c++ - 内部嵌套循环的实用语法未在嵌套循环中正确忽略

Question

我正在尝试实现以下代码，以查看如何在嵌套循环中管理OpenMP线程，在该嵌套循环中，每个内部/外部循环分别在函数及其调用者中实现。
每个循环都通过以下语句实现#pragma omp parallel for，我假设将忽略内部循环的pragma。
为了看到这一点，我在每个循环中打印了线程号。
然后，我可以看到以下内容，其中内部循环中的线程ID始终为零，与调用方所对应的线程号不同。为什么会这样？

Calling 0 from 0
Calling 2 from 1
Calling 6 from 4
Calling 8 from 6
Calling 4 from 2
Calling 7 from 5
Calling 5 from 3
    Calling 0 from 0  // Expecting 3
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
    Calling 0 from 0
    Calling 0 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
Calling 9 from 7
    Calling 1 from 0 // Expecting 7
    Calling 2 from 0
    Calling 3 from 0
    Calling 0 from 0
Calling 3 from 1
    Calling 0 from 0 // Expecting 1
    Calling 1 from 0
    Calling 2 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
    Calling 3 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0
Calling 1 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 0
    Calling 3 from 0

#include <vector>                                                                                                                                                                                                                                                          
#include <omp.h>
#include <iostream>
#include <cstdio>
#include <limits>
#include <cstdint>
#include <cinttypes>

using namespace std;

const size_t  kM = 4;

struct Mat
{
 int elem[kM];

 Mat(const Mat& copy)
 {
  for (size_t i = 0; i<kM; ++i)
   this->elem[i] = copy.elem[i];
 }
 Mat()
 {
  for (size_t i = 0; i<kM; ++i)
    elem[i] = 0;
 }

 void do_mat(Mat& m)
 {
  #pragma omp parallel for
  for (int i = 0; i<kM; ++i)
  {
    printf(" \tCalling %d from %d\n", i, omp_get_thread_num());
    elem[i] += m.elem[i];
  }
 }
};

int main ()
{
  const int kN = 10;
  vector<Mat> matrices(kN);

  Mat m;
  #pragma omp parallel for
  for (int i = 0; i < kN; i++)
  {
    int tid = omp_get_thread_num();
    printf("Calling %d from %d\n", i, tid);
    matrices[i].do_mat(m);
  }

  return 0;
}          

Answer 1

我不确定我了解您的期望是什么，但是您得到的结果是完全可以预期的。
默认情况下，OpenMP嵌套并行性是禁用的，这意味着任何嵌套parallel区域将创建与1个线程组成的线程组一样多，而外部线程遇到这些线程。
在您的情况下，最外面的parallel区域创建了一个由8个线程组成的团队。它们中的每一个都将到达最里面的parallel区域，并创建第二级的1线程团队。每个二级线程在其自己的团队中的排名为0，因此您具有打印的0。
使用通过g++ 9.3.0编译的完全相同的代码，通过设置2个环境变量OMP_NUM_THREADS和OMP_NESTED，我得到以下信息:

OMP_NUM_THREADS="2,3" OMP_NESTED=true ./a.out 
Calling 0 from 0
Calling 5 from 1
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 1
    Calling 0 from 0
    Calling 1 from 0
    Calling 3 from 2
    Calling 3 from 2
    Calling 2 from 1
Calling 6 from 1
Calling 1 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 3 from 2
    Calling 2 from 1
Calling 2 from 0
    Calling 0 from 0
    Calling 1 from 0
    Calling 2 from 1
    Calling 3 from 2
    Calling 0 from 0
    Calling 1 from 0
    Calling 3 from 2
    Calling 2 from 1
Calling 3 from 0
Calling 7 from 1
    Calling 0 from 0
    Calling 3 from 2
    Calling 2 from 1
    Calling 3 from 2
    Calling 0 from 0
    Calling 1 from 0
    Calling 1 from 0
    Calling 2 from 1
Calling 4 from 0
Calling 8 from 1
    Calling 0 from 0
    Calling 3 from 2
    Calling 2 from 1
    Calling 2 from 1
    Calling 0 from 0
    Calling 1 from 0
    Calling 3 from 2
    Calling 1 from 0
Calling 9 from 1
    Calling 2 from 1
    Calling 0 from 0
    Calling 1 from 0
    Calling 3 from 2

也许这与您期望看到的更好？