使用omp_set_num_threads()将线程数设置为2，但是omp_get_num_threads()返回1 [英] Set number of threads using omp_set_num_threads() to 2, but omp_get_num_threads() returns 1

问题描述

我使用OpenMP拥有以下C/C ++代码:

I have the following C/C++ code using OpenMP:

    int nProcessors=omp_get_max_threads();
    if(argv[4]!=NULL){
        printf("argv[4]: %s\n",argv[4]);
        nProcessors=atoi(argv[4]);
        printf("nProcessors: %d\n",nProcessors);
    }
    omp_set_num_threads(nProcessors);
    printf("omp_get_num_threads(): %d\n",omp_get_num_threads());
    exit(0);

如您所见，我正在尝试根据在命令行上传递的参数来设置要使用的处理器数量.

As you can see, I'm trying to set the number of processors to use based on an argument passed on the command line.

但是，我得到以下输出:

However, I'm getting the following output:

argv[4]: 2   //OK
nProcessors: 2   //OK
omp_get_num_threads(): 1   //WTF?!

omp_get_num_threads()为什么不返回2?!!!

Why isn't omp_get_num_threads() returning 2?!!!

如前所述，我在串行区域中调用omp_get_num_threads()，因此该函数返回1.

As has been pointed out, I'm calling omp_get_num_threads() in a serial region, hence the function returns 1.

但是，我有以下并行代码:

However, I have the following parallel code:

#pragma omp parallel for private(i,j,tid,_hash) firstprivate(firstTime) reduction(+:nChunksDetected)
    for(i=0;i<fileLen-CHUNKSIZE;i++){
        tid=omp_get_thread_num();
        printf("%d\n",tid);
        int nThreads=omp_get_num_threads();
        printf("%d\n",nThreads);
...

输出:

0   //tid
1   //nThreads - this should be 2!
0
1
0
1
0
1
...

推荐答案

omp_get_num_threads()调用在代码的串行部分返回1.参见链接

The omp_get_num_threads() call returns 1 in the serial section of the code. See Link

因此，您需要使用并行代码来获取正确的值，这里的代码应如下所示:

So you need to have parallel code to get the correct value, here how your code should look like:

#include <iostream>
#include <omp.h>

int main (int argc, const char * argv[])
{
    int nProcessors = omp_get_max_threads();

    std::cout<<nProcessors<<std::endl;

    omp_set_num_threads(nProcessors);

    std::cout<<omp_get_num_threads()<<std::endl;

#pragma omp parallel for 
    for(int i = 0; i < 5; i++){
        int tid = omp_get_thread_num();
        std::cout<<tid<<"\t tid"<<std::endl;
        int nThreads = omp_get_num_threads();
        std::cout<<nThreads<<"\t nThreads"<<std::endl;
    }

    exit(0);
}

此代码产生:

2

1
0    tid
2    nThreads
0    tid
2    nThreads
0    tid
2    nThreads
1    tid
2    nThreads
1    tid
2    nThreads

似乎您没有启用open mp或循环的形式不能被openmp并行化

It seems that you have either open mp not enabled or your loop is not in the form that can be parallized by openmp

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