# Usage

MPI is based on a single program, multiple data (SPMD) model, where multiple processes are launched running independent programs, which then communicate as necessary via messages.

As the main entry point for users, MPI.jl provides a high-level interface which loosely follows the MPI C API and is described in details in the following sections. The syntax should look familiar if you know MPI already, but some arguments may not be needed (e.g. the type or the number of elements of arrays, which are inferred automatically), others may be placed slightly differently, and others may be optional keyword arguments (e.g. for the index of the root process, or the source and destination of point-to-point communication functions).

In addition to the high-level interface, MPI.jl provides a low-level API which closely matches the MPI C API and from which it has been automatically generated. This is not intended for general usage, but it can be employed if a high-level wrapper is not yet available.

## Basic example

A script should include using MPI and MPI.Init() statements before calling any MPI operations, for example

# examples/01-hello.jl
using MPI
MPI.Init()

comm = MPI.COMM_WORLD
println("Hello world, I am $(MPI.Comm_rank(comm)) of$(MPI.Comm_size(comm))")
MPI.Barrier(comm)

Calling MPI.Finalize() at the end of the program is optional, as it will be called automatically when Julia exits.

The program can then be launched via an MPI launch command (typically mpiexec, mpirun or srun), e.g.

$mpiexec -n 3 julia --project examples/01-hello.jl Hello world, I am rank 0 of 3 Hello world, I am rank 2 of 3 Hello world, I am rank 1 of 3 The mpiexec function is provided for launching MPI programs from Julia itself. ## 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. $ julia
julia> using MPI
julia> MPI.install_mpiexecjl()

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:

$mpiexecjl --project=/path/to/project -n 20 julia script.jl ## CUDA-aware MPI support If your MPI implementation has been compiled with CUDA support, then CUDA.CuArrays (from the CUDA.jl package) can be passed directly as send and receive buffers for point-to-point and collective operations (they may also work with one-sided operations, but these are not often supported). Successfully running the alltoall_test_cuda.jl should confirm your MPI implementation to have the CUDA support enabled. Moreover, successfully running the alltoall_test_cuda_multigpu.jl should confirm your CUDA-aware MPI implementation to use multiple Nvidia GPUs (one GPU per rank). If using OpenMPI, the status of CUDA support can be checked via the MPI.has_cuda() function. ## ROCm-aware MPI support If your MPI implementation has been compiled with ROCm support (AMDGPU), then AMDGPU.ROCArrays (from the AMDGPU.jl package) can be passed directly as send and receive buffers for point-to-point and collective operations (they may also work with one-sided operations, but these are not often supported). Successfully running the alltoall_test_rocm.jl should confirm your MPI implementation to have the ROCm support (AMDGPU) enabled. Moreover, successfully running the alltoall_test_rocm_multigpu.jl should confirm your ROCm-aware MPI implementation to use multiple AMD GPUs (one GPU per rank). The status of ROCm (AMDGPU) support cannot currently be queried. ## Writing MPI tests It is recommended to use the mpiexec() wrapper when writing your package tests in runtests.jl: # test/runtests.jl using MPI using Test @testset "hello" begin n = 2 # number of processes mpiexec() do exe # MPI wrapper run($exe -n $n$(Base.julia_cmd()) [...]/01-hello.jl)
# alternatively:
# p = run(ignorestatus(...))
# @test success(p)
end
end