Why do I get comm.rank == 0 everytime when using mpirun?

1

I am trying to understand how Fenics works with mpirun. I execute the following in a terminal

mpirun -n 2 python3 check.py

where the contents of check.py are as follows

import dolfin as df

comm = df.MPI.comm_world
print('comm.size =', comm.size)

if comm.rank == 0:
    for i in range(5):
        print('Rank =', comm.rank, 'i =', i)

I get following output:

comm.size = 1
Rank = 0 i = 0
Rank = 0 i = 1
Rank = 0 i = 2
Rank = 0 i = 3
Rank = 0 i = 4
comm.size = 1
Rank = 0 i = 0
Rank = 0 i = 1
Rank = 0 i = 2
Rank = 0 i = 3
Rank = 0 i = 4

I can guess that I am getting 5 lines of output for each of the two processes. But why is the rank always printed as 0 and why is comm.size = 1?

2

It works as expected on my personal machine. There is something wrong with your MPI setup.

3

Thank you very much for confirming. I am running Fenics 2019.1.0 inside docker container. But I made my own docker container based on the original Dockerfile because I needed to install scipy inside it. I will try to use the original docker container and cross-check.

4

Take a look at this thread:

there was a similar issue there.

5

Thank you very much! I pulled the latest stable docker image and now I get the following output

comm.size = 2
Rank = 0 i = 0
Rank = 0 i = 1
Rank = 0 i = 2
Rank = 0 i = 3
Rank = 0 i = 4
comm.size = 2

I think this is a more reasonable output.