Question

我想知道每个线程在for循环中使用了多少时间。我希望time_taken对于每个线程都是私有的，以便他们可以在那里添加自己的时间。最好的转换，我想要每个线程的总时间，而不是while循环中每次迭代的时间。

double time_taken = 0.0;

while(delta >= epsilon) {
    delta = 0.0;

    double wtime = omp_get_wtime();

    #pragma omp parallel for reduction(+:delta)
    for (i = 0; i < workSize; i++) { 
        #do some work and change delta
    }
    time_taken += omp_get_wtime() - wtime
    printf("time taken by thread %d: %f\n", omp_get_thread_num(), time_taken);
}

我试图像这样在while循环中设置time_taken私有：

double time_taken = 0.0;

while(delta >= epsilon) {
    delta = 0.0;

    #pragma omp private(time_taken)
    {
        double wtime = omp_get_wtime();

        #pragma omp parallel for reduction(+:delta)
        for (i = 0; i < workSize; i++) { 
            #do some work and change delta
        }
        time_taken += opm_get_wtime() - wtime
        printf("time taken by thread %d: %f\n", omp_get_thread_num(), time_taken);
    }
}

我也研究了使用threadprivate，但我不知道如何正确使用它。

Answer 1

您可以尝试以下代码。这样会分配一个大小等于线程数的指针数组，然后每个线程都会分配私有内存，即没有错误共享。

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

int main(void) {
  double delta = 20, epsilon = 0;
  double **time_taken; 
  omp_set_dynamic(0);

  #pragma omp parallel
  {
    int it = omp_get_thread_num();
    #pragma omp single
    time_taken = malloc(sizeof *time_taken * omp_get_num_threads());
    time_taken[it] = malloc(sizeof *time_taken[omp_get_thread_num()]);
    *time_taken[it] = 0.0;
  }

  double sum = 0;
  while(delta >= epsilon) {
    #pragma omp parallel
    {
      int it = omp_get_thread_num();
      *time_taken[it] -= omp_get_wtime();
      #pragma omp for reduction(+: sum)
      for(int i= 0 ; i<2000000000; i++) sum += 1.0*i, delta -=1;
      *time_taken[it] += omp_get_wtime();
    }
  }
  printf("sum %f\n", sum);

  #pragma omp parallel
  {
    int it = omp_get_thread_num();
    printf("time taken by thread %d: %f\n", it, *time_taken[it]);
    free(time_taken[omp_get_thread_num()]);
    #pragma omp barrier
    #pragma omp single
    free(time_taken);
  }
}

并行for循环外的私有变量

1 个答案: