openMP:为什么当我使用“; #pragma omp parallel num_threads(4)" [英] openMP:why am I not getting different thread ids when i uses " #pragma omp parallel num_threads(4)"

问题描述

当我使用"#pragma omp parallel num_threads(4)"时，为什么没有得到不同的线程ID.在这种情况下，所有线程ID均为0. 但是，当我注释该行并使用默认线程数时，我得到了不同的线程ID. 注意:-变量我使用变量tid来获取线程ID.

Why am I not getting different thread ids when I uses " #pragma omp parallel num_threads(4)". All the thread ids are 0 in this case. But when I comment the line and use default number of threads, I got different thread ids. Note:- variable I used variable tid to get thread id.

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

int main (int argc, char *argv[]) 
{
int nthreads, tid;
int x = 0;

#pragma omp parallel num_threads(4)
#pragma omp parallel private(nthreads, tid)
  {
  /* Obtain thread number */
 tid = omp_get_thread_num();
  printf("Hello World from thread = %d\n", tid);

  // /* Only master thread does this */
   if (tid == 0) 
     {
     nthreads = omp_get_num_threads();
     printf("Number of threads = %d\n", nthreads);
     }

  }


}

以上代码的输出:-

Hello World from thread = 0
Hello World from thread = 0
Number of threads = 1
Hello World from thread = 0
Number of threads = 1
Hello World from thread = 0
Number of threads = 1
Number of threads = 1

当我评论上述行时输出:-

Output when I comment the line mentioned above:-

Hello World from thread = 3
Hello World from thread = 0
Number of threads = 4
Hello World from thread = 1
Hello World from thread = 2

推荐答案

您正在创建两个嵌套的并行区域.与此操作相同:

You are creating two nested parallel regions. It is the same as doing this:

#pragma omp parallel num_threads(4)
{
  #pragma omp parallel private(nthreads, tid)
  {
    /* Obtain thread number */
    tid = omp_get_thread_num();
    printf("Hello World from thread = %d\n", tid);

    // /* Only master thread does this */
    if (tid == 0) 
    {
      nthreads = omp_get_num_threads();
      printf("Number of threads = %d\n", nthreads);
    }
  }
}

omp_get_num_threads()返回最内部区域中的线程数.因此，您正在执行四个线程，每个线程都在执行一个线程.

omp_get_num_threads() returns the number of threads in the innermost region. So you are executing four threads, each of which is executing one thread.

内部并行区域仅执行一个线程，因为尚未启用嵌套并行性.您可以通过调用omp_set_nested(1)启用它.

The inner parallel region is only executing one thread, because you haven't enabled nested parallelism. You can enable it by calling omp_set_nested(1).

http://docs.oracle.com/cd/E19205-01 /819-5270/aewbi/index.html

如果要创建单个并行区域并指定两个属性，而不是创建两个嵌套的并行区域，则可以执行以下操作:

If instead of making two nested parallel regions, you wanted to make a single parallel region and specify two properties, you can do this:

#pragma omp parallel num_threads(4) private(nthreads,tid)
{
  .
  .
  .
}

这篇关于openMP:为什么当我使用“; #pragma omp parallel num_threads(4)"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！