pi_gather.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <math.h>
#include <time.h>
#include <mpi.h>

#define SEED     100
// #define NUM_ITER 1000000000

int main(int argc, char* argv[])
{
    int local_count = 0, flip = 1 << 24;
    int rank, num_ranks, i, iter, provided;
    double x, y, z, pi;
    
	MPI_Init_thread(&argc, &argv, MPI_THREAD_SINGLE, &provided);

	double start_time, stop_time, elapsed_time;
	start_time = MPI_Wtime();

	MPI_Comm_rank(MPI_COMM_WORLD, &rank);
	MPI_Comm_size(MPI_COMM_WORLD, &num_ranks);

    srand(rank*SEED); // Important: Multiply SEED by "rank" when you introduce MPI!
    
    flip = flip/num_ranks;
    // Calculate PI following a Monte Carlo method
    for (iter = 0; iter < flip; iter++)
    {
        // Generate random (X,Y) points
        x = (double)random() / (double)RAND_MAX;
        y = (double)random() / (double)RAND_MAX;
        z = sqrt((x*x) + (y*y));
        
        // Check if point is in unit circle
        if (z <= 1.0)
        {
            local_count++;
        }
    }
    int counts[num_ranks];
//    if (rank == 0){
//		counts = malloc(sizeof(int) * (num_ranks - 1));
//	}
    MPI_Gather(&local_count, 1, MPI_INT, &counts, 1, MPI_INT, 0, MPI_COMM_WORLD);
    
    if (rank == 0)
	{
//		int counts[num_ranks - 1];
	//	MPI_Request requests[num_ranks -1];
		int global_count = 0;
//		for (i = 1; i < num_ranks; i++)
//		{
//			MPI_Irecv(&counts[i-1], 1, MPI_INT, i, 0, MPI_COMM_WORLD, &requests[i - 1]);
//		}
//		MPI_Waitall(num_ranks - 1, requests, MPI_STATUSES_IGNORE);
//		MPI_Gather(&local_count, 1, MPI_INT, &counts, 1, MPI_INT, 0, MPI_COMM_WORLD);
//		global_count += local_count;
		for (i = 0; i < num_ranks; i++)
		{
			global_count += counts[i];
		}
    	pi = ((double) global_count / (double) (flip * num_ranks)) * 4.0;
		
	}
//	else
//	{
//		MPI_Send(&local_count, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
//	}
	stop_time = MPI_Wtime();
	elapsed_time = stop_time - start_time;

	if (rank == 0)
	{
		if (num_ranks==8)
		{
			printf("N\tti\t\tpi\t\tNiter\n");
		}
		printf("%i\t",num_ranks);
		printf("%f\t",elapsed_time);
		printf("%f\t", pi);
		printf("%i\n", flip);
	}
	MPI_Finalize();
    
    // Estimate Pi and display the result
    // pi = ((double)count / (double)NUM_ITER) * 4.0;
    
    //printf("The result is %f\n", pi);
    
    return 0;
}