OpenMP: Nested for-loop, barely any difference in execution time

Question

I am doing some image processing and have a nested for loop. I want to implement multiprocessing using OpenMP. The for loop looks like this, where I have added the pragma tags and declared some of the variables private as well.

int a,b,j, idx;
#pragma omp parallel for private(b,j,sumG,sumGI)
    for(a = 0; a < ny; ++a) 
    {
        for(b = 0; b < nx; ++b) 
        {
            idx = a*ny+b;
            if (imMask[idx] == 0) 
            {
                Wshw[idx] = 0;
                continue;
            }

            sumG = 0;
            sumGI = 0;

            for(j = a; j < ny; ++j) 
            {
                sumG += shadowM[j-a];
                sumGI += shadowM[j-a] * imBlurred[nx*j + b];
            }

            Wshw[idx] = sumGI / sumG;
        }
    }

The size of both nx and ny is large and I thought that, using OpenMP, I would get a descent decrease in execution time, instead there is almost no difference. Am I doing something wrong when I implement the multi-threading maybe?

You need to make idx private as well.

Z boson
– Z boson

2014-02-08 09:34:50 +00:00
Commented Feb 8, 2014 at 9:34 — Z boson
– Z boson, Commented Feb 8, 2014 at 9:34

Z boson · Accepted Answer · 2014-02-14 20:32:27Z

2

You have a race conditon in idx. You need to make it private as well.

However, instead you could try something like this.

int a,b,j, idx;
#pragma omp parallel for private(a,b,j,sumG,sumGI)
for(idx=0; idx<ny*nx; ++idx) { 
    if (imMask[idx] == 0) 
    {
        Wshw[idx] = 0;
        continue;
    }

    sumG = 0;
    sumGI = 0;
    a=idx/ny;
    b=idx%ny;
    for(j = a; j < ny; ++j) {
        sumG += shadowM[j-a];
        sumGI += shadowM[j-a] * imBlurred[nx*j + b];
    }

    Wshw[idx] = sumGI / sumG;
}

You might be able to simiply the inner loop as well as a functcion of idx instead a and b.

edited Feb 14, 2014 at 20:32

answered Feb 8, 2014 at 9:41

Z boson

34k14 gold badges132 silver badges238 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

OpenMP: Nested for-loop, barely any difference in execution time

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related