RE: [mpi-21] Proposal EH2: add const keyword to the C bindings

[Patrick Ohly <patrick.ohly@xxxxxxxxx>]

On Wed, 2008-01-09 at 13:51 -0800, Bronis R. de Supinski wrote:
> Not ONCE have I heard that "Restricted access to buffers
> of non-blocking MPI send operations was the source of my
> problem." Not only do implementations not take advantage
> of the opportunity,

Probably true, but is hard to verify for all existing implementations.
Others have already pointed out that this could change in the future. 

>  the restriction is NOT violated.

It is getting violated, sometimes intentionally to avoid extra memory
copies in the application when custom communicators would be even worse
(Pallas/Intel MPI Benchmarks did it for that reason until very
recently), sometimes unintentionally (applying the automated correctness
checking of the Intel Cluster Tools to HPCC 1.0.0 found such an
instance, see [1]).

>  This
> issue is not worth the time it is being given until someone
> can do more than state that it must be causing problems for
> "many applications and commercial applications." I see no
> reason to sacrifice any potential performance for something
> no one slips on.

My estimate is that quite a few applications will start to fail as soon
as implementations actually use this opportunity. This is not a healthy
state for MPI to be in, but it is less obvious what the right solution
is.

[1] http://www.linuxclustersinstitute.org/conferences/archive/2007/PDF/ohly_19839.pdf
    "Automated MPI Correctness Checking: What If There Were a Magic Option?"

-- 
Best Regards

Patrick Ohly
Senior Software Engineer

Intel GmbH
Software & Solutions Group                
Hermuelheimer Strasse 8a                  Phone: +49-2232-2090-30
50321 Bruehl                              Fax: +49-2232-2090-29
Germany


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