Question

我无法调整自己的想法以适应OpenMP的做事方式。

粗略地说，我想要的是：

for(int i=0; i<50; i++)
{
   doStuff();
   thread t;
   t.start(callback(i)); //each time around the loop create a thread to execute callback
}

我想我知道在c ++ 11中如何做到这一点，但我需要能够用OpenMP完成类似的工作。

Answer 1

最接近您想要的是OpenMP任务，可在OpenMP v3.0及更高版本的兼容编译器中使用。它就像：

#pragma omp parallel
{
    #pragma omp single
    for (int i = 0; i < 50; i++)
    {
        doStuff();
        #pragma omp task
        callback(i);
    }
}

此代码将使循环仅在一个线程中执行，它将创建50个OpenMP任务，这些任务将使用不同的参数调用callback()。然后它会在退出并行区域之前等待所有任务完成。任务将被执行的空闲线程挑选（可能是随机的）。 OpenMP在每个并行区域的末尾强加了一个隐式屏障，因为它的fork-join执行模型要求只有主线程在并行区域之外运行。

以下是示例程序（ompt.cpp）：

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

void callback (int i)
{
   printf("[%02d] Task stated with thread %d\n", i, omp_get_thread_num());
   sleep(1);
   printf("[%02d] Task finished\n", i);
}

int main (void)
{
   #pragma omp parallel
   {
      #pragma omp single
      for (int i = 0; i < 10; i++)
      {
         #pragma omp task
         callback(i);
         printf("Task %d created\n", i);
      }
   }
   printf("Parallel region ended\n");

   return 0;
}

编译和执行：

$ g++ -fopenmp -o ompt.x ompt.cpp
$ OMP_NUM_THREADS=4 ./ompt.x
Task 0 created
Task 1 created
Task 2 created
[01] Task stated with thread 3
[02] Task stated with thread 2
Task 3 created
Task 4 created
Task 5 created
Task 6 created
Task 7 created
[00] Task stated with thread 1
Task 8 created
Task 9 created
[03] Task stated with thread 0
[01] Task finished
[02] Task finished
[05] Task stated with thread 2
[04] Task stated with thread 3
[00] Task finished
[06] Task stated with thread 1
[03] Task finished
[07] Task stated with thread 0
[05] Task finished
[08] Task stated with thread 2
[04] Task finished
[09] Task stated with thread 3
[06] Task finished
[07] Task finished
[08] Task finished
[09] Task finished
Parallel region ended

请注意，任务的执行顺序与创建顺序不同。

GCC在早于4.4的版本中不支持OpenMP 3.0。无法识别的OpenMP指令被静默忽略，生成的可执行文件将以串行代码部分：

$ g++-4.3 -fopenmp -o ompt.x ompt.cpp
$ OMP_NUM_THREADS=4 ./ompt.x
[00] Task stated with thread 3
[00] Task finished
Task 0 created
[01] Task stated with thread 3
[01] Task finished
Task 1 created
[02] Task stated with thread 3
[02] Task finished
Task 2 created
[03] Task stated with thread 3
[03] Task finished
Task 3 created
[04] Task stated with thread 3
[04] Task finished
Task 4 created
[05] Task stated with thread 3
[05] Task finished
Task 5 created
[06] Task stated with thread 3
[06] Task finished
Task 6 created
[07] Task stated with thread 3
[07] Task finished
Task 7 created
[08] Task stated with thread 3
[08] Task finished
Task 8 created
[09] Task stated with thread 3
[09] Task finished
Task 9 created
Parallel region ended

Answer 2

例如，请查看http://en.wikipedia.org/wiki/OpenMP。

#pragma omp for

是你的朋友。 OpenMP不需要你考虑线程。您只需声明（！）要并行运行的内容，OpenMP兼容编译器在编译期间在代码中执行所需的转换。

OpenMP的规范也非常有用。他们很好地解释了可以做什么以及如何做：http://openmp.org/wp/openmp-specifications/

您的样本可能如下：

#pragma omp parallel for
for(int i=0; i<50; i++)
{
   doStuff();
   thread t;
   t.start(callback(i)); //each time around the loop create a thread to execute callback
}

for循环中的所有内容都是并行运行的。你必须注意数据依赖性。 'doStuff（）'函数在伪代码中运行，但在我的示例中将并行运行。您还需要指定哪些变量是线程私有的，以及那些也会进入#pragma语句的变量。

OpenMP - 在每个循环迭代中启动一个新线程

2 个答案: