This sounds right. It's been eons since I've looked at the GM code
in LAM, but the short sends are eager and the long sends use a
rendezvous protocol. Specifically, the main content of the message
won't be sent for a "long" message until the receiver ACKs that a
matching MPI receive has been posted.
This is, among other reasons, to help prevent resource exhaustion at
the receiver.
I believe that there is an SSI parameter to change the size of
short / long messages, but I don't recall what it is offhand. Did
you look in the LAM/MPI User's Guide? I seem to recall documenting
all such things in there...
On Mar 22, 2007, at 6:03 AM, Vartan Padaryan wrote:
> Hello everybody.
>
> I'm exploring performance of various communication patterns on
> Linux-cluster with LAM-7.1.1 over GM. A simple example with
> nonblocking
> communication produced strange results.
>
> Proc0: Isend; Some computation; Wait;
> Proc1: Recv;
>
> In case of buf_size <= 16KB that's OK. Execution time of Recv is
> stable
> ~100us. for 4KB and any communication mode of send (Isend, Issend,
> Ibsend, Irsend). But if buf_size > 16KB, execution time of Recv
> depends
> on weight of computation in Proc0, i.e. the more computations Proc0
> does, the longer Recv goes on.
> Is it correct behavior?
> Does LAM change protocol for nonblocking sends, if buf_size more
> than 16K?
> How can I get the same behavior for large buffer?
> Maybe my code isn't right (i use pselect for modeling delay between
> Isend and Wait)?
>
> WBR, Vartan Padaryan.
> _______________________________________________
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--
Jeff Squyres
Cisco Systems
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