From: Jaume Catarineu <jaume.catarineu@econ.upf.es>
Newsgroups: comp.parallel.mpi
Subject: Re: LAM question: what does mpirun -c2c ?
Date: Tue, 25 May 1999 17:03:08 +0200
Organization: CESCA News service. Catalunya
Message-Id: <374ABBAC.B258C000@econ.upf.es>
References: <7hp20m$gat$1@sunu789.rz.ruhr-uni-bochum.de>
    <7hp9g5$d9j3@puligny.idt.cdc.fr>
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> >The following happened:
> >The transfer-time are little smaller for x <= 2^15 byte , but
> >when each slave send back >= 2^16 byte the transfer-time
> >increased strongly.  (factor 13 larger than without the -c2c option)
> strange, on our 8 bi-P2 clusters on fast ethernet, -c2c make all much better.
> 
> One problem we had was about memory, when one one was
> short on RAM the performance were trashing above a few megs.
> 
> note that 32 or 64k is about a break in the slope of the delay curve,
> probably because it change protocole (UDP-> TCP...)
> 
> I don't know the internals of LAM but any comment would be appreciated.
> 
I've found a 1996 tech report that talks about this.

LAM 6.0 uses short message protocol upto 8192 bytes. It switches over
to long message protocol for longer messages. MPICH changes its
protocol at 16384 bytes. You can find the technical report explaining
this at http://www.mpi.nd.edu/lam/perf/lam60-perf.html

Salut!

Jaume,
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