Newsgroups: comp.parallel.mpi
From: rdaoud@magnus.acs.ohio-state.edu (Raja B Daoud)
Subject: Re: COW MPI Question
Organization: Ohio Supercomputer Center
Date: 6 Aug 1995 02:55:12 GMT
Message-ID: <401aug$qis@charm.magnus.acs.ohio-state.edu>

Greenup Cade <gree9031@buzzard.csrv.uidaho.edu> wrote:
>I've been working with a program that was converted from a Mas-Par (with 
>a ton of processors) to MPI (LAM) on a HP COW. [...]
>do they still put the information out on the network, [...] or is MPI
>smart enough to keep the communication internal to that single processor?

This is an implementation issue, not related to the MPI standard (though
MPI is smart! :-) ).  In LAM, processes running on the same node do not
use the network to communicate.  Note that sockets are used to do UNIX
IPC between them, but information doesn't go on the network.

You will (most probably) get a significant performance boost by modifying
the code to use 1 process per node.  This gets rid of the communication
(and context switching this generates) between the processes running on
the same nodes.

Regards,

--Raja

-=-
Raja Daoud				raja@tbag.osc.edu
Ohio Supercomputer Center		http://www.osc.edu/lam.html

