OpenMP - 在每次循環迭代中啓動一個新線程

Question

我在調整思路以適應OpenMP的做事方式時遇到了困難。

粗略地說，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這一點在C++ 11做的，但我需要能夠實現使用OpenMP類似的東西。

Answer 1

與您想要的最接近的是OpenMP任務，可用於OpenMP v3.0及更高版本的編譯器。它是這樣：

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

這段代碼可以使毛在一個線程只能執行，它會創建將調用callback()使用不同的參數50 OpenMP的任務。然後它會在退出並行區域之前等待所有任務完成。空閒線程將執行任務（可能是隨機的）。 OpenMP在每個並行區域的末尾施加了一個隱式屏障，因爲它的fork-join執行模型要求只有主線程在並行區域之外運行。

下面是一個示例程序（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; 
} 

編譯和執行：

$ 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，任務不會在他們創建相同的順序執行。

GCC在4.4以前的版本中不支持OpenMP 3.0。無法識別OpenMP指令會被忽略和生成的可執行文件會在連續的代碼段：

$ 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不需要你考慮線程。您只需聲明（！）您想要並行運行的內容，並且OpenMP兼容編譯器在編譯期間在代碼中執行所需的轉換。

OpenMP的規格也非常有用。他們解釋相當不錯可以做什麼，以及如何：http://openmp.org/wp/openmp-specifications/

你的樣本可能是這樣的：在for循環並行運行

#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。你必須注意數據依賴。 'doStuff（）'函數在您的僞代碼中按順序運行，但是會在我的示例中並行運行。你還需要指定哪些變量是線程私有的，以及類似於#pragma語句中的那些變量。