减少OpenMp中的数组 [英] Reducing on array in OpenMp

问题描述

我试图并行化以下程序，但不知道如何减少数组。我知道这是不可能这样做，但有另一个选择吗？谢谢。（我在m上添加了减少，这是错误的，但想要如何做的建议。）

I am trying to parallelize the following program, but don't know how to reduce on an array. I know it is not possible to do so, but is there an alternative? Thanks.(I added reduction on m which is wrong but would like to have an advice on how to do it.)

#include <iostream>
#include <stdio.h>
#include <time.h>
#include <omp.h>
using namespace std;

int A [] = {84, 30, 95, 94, 36, 73, 52, 23, 2, 13};
int S [10];
int n,m=0;
time_t start_time, end_time;

int main ()
{
start_time = time(NULL);
#pragma omp parallel for private (m)reduction(+:m)
for ( n=0 ; n<10 ; ++n )
{
    for (m=0; m<=n; ++m){
    S[n] += A[m];
    }
}
end_time = time(NULL);
cout << end_time-start_time;
}

推荐答案

使用OpenMP进行数组减少。在Fortran它甚至有构造为此。在C / C ++中，你必须自己做。

Yes it is possible to do an array reduction with OpenMP. In Fortran it even has construct for this. In C/C++ you have to do it yourself. Here are two ways to do it.

第一种方法为每个线程创建 S 的私有版本，并行，然后在临界区将它们合并到 S 中（见下面的代码）。第二种方法产生具有尺寸10 * nthreads的数组。并行填充此数组，然后将其合并到 S ，而不使用临界区。第二种方法复杂得多，并且可能有缓存问题，特别是在多插座系统上，如果你不小心。有关详情，请参阅填充直方图（数组减少）与OpenMP并行而不使用关键部分

The first method makes private version of S for each thread, fill them in parallel, and then merges them into S in a critical section (see the code below). The second method makes an array with dimentions 10*nthreads. Fills this array in parallel and then merges it into S without using a critical section. The second method is much more complicated and can have cache issues especially on multi-socket systems if you are not careful. For more details see this Fill histograms (array reduction) in parallel with OpenMP without using a critical section

第一种方法

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];
        }
    }
}

第二种方法

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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