Comment (by Roland Haas):

Adding the vader env setting

export OMPI_MCA_btl_vader_single_copy_mechanism=none

also helps prevent MPI hangs in OSX VirtualBox containers (OSX being the client) when using MacPorts as described in #2290 . The same hang still occurs in OSX Catalina VMs. This may be related to an actual bug on OpenMPI: https://github.com/open-mpi/ompi/issues/6568 and affects only MPI communication with large packages (which likely explains why not every single MPI using test would hang). The OpneMPI ticket provides what looks like a reproducer.

I would like to add this env variable setting (with a comment about Docker and OSX virtual machines) to the tutorial notebook (I will also use it for my own VMs to run the testsuite but that is somewhat unrelated).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2287/add-openmpi-env-vars-to-notebook-to-avoid