使用openMP的并行qsort

Question

I'm trying to parallelize qsort() function in c++ with openMP, but I have some problems. For of all, for a high number of element to sort (ex: V=1000) qsort doesn't work at all, I never exit this function (infinite loop). Then, I notice that my parallel version of qsort is much slower then the serial one. Anyone could help me? Here the code: https://dpaste.de/EaH0 .

here the average of time elapsed where kruscalP is the parallel version: thanks

Answer 1

You have multiple race conditions in your code when you write to A , exch0 , and exch1 which are all shared. Not only can this hurt performance it likely will give the wrong result. Unfortunately, you have to use a critical section to fix this which will also likely hurt performance but at least it will give the right result.

#pragma omp for    
for (i = 0; i < N-1; i += 2) {
    #pragma critical
    {
        if (A[i].weight > A[i+1].weight) {
            temp = A[i];
            A[i] = A[i+1];
            A[i+1] = temp;
            exch0 = 1;
        }
    }
}

The same applies to the next for loop. If you want to do this efficiently you have to change your algorithm. Maybe consider merge sort.