Frequently Asked Questions (FAQ)

How do I use MPI?

MPICH is a freely available, portable implementation of MPI, the standard for message-passing libraries.

Building Your Applications

To build applications with MPICH, you should replace direct references to compilers (gcc, g77, etc.) with references to the appropriate MPICH wrapper scripts (mpicc, mpif77, etc.). The wrapper scripts are intended to supply correct include and library paths and options automatically. The wrapper script names are:

mpicc the C compiler wrapper for gcc, pgcc, icc.
mpif77 the Fortran 77 compiler wrapper for f77, pgf77, ifc.
mpif90 the Fortran 90 compiler wrapper for pgf90, f90com.
mpiCC the C++ compiler wrapper for g++, pgCC, icc.

To troubleshoot problems compiling with a wrapper script, use the -show argument to see what commands it is executing, and try executing those commands manually. Try running mpicc -show alone to see the general effect of the wrapper script. Here's an example:

% mpicc -show test.c -o test
gcc -DUSE_STDARG -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_UNISTD_H=1 -DHAVE_STDARG_H=1 -DUSE_STDARG=1
-DMALLOC_RET_VOID=1 -L/usr/local/mpich/1.2.5.2/gcc/i686/lib  test.c -o test -lmpich

Running Your Applications

Start MPICH programs using the mpirun wrapper script.

The most common mpirun arguments are briefly described below:

Argument	Purpose
`-np N`	Request quantity N processors.
`-nolocal`	Do not run the job on the local node (example: master).
`-show`	Show what mpirun would do, but don't actually do it. Useful for troubleshooting.

For more information, please read man mpirun. Also note that some options have no effect on execution because they don't apply to the cluster's configuration.