使用 OpenMP 进行数组缩减导致 "user defined reduction not found for"

Question

我正在做一项学术工作，我必须从图像中获取直方图。

一切顺利，但是当我尝试使代码与 OpenMP 并行时，编译器 returns 给我这个错误：user defined reduction not found for 'histog'

我使用的代码是这样的：

void HistogramaParaleloRed(int *histog)
{

    #pragma omp parallel
    {
        #pragma omp for
        for (int i = 0; i < NG; i++)
        {
            histog[i] = 0;
        }

        #pragma omp for reduction(+ : histog)
        for (int i = 0; i < N; i++)
        {
            for (int j = 0; j < N; j++)
            {
                histog[IMAGEN[i][j]]++;
            }
        }
    }
}

Main中函数的调用是：HistogramaParaleloRed(histog_pal_red);

Answer 1

错误

user defined reduction not found for

可能是因为代码是使用不支持 OpenMP 4.5 数组缩减功能的编译器编译的（或者该编译器配置错误）或者因为您正在尝试缩减 naked 指针（就像你的例子一样）。在后者中，编译器无法告诉要减少多少元素。

因此，要么使用支持 OpenMP 5.0 的编译器并利用 数组部分 功能，如下所示：

void HistogramaParaleloRed(int *histog)
{

    #pragma omp parallel
    {
        #pragma omp for
        for (int i = 0; i < NG; i++)
        {
            histog[i] = 0;
        }

        #pragma omp for reduction(+ : histog[:N])
        for (int i = 0; i < N; i++)
        {
            for (int j = 0; j < N; j++)
            {
                histog[IMAGEN[i][j]]++;
            }
        }
    }
}

或者，您自己实施减少。

手动执行缩减

一种方法是在线程之间创建一个共享结构（即 thread_histog），然后每个线程更新它的位置，之后，线程将共享结构的值减少到原始 histog 数组中。

void HistogramaParaleloRed(int *histog, int number_threads)
{
    int thread_histog[number_threads][NG] = {{0}};
    #pragma omp parallel
    {
        int thread_id = omp_get_thread_num();
        #pragma omp for 
        for (int i = 0; i < N; i++)
          for (int j = 0; j < N; j++)
                thread_histog[thread_id][IMAGEN[i][j]]++;

       #pragma omp for no_wait
       for (int i = 0; i < NG; i++)
           for(int j = 0; j < number_threads; j++)
              histog[i] += thread_histog[j][i]
    }
}

另一种方法是创建一个锁数组，一个锁对应 histog 数组的每个元素。每当一个线程更新给定的 histog 位置时，首先获取与该位置对应的锁，这样其他线程就不会同时更新同一数组位置。

void HistogramaParaleloRed(int *histog)
{
    omp_lock_t locks[NG];
    #pragma omp parallel
    {
       #pragma omp for
       for (int i = 0; i < NG; i++)
            omp_init_lock(&locks[i]);

        int thread_id = omp_get_thread_num();
        #pragma omp for 
        for (int i = 0; i < N; i++)
          for (int j = 0; j < N; j++){
              int pos = IMAGEN[i][j]
              omp_set_lock(&locks[pos]);
              thread_histog[thread_id][pos]++; 
              omp_unset_lock(&locks[pos]);
          }

       #pragma omp for no_wait
       for (int i = 0; i < NG; i++)
            omp_destroy_lock(&locks[i]);
    }
}

Answer 2

错误是试图直接减少参数指针 int *histog。您必须改为减少本地 array，并将减少的结果复制到 histog.

void HistogramaParaleloRed(int *histog)
{
    int localHistog[NG];

    /*
     * your code as before, replacing histog with localHistog
     */

    #pragma omp parallel
    {
        #pragma omp for
        for (int i = 0; i < NG; i++)
        {
            localHistog[i] = 0;
        }

        #pragma omp for reduction(+ : localHistog)
        for (int i = 0; i < N; i++)
        {
            for (int j = 0; j < N; j++)
            {
                localHistog[IMAGEN[i][j]]++;
            }
        }

        /* 
         * copy localHistog into output histog
         */

        #pragma omp for
        for (int i = 0; i < NG; i++)
        {
            histog[i] = localHistog[i];
        }
    }
}

使用 OpenMP 进行数组缩减导致 "user defined reduction not found for"

Array reduction with OpenMP leads to "user defined reduction not found for"

c

parallel-processing

multithreading

openmp

reduction