progress, nonblockingnonblocking, progress A communication is enabled once a send and a matching receive have been enabled communication posted by two processes. The progress rule requires that once a communication is enabled, then either the send or the receive will proceed to completion (they might not both complete as the send might be matched by another receive or the receive might be matched by another send). Thus, a call to MPI_WAIT that completes a receive will eventually return if a matching send has been started, unless the send is satisfied by another receive. In particular, if the matching send is nonblocking, then the receive completes even if no complete-send call is made on the sender side.
Similarly, a call to MPI_WAIT that completes a send eventually returns if a matching receive has been started, unless the receive is satisfied by another send, and even if no complete-receive call is made on the receiving side.
If a call to MPI_TEST that completes a receive is repeatedly made with the same arguments, and a matching send has been started, then the call will eventually return flag = true, unless the send is satisfied by another receive. If a call to MPI_TEST that completes a send is repeatedly made with the same arguments, and a matching receive has been started, then the call will eventually return flag = true, unless the receive is satisfied by another send.
