(j3.2006) (SC22WG5.3658) N1755: Request for new features from MPI Forum

[Aleksandar Donev]

N.M. Maclaren wrote:

>> We should explicitly allow a variable with the ASYNCHRONOUS attribute to 
>> be modified or examined by means external to the processor, similarly to 
>> VOLATILE variables. If such a variable is modified or examined externally 
>> during a segment, that variable must not be referenced or define during 
>> that segment.
> 
> That will introduce performance implications, though perhaps less
> serious. 
Can you be more specific please?

> But I really don't like the idea of having to teach kiddies about
> coarrays when they want to learn MPI.
You don't have to. Segments make sense even without co-arrays, as a way 
to control optimizations (code motion) in order to facilitate 
interaction with "things" not visible to the compiler.

> What is actually needed is a way of allowing user code to set and clear
> a variable's pending state, in the sense used in 9.6.4.  While it would
> be a bit tricky to add to Fortran, it would be semantically clean, and
> be as efficient as possible.
Sure. But, I can bet that others will have a different feeling about 
what is "clean" and "efficient". This seems to be the problem here---we 
are not likely to make any progress...and the issue will remain. 
Imperfect is better than non-existent, IMO.

Best,
Aleks

-- 
Aleksandar Donev, Ph.D.
Lawrence Postdoctoral Fellow @ LLNL
High Performance Computational Materials Science and Chemistry
E-mail: donev1 at llnl.gov
Phone: (925) 424-6816  Fax: (925) 423-0785
Address: P.O.Box 808, L-367, Livermore, CA 94551-9900
Web: http://cherrypit.princeton.edu/donev