I want to run a Fortran program which calls a subroutine that I want to parallelize with MPI. I know this sounds complicated, but I want to be able to specify the number of processes for each call. What I would want to use is a structure like this:
program my_program
implicit none
!Define variables
nprocs = !formula for calculating number of processes.
call my_subroutine(output,nprocs,other input vars)
end my_program
I want to run my_subroutine with the same effect as this:
mpirun -nprocs my_subroutine.o, where my_subroutine has been compiled with 'other input vars.'
Is this possible? Please help. Thanks.
Ok. Sorry for not responding earlier. What I'm wondering is if I need to label the processes first and do something like this:
CALL MPI_COMM_RANK(MPI_COMM_WORLD,MYID,IERR)
before calling the subroutine. So if 0
Before you can call MPI_comm_rank, you have to call MPI_init. But in 2018, I would not call MPI in a Fortran program to start with. Fortran has parallel (SPMD distributed memory) execution built into the base language syntax. There is no need for MPI any more for a Fortran code.
SPMD is Single Program Multiple Data. It is a commonly used parallel programming model where a Single Program is replicated multiple times at execution time with each instance having its own data (Multiple Data). The instances execute asynchronously except where synchronization is specified. MPI, Shmem, and Fortran all use this model for parallel programming.
The program launch process automatically provides a number for each of the instances. In MPI it is a "rank", in Shmem a PE, and in Fortran an "image". Each model provides a procedure to return the number for the executing instance.
Yes, Fortran images are designed to be run in a distributed-memory environment. They are much more like ranks in MPI than threads in OpenMP. In my experience native Fortran (using images) is faster than MPI. It should scale at least as well - certainly up to many thousands of processes.
Images are a feature of the current Fortran standard. Support is included in the latest versions of gfortran, in the current Intel compiler, and in the Cray compiler. Possibly others that I'm not aware of.
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