#C
Copy this into loops.c
#include <stdio.h>
#include <mpi.h>
int main(int argc, char **argv)
{
int i, rank, nprocs, count, start, stop, nloops;
MPI_Init(&argc, &argv);
// get the number of processes, and the id of this process
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
// we want to perform 1000 iterations in total. Work out the
// number of iterations to perform per process...
count = 1000 / nprocs;
// we use the rank of this process to work out which
// iterations to perform.
start = rank * count;
stop = start + count;
// now perform the loop
nloops = 0;
for (i=start; i<stop; ++i)
{
++nloops;
}
printf("Process %d performed %d iterations of the loop.\n",
rank, nloops);
MPI_Finalize();
return 0;
}In this example we use MPI_Comm_size(MPI_COMM_WORLD, int *size) to put the number of processes in the MPI process team into the variable pointed to by size. This allows each process to work out how many interations of the loop to perform, e.g. if there are four processes in the team, then each only needs run 250 of the 1000 iterations to be performed. In this example, the only work performed in each iteration is increasing the process-local counter of the number of times that the loop has been performed (each process in the team has its own copy of nloops). By doing this, each process in the team counts up the number of times that it has performed the loop.
You can compile this program using;
mpicc loops.c -o loopsThis will produce the executable, loops.