On Aug 26, 2009, at 10:38 AM, Jeff Squyres (jsquyres) wrote:
> Yes, this could cause blocking. Specifically, the receiver may not
> advance any other senders until the matching Irecv is posted and is
> able to make progress.
I should clarify something else here -- for long messages where the
pipeline protocol is used, OB1 may need to be invoked repeatedly to
keep making progress on all the successive fragments. I.e., if a send
is long enough to entail many fragments, then OB1 may (read: likely
will) not progress *all* of them simultaneously. Hence, if you're
calling MPI_Test(), for example, to kick the progress engine, you may
have to call it a few times to get *all* the fragments processed.
How many fragments are processed in each call to progress can depend
on the speed of your hardware and network, etc.