Mixing ranks, tasks, progress and nonblocking collectives

Since the beginning, MPI has defined the rank as an implicit attribute associated with the MPI process' environment. In particular, each MPI process generally runs inside a given UNIX process and is associated with a fixed identifier in its WORLD communicator. However, this state of things is about to change with the rise of new abstractions such as MPI Sessions. In this paper, we propose to outline how such evolution could enable optimizations which were previously linked to specific MPI runtimes executing MPI processes in shared memory (e.g. thread-based MPI). By implementing runtime-level work-sharing through what we define as MPI tasks, enabling the ability to progress indifferently from stream context we show that there is potential for improved asynchronous progress. In the absence of a Session implementation, this assumption is validated in the context of a thread-based MPI where nonblocking Collective (NBC) were implemented on top of Extended Generic Requests progressed by any rank on the node thanks to an MPI extension enabling threads to dynamically share their MPI context.