How to parallelize monte carlo estimation of pi using pthreads?

Question

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

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

    unsigned long long in = 1;
    unsigned long long total = 2;
    double tol , change, new, secs , old = 0.0;
    struct timeval start , end;
    int threads ; /* ignored */

    if ( argc < 2) {
        exit (-1);
    }

    threads = atoi ( argv[1]);
    tol = atof ( argv[2]);
    if (( threads < 1) || ( tol < 0.0)) {
        exit (-1);
    }
    tol = tol *tol;

    srand48(clock());
    gettimeofday (&start , NULL);
    do
    {
        double x, y;
        x = drand48();
        y = drand48();
        total ++;
        if (( x*x + y*y) <= 1.00)
            in ++;
        new = 4.0 * (double)in/( double)total ;
        change = fabs (new - old);
        old = new;
    }while (change > tol );
    gettimeofday (&end, NULL);
    secs = (( double)end.tv_sec - (double)start.tv_sec )
    + (( double)end.tv_usec - (double)start.tv_usec )/1000000.0;
    printf ( ”Found estimate of pi of %.12f in %llu iterations , %.6f seconds.n n”,
            new, total - 2, secs );
}

The code above is a sequential program that takes an argument for the tolerance for how closely to estimate pi. Once the change in these old and new values fall below the tolerance it exits.

I have to parallelize this program in pthreads. I am not trying to have someone do it for me but rather to get some pointers and ideas to think about so that I may be able to this. the pthreads program will take number of threads and tolerance as it's argument and output the estimation. I am very new to parallel programs and don't really know where to start so I will take any advice. Thanks.

Answer 1

Each thread should keep its own in and total counts and use a better exit condition. Then add up the in and total values from each thread.