如何让 OpenMP 与 #pragma omp 任务一起工作？

Question

我在 Linux 终端上使用 OpenMP。我用标志 -fopenmp 编译我的程序。我使用的环境确实支持多线程。

    #pragma omp parallel num_threads(3)
    {
        #pragma omp task
            R11Inverted = compute_inverse(r11, half);
        #pragma omp task
            printf("5 I am thread # %d\n", omp_get_thread_num());
            R22Inverted = compute_inverse(r22, half);
    }

作为一个简单的最小测试，我想做的就是得到printf("5 I am thread # %d\n", omp_get_thread_num());来表明程序是运行三个线程。基本上 omp_get_thread_num() 应该 return 值 0、1 或 2。

The Output 上面的代码块被重复调用了很多次。然而每次的结果都显示线程没有工作，程序只使用一个线程（线程 0）。

Answer 1

您缺少代码块周围的 {} 以及单个子句。

尝试以下操作：

#pragma omp parallel num_threads(3)
{
        #pragma omp single
        {
            #pragma omp task
               R11Inverted = compute_inverse(r11, half);
            #pragma omp task
            {
              printf("5 I am thread # %d\n", omp_get_thread_num());
              R22Inverted = compute_inverse(r22, half);
            }
        }
}

However the results every time is showing that threading is not working and the program is only using a single thread (Thread 0).

如果您想检查它是否被多线程调用，请改用 omp_get_num_threads，returns 团队的线程数。

   #pragma omp parallel num_threads(3)
   {
            #pragma omp master
            {           
                printf("Total Threads # %d\n", omp_get_num_threads()); 
            }
            #pragma omp single
            {
                #pragma omp task
                   R11Inverted = compute_inverse(r11, half);
                #pragma omp task
                {
                  printf("5 I am thread # %d\n", omp_get_thread_num());
                  R22Inverted = compute_inverse(r22, half);
                }
            }
    }

---

如果上面的代码是递归调用的，那么需要适配如下：

   #pragma omp parallel num_threads(3)
   {
      #pragma omp master      
      printf("Total Threads # %d\n", omp_get_num_threads()); 
      #pragma omp single
      compute_inverse(...); // First call to the recursive function
   }

和 compute_inverse 函数内部：

   #pragma omp task
   R11Inverted = compute_inverse(r11, half);
   #pragma omp task
   {
      printf("5 I am thread # %d\n", omp_get_thread_num());
      R22Inverted = compute_inverse(r22, half);
   }