c - OpenMP 并行化 Pi 程序

Question

我一直在尝试使用 OpenMP 并行化以下代码，但没有成功。我在互联网上搜索了几个例子，但在多次执行程序后，没有一个给出相同的答案。

#include <stdio.h>
#include <omp.h>
#define NUM_THREADS 2

long num_steps = 100000;
double step    = 1.0/100000.0;

int main() {
int i;
double x, pi, sum = 0.0;
for(i = 0; i < num_steps; ++i) {
 x = (i-0.5)*step;
 sum += 4.0/(1.0+x*x);
}
pi = step*sum;
printf("PI value = %f\n", pi);

}

这是我迄今为止的解决方案:

int main (int argc, char **argv){

//Variables
int i=0, aux=0;
double step = 1.0/100000.0;
double  x=0.0, 
        pi=0.0, 
        sum = 0.0;

 #pragma omp parallel  shared(sum,i) private(x)
 {
    x = 0.0;
    sum = 0.0;

    #pragma omp for 
    for (i=0; i<num_steps; ++i) {
        x = (i-0.5)*step;

        #pragma omp critical
        sum += 4.0/(1.0+x*x);

    }
 } 

 /* All threads join master thread and terminate */
 pi= step*sum;
 printf("PI value = %f\n", pi);
}

Answer 1

请考虑对您的循环使用 OpenMP 官方网站中提到的相同指令:loop parallelism ，我不得不更改您代码中的许多行，希望这将成为您进一步熟悉 OpenMP 和 C 语言循环并行性的起点。

#include <stdio.h>
#include <omp.h>
#define NUM_STEPS 10000000

int main (int argc, char **argv){

//Variables
long int i, num_steps = NUM_STEPS;
double x, step, sum, pi; 
sum = 0.0;
step = 1.0 / (double) num_steps;
 #pragma omp parallel private(i,x)
 {
  #pragma omp for reduction(+:sum)
    for (i=0; i<num_steps; ++i) {
        x = (i+0.5)*steps;
        sum += 4.0/(1.0+x*x);

    }
 } 

 /* All threads join master thread and terminate */
 pi= steps*sum;
 printf("PI value = %.24f\n", pi);