OpenMPI hangs on large message

This happens with (Open MPI) 4.1.1. To reproduce the problem, run below script with

mpirun -n 2 python test_largemsg.py
cat test_largemsg.py

from mpi4py import MPI

...

comm = MPI.COMM_WORLD

...

rank = comm.Get_rank()

...

size = comm.Get_size()

...

import numpy as np

...

n = 20000

...

if rank == 0:

...

    data = np.arange(n, dtype='i')

...

else:

...

    data = np.empty(n, dtype='i')

...

comm.Bcast(data, root=0)

...

print(f'rank={rank} data[-1]={data[-1]}')

Solution: suppress openmpi tcp protocol with the following command:

mpirun -n 2 --mca btl ^tcp python test_largemsg.py

Note: This is NOT an issue with srun.