Im asking for a solution for execution time to my parallel OpenMP C code

Question

    pos = calloc(nbodies, sizeof(*pos));
    forces = calloc(nbodies, sizeof(*forces));
    //...more...
    printf("Calculating......\n");
    ene = 0.0;

    #pragma omp parallel shared(pos,forces,ene,i)
    {
        #pragma omp for private(j,k,d,d2,d3,rij)
        for(i=0; i<nbodies; ++i){
            for(j=i+1; j<nbodies; ++j) {
                d2 = 0.0;
                for(k=0; k<3; ++k) {
                    rij[k] = pos[i][k] - pos[j][k];
                    d2 += rij[k]*rij[k];
                }
                if (d2 <= cut2) {
                   d = sqrt(d2);
                   d3 = d*d2;
                   for(k=0; k<3; ++k) {
                        double f = -rij[k]/d3;
                        forces[i][k] += f;
                        #pragma omp atomic
                        forces[j][k] -= f;
                   }
                   #pragma omp atomic
                   ene += -1.0/d; 
               }
            }
        }
    }

. . . . . . . . Im using 2 threads for my parallel code and DevCpp program and OpenMP. My Parallel OpenMP C code runs at the same speed or much slower than the serial one! Is there any solution?

Answer 1

Introducing synchronization always has an overhead. But you only need this because you're trying to save a couple of operations. Ask yourself, is a factor of 2 work savings important when you have tens of cores to make the work parallel?

So maybe you should make the code a little more wasteful in scalar terms, meaning compute forces for all i,j, but more easily parallelized.