Hi, In my cluster I found problems running some MPI applications (for e.g. via mpi4py) and it is related to defaults in /etc/openmpi/4.0/openmpi-mca-params.conf. When I changed btl to ^openib or to vader,selft,tcp (in case tcp nodes) - it started to work, but in this case it doesn't use UCX anymore. Regards Rolandas