How to Use MPI

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Basics

This will give you a general overview of how to compile and execute a program that has been parallelized with MPI. Many of the options listed below are the same for both Open MPI and Intel MPI, however, be care if they do differentiate.


Loading the Correct Modules

Open MPI

To ensure that the Open MPI module is loaded, check the output of the following command:

$ module list

In case it has not been loaded yet, type

$ module load openmpi

If you are currently using IntelMPIt, type

$ module switch intelmpi openmpi

IntelMPI

Look for "intelmpi" in the output of this command:

$ module list

In case it did not appear in the output, you have to load the module by typing

$ module load intelmpi

If Open MPI has already been loaded, switch to IntelMPI like this:

$ module switch openmpi intelmpi


How to Compile MPI Code

There are two MPI compilers, one for C/C++ and one for Fortran, but they share most command line options. Depending on whether your code is written in C or Fortran, follow the instructions in one of the tables below. Make sure to replace the arguments inside <…> with specific values.

OpenMPI

Use the following command to specify the program you would like to compile (replace <src_file> with a path to your code, e. g. ./myprog.c).

C/C++ Fortran
$ mpicc <src_file> $ mpifort <src_file>

You can also type the command $ mpicc [options] or $ mpifort [options]. There are a few options that come with Open MPI, however, options are more important for running your program. The compiler options might be useful to fetch more information about the Open MPI module you're using.

Options Function
-showme:help print a short help message about the usage and lists all compiler options
-showme:version show Open MPI version


How to Run an MPI Executable

Ensure that your the correct MPI module is loaded (see here).

Don’t forget to put the “-np” or “-n” option as explained below. All the other options listed below are not mandatory.

Option Function
-np <num_procs> or -n <num_procs> number of processes to run
-path <path> look for executables in the directory specified
-q or –quiet suppress helpful messages
-output-filename <name> redirects output into the file <name>.<rank>
--help lists all options available with an explanation