pragma omp parallel for vs. pragma omp parallel [英] pragma omp parallel for vs. pragma omp parallel

问题描述

在带有openMP的C ++中，两者之间有什么区别

In C++ with openMP, is there any difference between

#pragma omp parallel for
for(int i=0; i<N; i++) {
   ...
}

和

#pragma omp parallel
for(int i=0; i<N; i++) {
   ...
}

?

谢谢！

推荐答案

#pragma omp parallel
for(int i=0; i<N; i++) {
   ...
}

此代码创建一个并行区域，每个单独的线程执行循环中的内容.换句话说，您完成N次完整循环，而不是N个线程将循环拆分并仅完成一次迭代.

This code creates a parallel region, and each individual thread executes what is in your loop. In other words, you do the complete loop N times, instead of N threads splitting up the loop and completing all iterations just once.

您可以这样做:

#pragma omp parallel
{
    #pragma omp for
    for( int i=0; i < N; ++i )
    {
    }

    #pragma omp for
    for( int i=0; i < N; ++i )
    {
    }
}

这将创建一个并行区域(又名一个fork/join，这很昂贵，因此您不想为每个循环都这样做)，并在该区域内并行运行多个循环.只要确保您已经有一个并行区域，就使用#pragma omp for而不是#pragma omp parrallel for，因为后者将意味着您的N个线程中的每个线程都产生了N个以上的线程来进行循环.

This will create one parallel region (aka one fork/join, which is expensive and therefore you don't want to do it for every loop) and run multiple loops in parallel within that region. Just make sure if you already have a parallel region you use #pragma omp for as opposed to #pragma omp parrallel for as the latter will mean that each of your N threads spawns N more threads to do the loop.

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