OpenMP - 在每次循环迭代中启动一个新线程

Question

我在调整思路以适应OpenMP的做事方式时遇到了困难。

粗略地说，我要的是：

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 
} 

我想我知道如何做到这一点在C++ 11做的，但我需要能够实现使用OpenMP类似的东西。

Answer 1

与您想要的最接近的是OpenMP任务，可用于OpenMP v3.0及更高版本的编译器。它是这样：

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

这段代码可以使毛在一个线程只能执行，它会创建将调用callback()使用不同的参数50 OpenMP的任务。然后它会在退出并行区域之前等待所有任务完成。空闲线程将执行任务（可能是随机的）。 OpenMP在每个并行区域的末尾施加了一个隐式屏障，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。

下面是一个示例程序（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 

注意，任务不会在他们创建相同的顺序执行。

GCC在4.4以前的版本中不支持OpenMP 3.0。无法识别OpenMP指令会被忽略和生成的可执行文件会在连续的代码段：

$ 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 

Answer 2

例如看看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（！）您想要并行运行的内容，并且OpenMP兼容编译器在编译期间在代码中执行所需的转换。

OpenMP的规格也非常有用。他们解释相当不错可以做什么，以及如何：http://openmp.org/wp/openmp-specifications/

你的样本可能是这样的：在for循环并行运行

#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 
} 

一切。你必须注意数据依赖。 'doStuff（）'函数在您的伪代码中按顺序运行，但是会在我的示例中并行运行。你还需要指定哪些变量是线程私有的，以及类似于#pragma语句中的那些变量。