Question

What is the difference in combining 2 for loops and parallizing together and parallizing separately

Example
1. not paralleling together

#pragma omp parallel for
for(i = 0; i < 100; i++) {
//.... some code
}

#pragma omp parallel for
for(i = 0; i < 1000; i++) {
//.... some code
}

2. paralleling together

#pragma omp parallel
{
#pragma omp for
for(i = 0; i < 100; i++) {
//.... some code
}

#pragma omp for
for(i = 0; i < 1000; i++) {
//.... some code
}
}

which code is better and why????

Answer 1

One might expect a small win in the second, because one is fork/joining (or the functional equivalent) the OMP threads twice, rather than once. Whether it makes any actual difference for your code is an empirical question best answered by measurement.

The second can also have a more significant advantage if the work in the two loops are independant, and you can start the second at any time, and there’s reason to expect some load imbalance in the first loop. In that case, you can add a nowait clause to the firs tomp for and, rather than all threads waiting until the for loop ends, whoever’s done first can immediately go on to start working on the second loop. Or, one could put the two chunks of codes each in a section, or task. In general, you have a lot of control over what threads do and how they do it within a parallel section; whereas once you end the parallel section, you lose that flexibility – everything has to join together and you’re done.