANT: no code in here
}

If it doesn't (e.g. only OpenMP 2.5 is supported), there is a simple workaround:

#pragma omp parallel for schedule(dynamic,1)
for (int xy = 0; xy < x_max*y_max; ++xy) {
    int x = xy / y_max;
    int y = xy % y_max;
    //parallelize this code here
}

You can enable nested parallelism with `omp_set_nested(1);` and your nested `omp parallel for` code will work but that might not be the best idea.

By the way, why the dynamic scheduling? Is every loop iteration evaluated in non-constant time?

Problem

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 schedule(dynamic,1) for (int y = 0; y < y_max; ++y) { //parallelize this code here } //IMPORTANT: no code in here } ``` is this equivalent to: ``` for (int x = 0; x < x_max; ++x) { #pragma omp parallel for schedule(dynamic,1) for (int y = 0; y < y_max; ++y) { //parallelize this code here } //IMPORTANT: no code in here } ``` Is the outer parallel for doing anything other than creating a new task?

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