#C++
Copy this into loops.cpp
#include <iostream>
#include <mpi.h>
int main(int argc, char **argv)
{
MPI::Init(argc, argv);
// get the number of processes, and the id of this process
int rank = MPI::COMM_WORLD.Get_rank();
int nprocs = MPI::COMM_WORLD.Get_size();
// we want to perform 1000 iterations in total. Work out the
// number of iterations to perform per process...
int count = 1000 / nprocs;
// we use the rank of this process to work out which
// iterations to perform.
int start = rank * count;
int stop = start + count;
// now perform the loop
int nloops = 0;
for (int i=start; i<stop; ++i)
{
++nloops;
}
std::cout << "Process " << rank << " performed " << nloops
<< " iterations of the loop.\n";
MPI::Finalize();
return 0;
}In this example we use MPI::COMM_WORLD.Get_size() to return the number of processes in the MPI process team. 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;
mpicxx loops.cpp -o loopsThis will produce the executable, loops.