OpenMP 如何处理嵌套循环?

How does OpenMP handle nested loops?(OpenMP 如何处理嵌套循环?)

问题描述

以下代码只是并行化第一个(外部)循环，还是并行化整个嵌套循环?

Does the following code just parallelize the first (outer) loops, or it parallelize the entire nested loops?

    #pragma omp parallel for
    for (int i=0;i<N;i++)
    { 
      for (int j=0;j<M;j++)
      {
       //do task(i,j)//
      }
    }

我只想确定上面的代码是否会并行化整个嵌套的 for 循环(因此一个线程直接与 task(i,j) 相关)，或者它只并行化外部 for 循环(从而确保，对于每个循环索引为 i 的并行线程，其内部循环将在单个线程中依次完成，这非常重要).

I just want to make sure if the above code will parallelize the entire nested for-loops (thus one thread directly related task(i,j)), or it only parallelizes the outer for-loop (thus it ensures that, for each parrallel thread with loop index i, its inner loop will be done sequentially in a single thread, which is very import).

推荐答案

您编写的行将仅并行化外循环.要并行化两者，您需要添加一个 collapse 子句:

The lines you have written will parallelize only the outer loop. To parallelize both you need to add a collapse clause:

#pragma omp parallel for collapse(2)
    for (int i=0;i<N;i++)
    { 
      for (int j=0;j<M;j++)
      {
       //do task(i,j)//
      }
    }

您可能需要查看 OpenMP 3.1 规范(第 2.5.1 节)以了解更多详细信息.

You may want to check OpenMP 3.1 specifications (sec 2.5.1) for more details.

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