# Configuration

This is suitable for most single-node use cases, but for larger systems, such as HPC clusters or multi-GPU machines, you will probably want to configure against a system-provided MPI implementation in order to exploit feature such as fast network interfaces and CUDA-aware MPI interfaces.

MPI.jl will attempt to detect when you are running on a HPC cluster, and warn the user about this. To disable this warning, set the environment variable JULIA_MPI_CLUSTER_WARN=n.

## Using a system-provided MPI

### Requirements

MPI.jl requires a shared library installation of a C MPI library, supporting the MPI 3.0 standard or later.

### Building

To use the the system MPI, set the environment variable JULIA_MPI_BINARY=system and run Pkg.build("MPI"). This can be done by:

julia --project -e 'ENV["JULIA_MPI_BINARY"]="system"; using Pkg; Pkg.build("MPI"; verbose=true)'

This will attempt find and identify any available MPI implementation.

The MPI standard doesn't specify the exact application binary interface (ABI). The build script will attempt to build a small C program to determine the appropriate type definitions and constants. This requires a compatible C compiler (mpicc by default).

The following implementations should work:

If the implementation is changed, you will need to re-run Pkg.build("MPI").

### Environment variables

The following optional environment variables can be used to control certain aspects of the build script and other library behaviour. The results of these will be cached in a configuration file located at ~/.julia/prefs/MPI.toml and so can be used for subsequent MPI builds.

• JULIA_MPI_BINARY: can be set to either the empty string (to use the default implementations above) or system (to use a system-provided implementation).
• JULIA_MPI_PATH: the top-level installation directory of MPI. i.e. the library should be located in ${JULIA_MPI_PATH}/lib and mpiexec in ${JULIA_MPI_PATH}/bin
• JULIA_MPI_LIBRARY: the library name or full path of the MPI shared library. By default, it will attempt to look for common MPI library names in the standard library paths (e.g. libmpi, libmpich, msmpi).
• JULIA_MPIEXEC: the name (or full path) of the MPI launcher executable. The default is mpiexec, but some clusters require using the scheduler launcher interface (e.g. srun on Slurm, aprun on PBS).
• JULIA_MPIEXEC_ARGS: Additional arguments to be passed to MPI launcher.

The following variables are also queried:

• JULIA_MPI_INCLUDE_PATH: the directory containing the MPI header files.
• JULIA_MPI_CFLAGS: C flags passed to the constant generation build (default: -lmpi)
• JULIA_MPICC: MPI C compiler (default: mpicc)

The test suite can also be modified by the following variables:

• JULIA_MPIEXEC_TEST_ARGS: Additional arguments to be passed to the MPI launcher for the tests only.
• JULIA_MPI_TEST_ARRAYTYPE: Set to CuArray to test the CUDA-aware interface with CUDA.CuArray buffers.

## Julia wrapper for mpiexec

Since you can configure MPI.jl to use one of several MPI implementations, you may have different Julia projects using different implementation. Thus, it may be cumbersome to find out which mpiexec executable is associated to a specific project. To make this easy, on Unix-based systems MPI.jl comes with a thin project-aware wrapper around mpiexec, called mpiexecjl.

### Installation

You can install mpiexecjl with MPI.install_mpiexecjl(). The default destination directory is joinpath(DEPOT_PATH[1], "bin"), which usually translates to ~/.julia/bin, but check the value on your system. You can also tell MPI.install_mpiexecjl to install to a different directory.

To quickly call this wrapper we recommend you to add the destination directory to your PATH environment variable.

### Usage

mpiexecjl has the same syntax as the mpiexec binary that will be called, but it takes in addition a --project option to call the specific binary associated to the MPI.jl version in the given project. If no --project flag is used, the MPI.jl in the global Julia environment will be used instead.

After installing mpiexecjl and adding its directory to PATH, you can run it with:

\$ mpiexecjl --project=/path/to/project -n 20 julia script.jl`