# 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 features such as fast network interfaces and CUDA-aware or ROCm-aware MPI interfaces.

The MPIPreferences.jl package allows the user to choose which MPI implementation to use in MPI.jl. It uses Preferences.jl to configure the MPI backend for each project separately. This provides a single source of truth that can be used for JLL packages (Julia packages providing C libraries) that link against MPI. It can be installed by

julia -e 'using Pkg; Pkg.add("MPIPreferences")'

## Using a system-provided MPI backend

### Requirements

MPI.jl requires a shared library installation of a C MPI library, supporting the MPI 3.0 standard or later. The following MPI implementations should work out-of-the-box with MPI.jl:

### Configuration

Run MPIPreferences.use_system_binary(). This will attempt to locate and to identify any available MPI implementation, and create a file called LocalPreferences.toml adjacent to the current Project.toml.

julia --project -e 'using MPIPreferences; MPIPreferences.use_system_binary()'

If the implementation is changed, you will need to call this function again. See the MPIPreferences.use_system_binary documentation for specific options.

Note

You can copy LocalPreferences.toml to a different project folder, but you must list MPIPreferences in the [extras] or [deps] section of the Project.toml for the settings to take effect.

Note

Due to a bug in Julia (until v1.6.5 and v1.7.1), getting preferences from transitive dependencies is broken (Preferences.jl#24). To fix this update your version of Julia, or add MPIPreferences as a direct dependency to your project.

### Notes to HPC cluster adminstators

Preferences are merged across the Julia load path, such that it is feasible to provide a module file that appends a path to JULIA_LOAD_PATH variable that contains system-wide preferences. The steps are as follows:

1. Run MPIPreferences.use_system_binary(), which will generate a file LocalPreferences.toml containing something like the following:

[MPIPreferences]
abi = "OpenMPI"
binary = "system"
libmpi = "/software/mpi/lib/libmpi.so"
mpiexec = "/software/mpi/bin/mpiexec"
2. Create a file called Project.toml or JuliaProject.toml in a central location, for example /software/mpi/julia or in the same directory as the MPI library module, and add the following contents:

[extras]
MPIPreferences = "3da0fdf6-3ccc-4f1b-acd9-58baa6c99267"

[preferences.MPIPreferences]
abi = "OpenMPI"
binary = "system"
libmpi = "/software/mpi/lib/libmpi.so"
mpiexec = "/software/mpi/bin/mpiexec"

updating the contents of the [preferences.MPIPreferences] section match those of the [MPIPreferences] in LocalPreferences.toml.

3. Append the directory containing the file to the JULIA_LOAD_PATH environment variable, with a colon (:) separator.

If this variable is not already set, it should be prefixed with a colon to ensure correct behavior of the Julia load path, e.g. JULIA_LOAD_PATH=":/software/mpi/julia". If using environment modules, this can be achieved with

append-path  -d {} JULIA_LOAD_PATH :/software/mpi/julia

in the corresponding module file (preferably the module file for the MPI installation or for Julia).

The user can still provide differing MPI configurations for each Julia project that will take precedent by modifying the local Project.toml or by providing a LocalPreferences.toml file.

## Using an alternative JLL-provided MPI library

The following MPI implementations are provided as JLL packages and automatically obtained when installing MPI.jl:

• MicrosoftMPI_jll: Microsoft MPI Default for Windows
• MPICH_jll: MPICH. Default for all other systems
• OpenMPI_jll: Open MPI
• MPItrampoline_jll: MPItrampoline: an MPI forwarding layer.

Call MPIPreferences.use_jll_binary, for example

julia --project -e 'using MPIPreferences; MPIPreferences.use_jll_binary("MPItrampoline_jll")'

## Configuration of the MPI.jl testsuite

### Testing against a different MPI implementation

The LocalPreferences.toml must be located within the test folder, you can either create it in place or copy it into place.

~/MPI> julia --project=test
julia> using MPIPreferences
julia> MPIPreferences.use_system_binary()
~/MPI> rm test/Manifest.toml
~/MPI> julia --project
(MPI) pkg> test

### Environment variables

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

• JULIA_MPI_TEST_NPROCS: How many ranks to use within the tests
• JULIA_MPI_TEST_ARRAYTYPE: Set to CuArray or ROCArray to test the CUDA-aware interface with CUDA.CuArray or the ROCm-aware interface with AMDGPU.ROCArray or buffers.
• JULIA_MPI_TEST_BINARY: Check that the specified MPI binary is used for the tests
• JULIA_MPI_TEST_ABI: Check that the specified MPI ABI is used for the tests

## Migration from MPI.jl version 0.19 or earlier

Prior to MPI.jl version 0.20, environment variables were used to configure which MPI library to use. These have now been removed and no longer have any effect:

• JULIA_MPI_BINARY
• JULIA_MPIEXEC
• JULIA_MPIEXEC_ARGS
• JULIA_MPI_INCLUDE_PATH
• JULIA_MPI_CFLAGS
• JULIA_MPICC