OpenMP,用于循环内部部分

Question

我想运行以下代码(如下).我想生成两个独立的线程,每个线程都会运行并行for循环.不幸的是,我收到了一个错误.显然,并行for不能在里面产生section.怎么解决？

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

int main()
{

omp_set_num_threads(10);

#pragma omp parallel    
#pragma omp sections
  {
#pragma omp section
#pragma omp for
    for(int i=0; i<5; i++) {
        printf("x %d\n", i);
    }

#pragma omp section
#pragma omp for
    for(int i=0; i<5; i++) {
        printf(". %d\n", i);
    }
  } // end parallel and end sections
}

而错误:

main.cpp: In function ‘int main()’:
main.cpp:14:9: warning: work-sharing region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region [enabled by default]
main.cpp:20:9: warning: work-sharing region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region [enabled by default]

Answer 1

在这里,您必须使用嵌套并行性.与这个问题omp for的sections是,在范围内的所有线程都参加了omp for,而且他们显然不-它们是由部分破碎.所以你必须引入函数,并在函数内做嵌套的并行.

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

void doTask1(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf("x %d %d %d\n", i, tid, gtid);
        }
    }
}

void doTask2(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf(". %d %d %d\n", i, tid, gtid);
        }
    }
}


int main()
{
    omp_set_num_threads(2);
    omp_set_nested(1);

#pragma omp parallel    
    {
        int gtid = omp_get_thread_num();
#pragma omp sections
        {
#pragma omp section
            doTask1(gtid);

#pragma omp section
            doTask2(gtid);
        } // end parallel and end sections
    }
}

Answer 2

OpenMP 无法在并行区域内创建并行区域。这是因为 OpenMP 在程序开始时创建了 num_threads 个并行线程，在非并行区域中其他线程不被使用并处于休眠状态。他们这样做是因为与唤醒睡眠线程相比，频繁生成新线程相当慢。

因此，您应该仅并行化循环：

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

int main()
{

omp_set_num_threads(10);

#pragma omp parallel for
    for(int i=0; i<5; i++) {
        printf("x %d\n", i);
    }

#pragma omp parallel for
    for(int i=0; i<5; i++) {
        printf(". %d\n", i);
    }
}