vecdot.c

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

/* Define length of dot product vectors */
#define VECLEN 6000000000

int main (int argc, char* argv[])
{
	/* MPI Initialization */
	int myid, numprocs;
	MPI_Init (&argc, &argv);
	MPI_Comm_size (MPI_COMM_WORLD, &numprocs);
	MPI_Comm_rank (MPI_COMM_WORLD, &myid);

	long int i, len = VECLEN / numprocs;
	double *a, *b;
	double mysum, allsum;

	/* 
	   Each MPI task performs the dot product, obtains its partial sum, and then calls
	   MPI_Reduce to obtain the global sum.
	   */
	if (myid == 0)
		printf("Starting omp_dotprod_mpi. Using %d tasks...\n",numprocs);

	/* Assign storage for dot product vectors */
	a = (double*) malloc (len*sizeof(double));
	b = (double*) malloc (len*sizeof(double));

	/* Initialize dot product vectors */
	for (i=0; i<len; i++) {
		a[i]=1.0;
		b[i]=a[i];
	}

	/* Perform the dot product */
	mysum = 0.0;
	for (i=0; i<len; i++) 
	{
		mysum += a[i] * b[i];
	}

	printf("Task %d partial sum = %f\n",myid, mysum);

	/* After the dot product, perform a summation of results on each node */
	MPI_Reduce (&mysum, &allsum, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
	if (myid == 0) 
		printf ("Done. MPI version: global sum  =  %f \n", allsum);

	free (a);
	free (b);
	MPI_Finalize();
}