Looks like the issue is that mpi4py by default uses THREAD_MULTIPLE, which ucx
does not support. It would be nice if the OpenMPI pml selection code provided
information on what exactly caused ucx initialization to fail, but at least I
know how to work around my problem now.
I've been happily using OpenMPI 4.1.4 for a while, but I've run into a weird
new problem. I mainly use it with ucx, typically running with the mpirun flags
--bind-to core --report-bindings --mca pml ucx --mca osc ucx --mca btl
^vader,tcp,openib
and with our compiled Fortran codes it seems to work
