Newsgroups: comp.parallel.mpi
From: salo@mrjones.engr.sgi.com (Eric Salo)
Subject: Re: Clarifications on Ready/Sync modes
Organization: Silicon Graphics, Inc.  Mountain View, CA
Date: 12 Aug 1995 01:43:13 GMT
Message-ID: <40h0vh$ioo@murrow.corp.sgi.com>

As I understand it, a synchronous send is allowed to return only after the
following two things are true:

	1) a matching receive has been posted
	2) the send buffer may safely be modified

So, it may or may not be the case that the data has actually been transfered
to the receiver when the send returns. This is the same thing that happens in
the standard send.

In ready mode, the application programmer is asserting to the MPI library
that the process at the receiving end has already posted the receive for
that message. The idea here is that the MPI library may then be able to
perform some optimizations that otherwise would not be possible, but it
seems pretty dubious to me; I personally believe that ready mode is
essentially useless.

(Actually, while we're on the subject, I happen to feel that way about *all*
of the send calls except for MPI_Isend. But that's a whole other can of
worms...)

Eric Salo         Silicon Graphics Inc.             "Do you know what the
(415)390-2998     2011 N. Shoreline Blvd, 7L-802     last Xon said, just
salo@sgi.com      Mountain View, CA   94043-1389     before he died?"

