Slurm and mpi4py : repeat n times the operation with one process instead of doing it one time with n process

Question

I'm a new user of Slurm and mpi4py so I wanted to test a little code I found here : https://researchcomputing.princeton.edu/mpi4py

My python code test.py is the following :

from mpi4py import MPI
import sys

def print_hello(rank, size, name):
  msg = "Hello World! I am process {0} of {1} on {2}.\n"
  sys.stdout.write(msg.format(rank, size, name))

if __name__ == "__main__":
  size = MPI.COMM_WORLD.Get_size()
  rank = MPI.COMM_WORLD.Get_rank()
  name = MPI.Get_processor_name()

  print_hello(rank, size, name)

And my bash script is :

#!/bin/bash
#SBATCH --job-name=mpi4py-test 
#SBATCH --nodes=1                
#SBATCH --ntasks=3              
#SBATCH --cpus-per-task=1 

srun python test.py

When running sbatch run.sh I expected to get something like :

Hello World! I am process 0 of 3 on node1.
Hello World! I am process 1 of 3 on node1.
Hello World! I am process 2 of 3 on node1.

However I get :

Hello World! I am process 0 of 1 on node1.
Hello World! I am process 0 of 1 on node1.
Hello World! I am process 0 of 1 on node1.

And if I change srun python test.py by srun mpiexec -n 3 python test.py I get :

Hello World! I am process 0 of 3 on node1.
Hello World! I am process 2 of 3 on node1.
Hello World! I am process 1 of 3 on node1.
Hello World! I am process 1 of 3 on node1.
Hello World! I am process 0 of 3 on node1.
Hello World! I am process 2 of 3 on node1.
Hello World! I am process 0 of 3 on node1.
Hello World! I am process 1 of 3 on node1.
Hello World! I am process 2 of 3 on node1.

The process was executed 3 times but I just want it to be executed once. Thanks in advance.

Answer 1

It can be because either

• Mpi4py is using a version of MPI that was not compiled with Slurm support ; or
• you have a very old OpenMPI ; or
• you have a very old Slurm.

Running

mpiexec -n 3 python test.py

will probably get you what you want.