How to Perform Array Reduction in OpenMP?

Array reduction in OpenMP

Question:

How to perform it in OpenMP for arrays Reduction operation?

Answer:

OpenMP does support array reduction. There are two ways to do this:

Method 1: Use "private" variables

to create a private copy of S for each thread, populate them in parallel, and then Merge them into S in the area:

int A[] = {84, 30, 95, 94, 36, 73, 52, 23, 2, 13};
int S[10] = {0};
#pragma omp parallel
{
    int S_private[10] = {0};
    #pragma omp for
    for (int n = 0; n < 10; ++n)
    {
        for (int m = 0; m <= n; ++m)
        {
            S_private[n] += A[m];
        }
    }
    #pragma omp critical
    {
        for (int n = 0; n < 10; ++n)
        {
            S[n] += S_private[n];
        }
    }
}

method 2: Create an array of dimension 10*nthreads using multidimensional array

, fill it in parallel, and then merge it into S without critical section:

int A[] = {84, 30, 95, 94, 36, 73, 52, 23, 2, 13};
int S[10] = {0};
int *S_private;
#pragma omp parallel
{
    const int nthreads = omp_get_num_threads();
    const int ithread = omp_get_thread_num();

    #pragma omp single
    {
        S_private = new int[10 * nthreads];
        for (int i = 0; i < (10 * nthreads); i++)
            S_private[i] = 0;
    }
    #pragma omp for
    for (int n = 0; n < 10; ++n)
    {
        for (int m = 0; m <= n; ++m)
        {
            S_private[ithread * 10 + n] += A[m];
        }
    }
    #pragma omp for
    for (int i = 0; i < 10; i++)
    {
        for (int t = 0; t < nthreads; t++)
        {
            S[i] += S_private[10 * t + i];
        }
    }
}
delete[] S_private;

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