In reply to motonacciu (First Name).

Correct -- in that case, I'd say that the app developer probably should have coalesced the 4 messages into 1.

But just to clarify your point: hardware send queues are typically fairly deep, capable of holding thousands of pending sends. So the coalescing code in Open MPI probably won't be triggered by just 4 sends. It'll typically be triggered by ping-pong benchmarks (yet another reason benchmarks are not good indicators of real performance!) and other sending-many-thousands-of-sends-at-a-time types of codes.