OpenMPI is an implementation of the Message Passing Interface (MPI) specification for writing distributed computing applications. MPI provides a message passing interface without shared-memory access, while OpenMP exposes shared-memory paradigms. Both can be combined to allow shared-memory access for ranks on the same MPI node.


On FreeBSD, install OpenMPI 3 with:

$ pkg install net/openmpi3

It may be necessary to add the OpenMPI path to the system search path manually. In KornShell, override the environment variable PATH with:

$ export PATH="$PATH:/usr/local/mpi/openmpi3/bin/"

Add the export statement to your profile file (for example, ~/.kshrc) to make the changes permanent. Optionally, we may change the default Fortran compiler invoked by OpenMPI:

$ export OMPI_FC="gfortran12"
$ mpifort --showme:command

The argument --showme lets us inspect the OpenMP build options, for instance:

$ mpifort --showme:link
-fexceptions -pthread -I/usr/local/mpi/openmpi3/lib -Wl,-rpath=-Wl,-rpath=/usr/local/lib/gcc12
-Wl,-rpath -Wl,/usr/local/mpi/openmpi3/lib -Wl,--enable-new-dtags -L/usr/local/mpi/openmpi3/lib
-lmpi_usempi -lmpi_mpifh -lmpi


This basic example program consists of a master routine, a worker routine, and the main program for calling OpenMPI:

! example.f90
module mpi_master
! The OpenMPI master process module.
    implicit none
    public :: master
    subroutine master(nworkers)
        character(len=*), parameter :: FMT = '("--- Master [", i4, 5(a, i2.2), "]")'

        integer, intent(in) :: nworkers
        integer             :: dt(8)

        call date_and_time(values=dt)
        print FMT, dt(1), '/', dt(2), '/', dt(3), ' ', dt(5), ':', dt(6), ':', dt(7)
    end subroutine master
end module mpi_master

module mpi_worker
! The OpenMPI worker process module.
    implicit none
    public :: worker
    subroutine worker(nworkers, rank)
        integer, intent(in) :: nworkers
        integer, intent(in) :: rank

        print '(a, i0)', '>>> Hello from worker ', rank
        call sleep(1)
    end subroutine worker
end module mpi_worker

program main
! Example program that calls OpenMPI.
    use, intrinsic :: iso_fortran_env, only: real64
    use :: mpi
    use :: mpi_master
    use :: mpi_worker
    implicit none
    integer           :: master_rank
    integer           :: nproc
    integer           :: nworkers
    integer           :: rank
    integer           :: rc
    real(kind=real64) :: t1, t2

    ! Initialise OpenMPI communication infrastructure.
    call mpi_init(rc)

    ! Get number of active processes.
    call mpi_comm_size(mpi_comm_world, nproc, rc)

    master_rank = 0
    nworkers    = nproc - 1

    ! Identify process.
    call mpi_comm_rank(mpi_comm_world, rank, rc)

    if (rank == master_rank) then
        ! Master process.
        t1 = mpi_wtime()
        call master(nworkers)
        t2 = mpi_wtime()

        print '("--- Timing: ", f6.4, " sec on ", i0, " workers")', t2 - t1, nworkers
        ! Worker process.
        call worker(nworkers, rank)

    ! Terminate OpenMPI communication infrastructure.
    call mpi_finalize(rc)
end program main

Inside our Fortran project directory, we have to create a file hostfile with the following contents:

localhost slots=25

We are then ready to compile and run the example OpenMPI program. Instead of calling GNU Fortran directly, we have to run the compiler wrapper mpifort. Afterwards, a runtime environment for program execution has to be created with mpirun:

$ mpifort -o example example.f90
$ mpirun --hostfile ./hostfile -np 4 ./example
>>> Hello from worker 2
>>> Hello from worker 3
>>> Hello from worker 1
--- Master [2020/01/13 19:09:14]
--- Timing: 0.0011 sec on 3 workers

The argument -np sets the number of processes: one master and three workers. Please note that mpirun requires at least the loopback device to be available. In doubt, make sure you have an ethernet or WiFi adapter enabled.


To simply the build process, we can write a short Makefile that handles compilation and execution for us:


FC       = mpifort
HOSTFILE = hostfile
NPROC    = 4
TARGET   = example

.PHONY: all clean run

all: $(TARGET)

	$(FC) -o $(TARGET) example.f90

	mpirun --hostfile ./$(HOSTFILE) -np $(NPROC) ./$(TARGET)

	rm *.mod $(TARGET)

Now, we just have to run BSD make or GNU make:

$ make
mpifort -o example example.f90
$ make run
mpirun --hostfile ./hostfile -np 4 ./example
>>> Hello from worker 1
>>> Hello from worker 3
>>> Hello from worker 2
--- Master [2020/01/13 19:50:33]
--- Timing: 0.0010 sec on 3 workers