OpenMP parallel integration to compute Pi.

pi.c

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define MAX_THREADS 8

static long steps = 1000000000;
double step;

int main (int argc, const char *argv[]) {

    int i,j;
    double x;
    double pi, sum = 0.0;
    double start, delta;

    step = 1.0/(double) steps;

    // Compute parallel compute times for 1-MAX_THREADS
    for (j=1; j<= MAX_THREADS; j++) {

        printf(" running on %d threads: ", j);

        // This is the beginning of a single PI computation 
        omp_set_num_threads(j);

        sum = 0.0;
        double start = omp_get_wtime();


        #pragma omp parallel for reduction(+:sum) private(x)
        for (i=0; i < steps; i++) {
            x = (i+0.5)*step;
            sum += 4.0 / (1.0+x*x); 
        }

        // Out of the parallel region, finialize computation
        pi = step * sum;
        delta = omp_get_wtime() - start;
        printf("PI = %.16g computed in %.4g seconds\n", pi, delta);

    }
    

}

Hi, correct me if I wrong but this is limited by the double type, so technically you can't calculate 20 digits.

Another implementation:

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 8

static long steps = 1000000000;
double step;

int main(int argc, const char* argv[]) {

	double pi = 0.0;
	double start, delta, sum[NUM_THREADS];
	start = omp_get_wtime();
	step = 1.0 / (double)steps;
	omp_set_num_threads(NUM_THREADS);
#pragma omp parallel
	{
		double x;
		int id, i;
		id = omp_get_thread_num();
		for (i = id, sum[id] = 0.0; i < steps; i = i + NUM_THREADS) {
			x = (i + 0.5) * step;
			sum[id] += 4.0 / (1.0 + x * x);
		}
	}
	for (int i = 0; i < NUM_THREADS; i++) {
		pi += sum[i] * step;
	}
	delta = omp_get_wtime() - start;
	printf("PI = %.16g computed in %.4g seconds with %d threads.", pi, delta, NUM_THREADS);
}

hello, so why did you define the number of threads "NUM_THREADS" ?