In reply to Heiko.

FWIW, MPI implementations are supposed to make progress on non-blocking communications (collective and otherwise) even while not in the MPI library (e.g., via MPI_TEST and friends).

That being said, it's still early days for non-blocking collective operation implementations; other than some hardware offload implementations, there has been limited availability of implementations that are truly asynchronous and do not require dipping into the MPI library. Give it some time.