View source for Mpi4py hello world

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This example shows how to use MPI library in a Python script by using the package [https://mpi4py.readthedocs.io/en/stable/index.html mpi4py].

== Load modules ==
On the login node, load the new software stack

 $ env2lmod

Or, set the new software stack as the default

 $ set_software_stack.sh new

Load necessary modules

 $ module load gcc/6.3.0 openmpi/4.0.2 python/3.8.5 eth_proxy

If not provided, install the package mpi4py locally

 $ pip install --user mpi4py

== mpi4py Hello World ==
Go to $SCRATCH and create a work directory
 $ cd $SCRATCH
 $ mkdir mpi4py
 $ cd mpi4py
 $ pwd
 /cluster/jarunanp/scratch/mpi4py

Create a file called ''hello_mpi4py.py'' and add these lines
 from mpi4py import MPI
 
 comm = MPI.COMM_WORLD
 rank = comm.Get_rank()
 size = comm.Get_size()
 procname = MPI.Get_processor_name()
 
 print("Hello world from processor {}, rank {:d} out of {:d} processors".format(procname, rank, size))

== Get an interactive session on compute nodes == 
Request 4 processors on 4 nodes, one processor on each node
 $ bsub -n 4 -R "span[ptile=1]" -Is bash
 Generic job.
 Job <187694829> is submitted to queue <normal.4h>.
 <<Waiting for dispatch ...>>
 <<Starting on eu-ms-002-19>>
 FILE: /sys/fs/cgroup/cpuset/lsf/euler/job.187694829.29543.1633967743/tasks
 [jarunanp@eu-ms-002-19 mpi4py]$

Run the Python script with '''mpirun'''
 [jarunanp@eu-ms-002-19 mpi4py]$ mpirun -np 4 python hello_mpi4py.py
 Hello world from processor eu-ms-002-19, rank 0 out of 4 processors
 Hello world from processor eu-ms-004-25, rank 1 out of 4 processors
 Hello world from processor eu-ms-005-34, rank 2 out of 4 processors
 Hello world from processor eu-ms-005-42, rank 3 out of 4 processors
 [jarunanp@eu-ms-002-19 mpi4py]$

== Submit a job from the command line ==
 $ bsub -n 4 -R "span[ptile=1]" "mpirun -n 4 python hello_mpi4py.py"

You should see similar screen output in the LSF output file.

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