并行编程Openmp中的Pi数

Question

大家好我想在openmp中计算pi的数量，但有些不对劲。你能告诉我哪个部分我做错了吗？

正如您在下面看到的那样，假设减少的时间却没有。

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

#define MAX_THREADS 4

static long num_steps = 100000000;
double step;
int main()
{
    int i, j;
    double pi, full_sum = 0.0;
    double start_time, run_time;
    double sum[MAX_THREADS];

    step = 1.0 / (double)num_steps;


    for (j = 1; j <= MAX_THREADS; j++){
        omp_set_num_threads(j);
        full_sum = 0.0;
        start_time = omp_get_wtime();
#pragma omp parallel private(i)
        {
            int id = omp_get_thread_num();
            int numthreads = omp_get_num_threads();
            double x;

            double partial_sum = 0;

#pragma omp single
            printf(" num_threads = %d", numthreads);

            for (i = id; i< num_steps; i += numthreads){
                x = (i + 0.5)*step;
                partial_sum += +4.0 / (1.0 + x*x);
            }
#pragma omp critical
            full_sum += partial_sum;
        }

        pi = step * full_sum;
        run_time = omp_get_wtime() - start_time;
        printf("\n pi is %f in %f seconds %d threds \n ", pi, run_time, j);
    }
}