[Beowulf] Re: Re: Home beowulf - NIC latencies
joachim at ccrl-nece.de
Mon Feb 14 14:18:51 EST 2005
Rob Ross wrote:
> Making a sequence of MPI_Isends followed by a MPI_Wait go faster than a
> sequence of MPI_Sends isn't hard, particularly if the messages are to
> different ranks. I would guess that every implementation will provide
> better performance in the case where the user tells the implementation
> about all these concurrent operations and then MPI_Waits on the bunch.
In this case, the user should think about MPI_Alltoall(v) - there are
MPI implementations which do this in a smarter way than
Isend/Irecv/Waitall to achieve much better performance than using the
naive approach. Especially if you go to large process numbers, some
coordination can help a lot, even for a full bisection network like a
single-stage full crossbar...
Generally, collectives are there to let the library know what kind of
communication is coming next. All speculations in the library based on
monitoring and predicting non-collective communication will probably
only do good in the matching micro-benchmark (my personal experience).
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de
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