# Configuration

By default, MPI.jl will download and link against the following MPI implementations:

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

The way how MPI.jl is configured has changed with MPI.jl v0.20. See Migration from MPI.jl v0.19 or earlier for more information on how to migrate your configuration from earlier MPI.jl versions.

## 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 administrators

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]
_format = "1.0"
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]
_format = "1.0"
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")'

If you omit the JLL binary name, the default is selected for the respective operating system.

## 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 v0.19 or earlier

For MPI.jl v0.20, environment variables were used to configure which MPI library to use. These have been removed and no longer have any effect. The following subsections explain how to the same effects can be achieved with v0.20 or later.

Note

Please refer to Notes to HPC cluster administrators if you want to migrate your MPI.jl preferences on a cluster with a centrally managed MPI.jl configuration.

### JULIA_MPI_BINARY

Use MPIPreferences.use_system_binary to use a system-provided MPI binary as described here. To switch back or select a different JLL-provided MPI binary, use MPIPreferences.use_jll_binary as described here.

### JULIA_MPI_PATH

Removed without replacement.

### JULIA_MPI_LIBRARY

Use MPIPreferences.use_system_binary with keyword argument library_names to specify possible, non-standard library names. Alternatively, you can also specify the full path to the library.

### JULIA_MPI_ABI

Use MPIPreferences.use_system_binary with keyword argument abi to specify which ABI to use. See MPIPreferences.abi for possible values.

### JULIA_MPIEXEC

Use MPIPreferences.use_system_binary with keyword argument mpiexec to specify the MPI launcher executable.

### JULIA_MPIEXEC_ARGS

Use MPIPreferences.use_system_binary with keyword argument mpiexec, and pass a Cmd object to set the MPI launcher executable and to include specific command line options.

### JULIA_MPI_INCLUDE_PATH

Removed without replacement. Automatic generation of a constants file for unknown MPI ABIs is not supported anymore. See also #574.

### JULIA_MPI_CFLAGS

Removed without replacement. Automatic generation of a constants file for unknown MPI ABIs is not supported anymore. See also #574.

### JULIA_MPICC

Removed without replacement. Automatic generation of a constants file for unknown MPI ABIs is not supported anymore. See also #574.