# Environment

## Launching MPI programs

MPI.mpiexecFunction
mpiexec(fn)

A wrapper function for the MPI launcher executable. Calls fn(cmd), where cmd is a Cmd object of the MPI launcher.

Environment Variables

The behaviour of mpiexec can be controlled by the following environment variables:

• JULIA_MPIEXEC: the name or path of the launcher executable (set at compile time).
• JULIA_MPIEXEC_ARGS: additional arguments that are passed to the launcher. These are space seperated, supporting the same quoting rules as Julia Cmd objects. These can be modified at run time.

Usage

julia> mpiexec(cmd -> run(\$cmd -n 3 echo hello world));
hello world
hello world
hello world
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MPI.install_mpiexecjlFunction
MPI.install_mpiexecjl(; command::String = "mpiexecjl",
destdir::String = joinpath(DEPOT_PATH[1], "bin"),
force::Bool = false, verbose::Bool = true)

Install the mpiexec wrapper to destdir directory, with filename command. Set force to true to overwrite an existing destination file with the same path. If verbose is true, the installation prints information about the progress of the process.

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## Enums

MPI.ThreadLevelType
ThreadLevel

An Enum denoting the level of threading support in the current process:

• MPI.THREAD_SINGLE: Only one thread will execute.

• MPI.THREAD_FUNNELED: The process may be multi-threaded, but the application must ensure that only the main thread makes MPI calls. See Is_thread_main.

• MPI.THREAD_SERIALIZED: The process may be multi-threaded, and multiple threads may make MPI calls, but only one at a time (i.e. all MPI calls are serialized).

• MPI.THREAD_MULTIPLE: Multiple threads may call MPI, with no restrictions.

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## Functions

MPI.AbortFunction
Abort(comm::Comm, errcode::Integer)

Make a “best attempt” to abort all tasks in the group of comm. This function does not require that the invoking environment take any action with the error code. However, a Unix or POSIX environment should handle this as a return errorcode from the main program.

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MPI.InitFunction
Init(;finalize_atexit=true, errors_return=true)

Initialize MPI in the current process. The keyword options:

All MPI programs must contain exactly one call to MPI.Init or MPI.Init_thread. In particular, note that it is not valid to call MPI.Init or MPI.Init_thread again after calling MPI.Finalize.

The only MPI functions that may be called before MPI.Init/MPI.Init_thread are MPI.Initialized and MPI.Finalized.

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MPI.Init_threadFunction
Init_thread(required::ThreadLevel; finalize_atexit=true, errors_return=true)

Initialize MPI and the MPI thread environment in the current process. The argument specifies the required level of threading support, see ThreadLevel.

The keyword options are:

The function will return the provided ThreadLevel, and values may be compared via inequalities, i.e.

provided = Init_thread(required)
@assert provided >= required

All MPI programs must contain exactly one call to MPI.Init or MPI.Init_thread. In particular, note that it is not valid to call MPI.Init or MPI.Init_thread again after calling MPI.Finalize.

The only MPI functions that may be called before MPI.Init/MPI.Init_thread are MPI.Initialized and MPI.Finalized.

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MPI.FinalizeFunction
Finalize()

Marks MPI state for cleanup. This should be called after MPI.Init or MPI.Init_thread, and can be called at most once. No further MPI calls (other than Initialized or Finalized) should be made after it is called.

MPI.Init and MPI.Init_thread will automatically insert a hook to call this function when Julia exits, if it hasn't already been called.

MPI.universe_sizeFunction
universe_size()
The total number of available slots, or nothing if it is not defined. This is determined by the MPI_UNIVERSE_SIZE attribute of COMM_WORLD.
This is typically dependent on the MPI implementation: for MPICH-based implementations, this is specified by the -usize argument. OpenMPI defines a default value based on the number of processes available.