Why is MPI (perceived to be) so complex?: Part 1—Does strong progress simplify MPI?

Strong progress is optional in MPI. MPI allows implementations where progress (for example, updating the message-transport state machines or interaction with network devices) is only made during certain MPI procedure calls. Generally speaking, strong progress implies the ability to achieve progress (to transport data through the network from senders to receivers and exchange protocol messages) without explicit calls from user processes to MPI procedures. For instance, data given to a send procedure that matches a pre-posted receive on the receiving process is moved from source to destination in due course regardless of how often (including zero times) the sender or receiver processes call MPI in the meantime. Further, nonblocking operations and persistent collective operations work ‘in the background’ of user processes once all processes in the communicator’s group have performed the starting step for the operation. Overall, strong progress is meant to enhance the potential for overlap of communication and computation and improve predictability of procedure execution times by eliminating progress effort from user threads. This paper posits that strong progress is desirable as an MPI implementation property and examines whether strong progress: This paper explores such possibilities and sets forth principles that underpin MPI and interactions with normal and fault modes of operation. The key contribution of this paper is the conclusion that, whether measured by absolute performance, by performance portability, or by interface simplicity, strong progress in MPI is no worse than weak progress and, in most scenarios, has more potential to fulfil the aforementioned desirable attributes.