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Subject: Re: [OMPI users] Debugging memory use of Open MPI
From: Shaun Jackman (sjackman_at_[hidden])
Date: 2009-04-09 19:14:07


Eugene Loh wrote:
> I'm no expert, but I think it's something like this:
>
> 1) If the messages are short, they're sent over to the receiver. If the
> receiver does not expect them (no MPI_Irecv posted), it buffers them up.
>
> 2) If the messages are long, only a little bit is sent over to the
> receiver. The receiver will take in that little bit, but until an
> MPI_Irecv is posted it will not signal the sender that any more can be sent.
>
> Are these messages being sent over TCP between nodes? How long are they?

Each message is 2500 bytes, as I mentioned previously. In fact, each
message is composed of one hundred 25-byte operations that have been
queued up at the application level and sent with a single MPI_Send. It
would depend on the nature of the application of course, but is there
any reason to believe that 100 individual MPI_Send would be any
faster? Or is there a better way to queue up messages for a batch
transmission?

Cheers,
Shaun