mpi_pi_cpu.c

/*
* mpipi.c
* Neil Gershenfeld 2/5/11
* Erik Strand 3/6/2021
* use MPI to evaluate pi by summation
*/

#include <stdio.h>
#include <mpi.h>
#include <sys/time.h>

// number of terms evaluated by each process
#define NPTS 250000000

void main(int argc, char** argv) {
    // variables used by all ranks
    int rank;
    long unsigned int i, istart, iend;
    double sum, pi;

    // variables used only by rank 0
    unsigned long int start_time, end_time;
    struct timeval start, end;
    int nproc;
    unsigned long int total_terms;
    double mflops;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0) {
        MPI_Comm_size(MPI_COMM_WORLD, &nproc);
        total_terms = nproc;
        total_terms *= NPTS;
        gettimeofday(&start, NULL);
    }

    istart = (long unsigned int)NPTS * rank + 1;
    iend = istart + NPTS;
    sum = 0.0;
    for (i = istart; i <= iend; ++i) {
        sum += 0.5 / ((i - 0.75) * (i - 0.25));
    }
    MPI_Reduce(&sum, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);

    if (rank == 0) {
        gettimeofday(&end, NULL);
        start_time = start.tv_sec * 1e6 + start.tv_usec;
        end_time = end.tv_sec * 1e6 + end.tv_usec;
        mflops = (double)nproc * NPTS * 5.0 / (end_time - start_time);
        printf("processes = %d, terms per process = %ldM, total terms = %ldM\n", nproc, NPTS / 1000000, total_terms / 1000000);
        printf("time = %fs, estimated GFlops = %f\n", (end_time - start_time) / 1.0e6, mflops / 1000);
        printf("pi ~ %f\n", pi);
    }

    MPI_Finalize();
}