Task concurrency using nested parallelism with OpenMP

concurrency.cpp

#include <cstdio>
#include <chrono>
#include <omp.h>

void viewer( int& a, bool& stop )
{
    printf("thread %d start viewer\n", omp_get_thread_num());
    auto start = std::chrono::high_resolution_clock::now();
    while( !stop )
    {
        auto now = std::chrono::high_resolution_clock::now();
        long int elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - start).count();
        // print every 100 milliseconds
        if ( elapsed % 100 == 0 )
        {
            // printf("thread %d a = %d at %ld ms\n",omp_get_thread_num(), a, elapsed);
        }
    }
    printf("thread %d stop viewer\n", omp_get_thread_num());
}

void render( int& a, bool& stop )
{
    printf("thread %d start render\n", omp_get_thread_num());
    int loop = 0;
    while( true )
    {   
        printf("loop %d\n", loop);
        int mod = 300000;
        // all remaining threads should be used here
        #pragma omp parallel for schedule(dynamic, 16)
        for(int i = 0; i < 1000 * 1000; ++i)
        {
            #pragma omp atomic
            a++;

            if( i % mod == 0 )
            {
                printf("thread %d working %d available %d\n", omp_get_thread_num(), omp_get_num_threads(), omp_get_max_threads() - omp_get_num_threads());
            }

        }
        if( a > 100000000 ) break;
        loop++;
    }
    // stop live viewer at the end of render
    stop = true; 
    printf("thread %d stop render\n", omp_get_thread_num());
}

int main( )
{
    // max threads
    int nb = omp_get_max_threads();
    printf("threads %d\n", nb);

    // enable nested parallelism
    omp_set_nested(1);

    // var
    int a = 0;
    bool stop = false;

    // 2 parallel tasks : 1 use nested parallelism
    #pragma omp parallel sections num_threads(2)
    {
        // live viewer
        #pragma omp section
        {
            viewer(a, stop);
        }
        // render
        #pragma omp section
        {
            render(a, stop);   
        }
    }
    
    printf("a %d\n", a);
    return 0;
}