OpenMP, for loop inside section

Question

I would like to run the following code (below). I want to spawn two independent threads, each one would run a parallel for loop. Unfortunately, I get an error. Apparently, p

Answer 1

OpenMP cannot create parallel regions inside parallel regions. This is due to the fact that OpenMP create at the beginning of the program num_threads parallel threads, in non parallel regions the others are not used and sleep. They have done this, as the frequent generation of new threads is quite slow compared to waking sleeping threads.

Therefore you should parallelize only the loops:

#include <omp.h>
#include "stdio.h"

int main()
{

omp_set_num_threads(10);

#pragma omp parallel for
    for(int i=0; i<5; i++) {
        printf("x %d\n", i);
    }

#pragma omp parallel for
    for(int i=0; i<5; i++) {
        printf(". %d\n", i);
    }
}

Answer 2

Practically, optimal number of threads is equal to number of available CPU cores. So, every parallel for should be handled in all available cores, which is impossible inside of omp sections. So, what you are trying to achieve, is not optimal. tune2fs' suggestion to execute two loops without sections makes sense and gives the best possible performance. You can execute parallel loops inside of another functions, but this "cheating" doesn't give performance boost.

Answer 3

Here you have to use nested parallelism. The problem with the omp for in the sections is that all the threads in scope have to take part in the omp for, and they clearly don't — they're broken up by sections. So you have to introduce functions, and do nested paralleism within the functions.

#include <stdio.h>
#include <omp.h>

void doTask1(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf("x %d %d %d\n", i, tid, gtid);
        }
    }
}

void doTask2(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf(". %d %d %d\n", i, tid, gtid);
        }
    }
}


int main()
{
    omp_set_num_threads(2);
    omp_set_nested(1);

#pragma omp parallel    
    {
        int gtid = omp_get_thread_num();
#pragma omp sections
        {
#pragma omp section
            doTask1(gtid);

#pragma omp section
            doTask2(gtid);
        } // end parallel and end sections
    }
}