Fortran MPI Examples

A few very basic examples, perhaps use to test MPI-3 Fortran library functionality.

Free, popular MPI-3 interfaces for C and Fortran include:

• OpenMPI (Linux, Mac)
• MPICH (Linux, Mac)
• Intel MPI (Linux, Windows)

Build and self-test:

cmake --workflow --preset default

or step-by-step

cmake -Bbuild

cmake --build build

ctest --test-dir build -V

This repo also gives an example of a workaround for OpenMPI 4.x and mpiexec race condition with large CPU count by setting TMPDIR to a short path name so as not to exceed 100 characters for UNIX sockets.

Message Passing

Pass data between two MPI threads. In this usage, MPI_Recv blocks waiting for MPI_Send

mpiexec -np 2 mpi/mpi_pass

Notes

See OpenMPI docs re: setting PATH and LD_LIBRARY_PATH if CMake has trouble finding OpenMPI for a compiler.

• https://hpc-forge.cineca.it/files/CoursesDev/public/2017/MasterCS/CalcoloParallelo/MPI_Master2017.pdf

Cray

FindMPI.cmake on Cray defines a rather sparse imported target MPI::MPI_C and MPI::MPI_Fortran like

-- Target: MPI::MPI_Fortran properties
-- MPI::MPI_Fortran IMPORTED = TRUE
-- MPI::MPI_Fortran NAME = MPI::MPI_Fortran
-- MPI::MPI_Fortran SYSTEM = ON
-- MPI::MPI_Fortran TYPE = INTERFACE_LIBRARY

-- Target: MPI::MPI_C properties
-- MPI::MPI_C IMPORTED = TRUE
-- MPI::MPI_C NAME = MPI::MPI_C
-- MPI::MPI_C SYSTEM = ON
-- MPI::MPI_C TYPE = INTERFACE_LIBRARY

while on most non-Cray platforms the typical properties are also defined: INTERFACE_INCLUDE_DIRECTORIES, INTERFACE_LINK_LIBRARIES, INTERFACE_LINK_OPTIONS. Also on Cray. these typically defined variables are empty: MPI_Fortran_LIBRARIES, MPI_Fortran_MODULE_DIR, MPI_Fortran_INCLUDE_DIRS, MPI_Fortran_COMPILE_OPTIONS, MPI_Fortran_LINK_FLAGS.

MPI-3 Fortran

If "mpi_f08.mod" is not found, typically the MPI include directories are also missing. You can manually check if mpi_f08.mod is preset under the include "-I" directories from:

mpif90 -show