在OpenMP中计算矩阵的条目 [英] Computing entries of a matrix in OpenMP

问题描述

我对openMP非常陌生，但是我试图编写一个简单的程序来并行生成矩阵的项，即对于N乘M的矩阵A，令A(i，j)= i * j.下面是一个最小的示例:

I am very new to openMP, but am trying to write a simple program that generates the entries of matrix in parallel, namely for the N by M matrix A, let A(i,j) = i*j. A minimal example is included below:

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>

int main(int argc, 
         char **argv)
{
    int i, j, N, M;

    N = 20; 
    M = 20;

    int* A;
    A = (int*) calloc(N*M, sizeof(int));

    // compute entries of A in parallel 
    #pragma omp parallel for shared(A)
    for (i = 0; i < N; ++i){
        for (j = 0; j < M; ++j){
            A[i*M + j] = i*j;
        }
    }

    // print parallel results
    for (i = 0; i < N; ++i){
        for (j = 0; j < M; ++j){
            printf("%d ", A[i*M + j]);
        }
        printf("\n");
    }

    free(A);

    return 0;
}

结果并不总是正确的.从理论上讲，我只是并行化外部循环，并且for循环的每次迭代都不会修改其他迭代将修改的条目.但是我不确定如何将其转换为openMP.当对向量数组执行类似的过程时(例如，仅一个for循环)，似乎没有问题，例如

The results are not always correct. In theory, I am only parallelizing the outer loop, and each iteration of the for loop does not modify the entries that the other iterations will modify. But I am not sure how to translate this to openMP. When doing a similar procedure for a vector array (i.e. just one for loop), there seems to be no issue, e.g.

#pragma omp parallel for
for (i = 0; i < N; ++i)
{
    v[i] = i*i;
}

有人可以向我解释如何解决此问题吗?

Can someone explain to me how to fix this?

推荐答案

例如这个
http://supercomputingblog.com/openmp/tutorial-parallel-for -loops-with-openmp/

在并行化部分之外声明变量很危险.
可以通过显式地将内部循环的循环变量设为私有来消除它.

The declaration of variables outside of a parallelized part is dangerous.
It can be defused by explicitly making the loop variable of the inner loop private.

为此，请更改此

#pragma omp parallel for shared(A)

到

#pragma omp parallel for private(j) shared(A)

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