OpenMP - 在每个循环迭代中启动一个新线程

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
}

我想我知道如何在c ++ 11中完成这项工作,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,并将创建50个将callback()使用不同参数调用的OpenMP任务.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.

这是一个示例程序(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不支持早于4.4版本的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是串行代码段:

$ 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