Newsgroups: comp.parallel.mpi
From: Steve Barnard <steve@megafauna.com>
Reply-To: steve@megafauna.com
Subject: Re: MPI Non blocking Sends, send buffer not readable or unclear standard?
Organization: megafauna
Date: Sat, 25 May 1996 16:54:00 -0800
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Message-ID: <31A7ABA8.4E1D@megafauna.com>

Eric Salo wrote:
> 
> The standard is correct as written, strictly speaking you are not allowed
> to even read the contents of your own send buffer until the send operation
> completes. For blocking sends, of course, this is not an issue. If you want
> to send the same data to multiple processes at once, use MPI_Bcast().
> 
> This restriction was put in to allow implementors more flexibility to use
> (for example) special hardware with dubious cache-coherency properties. On
> the other hand, I am personally unaware of any specific implementations for
> which reading the send buffer early would cause erroneous behavior.
> 
> Eric Salo         Silicon Graphics Inc.             "Do you know what the
> (415)933-2998     2011 N. Shoreline Blvd, 7L-802     last Xon said, just
> salo@sgi.com      Mountain View, CA   94043-1389     before he died?"

Eric,

If this is the case, it was a very poor decision by the standards 
committee.  I'll back this up with two points:

- The fact that there are no implementations that take advantage of 
this "loophole" (that either you or I am aware of) amounts to a de 
facto case that it isn't necessary.

- I am personally aware of at least one application that almost 
*requires* read-only access for efficiency.  I can give details if 
necessary.

	Steve Barnard

