How to make/configure an MPI project to run on multiple processes? - c

Question

I have a small piece of code that should run on multilple processes, which is :
#include <stdio.h>
#include "mpi.h"
main(int argc, char **argv)
{
int ierr, num_procs, my_id;
ierr = MPI_Init(&argc, &argv);
/* find out MY process ID, and how many processes were started. */
ierr = MPI_Comm_rank(MPI_COMM_WORLD, &my_id);
ierr = MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
printf("Hello world! I'm process %i out of %i processes\n",
my_id, num_procs);
ierr = MPI_Finalize();
}
the output is :
Hello world! I'm process 0 out of 1 processes. although it should run on more than one process
We edited the run configurations arguments to "-np 2" so it would run on 2 processes but it always gives us 1 process no matter what the value is.
The used environment is:
Eclipse Juno on Ubuntu 12.04
Source of code: http://condor.cc.ku.edu/~grobe/docs/intro-MPI-C.shtml[^]

Answer 1

It seems that you are trying to launch your MPI application directly, i.e. starting the compiled executable with -np 2, like this:
$ ./my_app -np 2
That's not the right way of launching MPI programs. Instead, you should call your MPI implementation's launcher (usually named mpirun or mpiexec), and pass the name of your executable and -np 2 to that. So if your executable is called my_app, then instead of the above command, you should run:
$ mpirun -np 2 ./my_app
Consult your MPI implementation's documentation for the specifics.

Answer 2

Some small points about mpirun or mpiexec commands:
If you are trying to run you app on multiple nodes, make sure that your app is copied to all the nodes.In addition, make sure that all needed binary files (executable & libraries) could be found directly from command line. I personally, prefer to run my MPI programs within a shell script like this:
#!/bin/sh
PATH=/path/to/all/executable/files \
LD_LIBRARY_PATH=/path/to/all/libraries \
mpirun -np 4 ./my_app arg1 arg2 arg3