Exclude some part of Python Syntax from MPI

Question

I'm currently facing a problem and I need your help! So I have a code that will call a Linux Terminal command from a python code. But, I also have some part of the code that needs to be run in parallel to accelerate the computation.

Roughly, the code (say, the name is code.py) will looks like this:

import parallel_func, cli_func

parallel_func() #this part is supposed to run in parallel when using mpirun 
cli_func() #this part is supposed to run without mpi

The problem is, if I run mpirun -np 4 python code.py, the parallel_func will do well, but the cli_func will run multiple cli_program that will overburden the cpu.

So, are there any way I can do to isolate that cli_func part from the MPI?

Thank you for your help!

edit: I've did the solution that tobias replied below (using mpi rank selection) but it results in running one rank with the cli_func with the rest of the cpu still running, limitting the usable processor for the cli program.

edit: I found the makeshift solution using this mpi4py: substantial slowdown by idle cores That fixed it all!

Tobias Tobias · Accepted Answer · 2020-07-01T17:09:50

First off, whenever you run a program using MPI, you will always have n instances of your program running in parallel. Hence, what you probably want to achieve is that only one of those n programs running simultaneously executed the cli_func() part.

The n programs running in parallel all have a rank, a number between 0 and n-1, so that you can distinguish the programs running. You can use that rank to make only one program perform a certain action. If you use the mpi4py library, this looks like so:

import parallel_func, cli_func

from mpi4py import MPI

comm = MPI.COMM_WORLD
rank = comm.Get_rank()

parallel_func()
if rank == 0:
    cli_func()

The condition rank == 0 makes sure that only the instance with rank 0 executes the respective code. But you can also choose any other rank, of course, or have different instances do different things.

if rank == 0:
    print("First")
elif rank == 1:
    print("Second!")

You did not specify in your question what MPI Python package you are using. Depending on this, the exact syntax might differ, although the idea will still be the same.

Exclude some part of Python Syntax from MPI

1 Answers