omp_set_num_threads不工作 [英] omp_set_num_threads not working

问题描述

我有以下C / C ++代码：

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

正如你所看到的，我试图根据参数设置要使用的处理器数

但是，我得到以下输出：

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

为什么 omp_get_num_threads（） 2？!!!

请帮助！

---

：正如已经指出的，我在串行区域调用omp_get_num_threads（），因此函数返回1。

但是，我有以下并行代码： p>

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

其输出：

$ b b

  0 // tid 
 1 // nThreads  - 这应该是2！ 
 0 
 1 
 0 
 1 
 0 
 1 
 ... 
  

解决方案

omp_get_num_threads（）调用在序列节的代码。请参见链接

所以你需要有并行代码来获取正确的值，这里代码如下：

  #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 
  

看来你已经打开mp没有启用，或者你的循环不是可以被openmp

I have the following C/C++ code:

    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);

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

However, I'm getting the following output:

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

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

Please help!

---

Edit: as has been pointed out, I'm calling omp_get_num_threads() in a serial region hence the function returning 1.

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);
...

which outputs:

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

解决方案

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

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);

}

This code produces:

2

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

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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