I have a certain Fortran program which is parallelized using MPI. For a certain application I know want to run 2 fairly (but not absolutely) identical instances of this program. These 2 concurrently running programs would have to communicate with each other during runtime.
I thought one of the easiest ways to do this would be to define a global communicator (eg. MPI_COMM_WORLD_GLOBAL) which spans all the processes. I would then proceed to redefine the usual global communicator MPI_COMM_WORLD to be the total amount of processes used by this instance of the program (note that, although I talk of multiple instances of the program, there would only be 1 real program running in this case).
I would like to keep MPI_COMM_WORLD to be the total amount of processes used for 'this' instance of the program, as to avoid changing the communicators throughout my code.
My question is therefore, is there a straightforward way of redefining MPI_COMM_WORLD in Fortran? Or is my proposed approach bad practice and should I look into other methods.
Thanks in advance.
MPI_COMM_WORLD can be just a constant or a macro. Trying to redefine it is asking for trouble.
In my OpenMPI it is a parameter with value 9.
You would have to make your own variable with that name which shadows that one from module mpi or mpif.h, e.g., by using some custom module or include file.
It shouldn't be that difficult to refactor your code to use a variable for a communicator instead of the hard-coded MPI_COMM_WORLD using some modern text editor or IDE. With some tools it can be as simple as one global Search and Replace and declaration and definition of the constant (in a module).