(j3.2006) [MPI3 Fortran] Feedback from Fortran J3 meeting

[Craig Rasmussen]

This may not be as bad as it seems as VOLATILE can be limited to a  
BLOCK construct (sorry, it's just BLOCK not BEGIN BLOCK as I have  
below).  Since the caller won't reference the memory between MPI_Irecv  
and MPI_Wait, this shouldn't be a problem for VOLATILE on the actual.   
Plus, since the implementation will likely be in C (which can ignore  
the VOLATILE dummy), there shouldn't be a performance hit on the  
callee either.

I was skeptical about this as well, but Aleks reminded me that the  
effects of VOLATILE could be limited to a BLOCK construct.

Cheers,
Craig


On May 27, 2008, at 9:55 AM, Dan Nagle wrote:

> Hi,
>
> I think we blundered with 08-185r1.
>
> Specifically, VOLATILE is exactly the wrong attribute.
> VOLATILE means "always go to memory" for this variable.
> The MPI specification says "don't touch the buf memory" between
> the MPI async send/recv and the MPI wait.  (The other actual args
> may be treated ordinarily.)
>
> What Fortran should say is "between these two subroutine references,
> don't read/write this actual argument" and I don't think we currently
> have a way to say that.  :-(
>
> It's not so much that VOLATILE is too big a hammer,
> it's that a hammer is exactly the wrong tool for this task.
>
> So I think we talked ourselves into a blunder with this one.
> Never design things quickly.  :-)
>
> On May 27, 2008, at 11:43 AM, Craig Rasmussen wrote:
>
>> Two weeks ago I attended the Fortran J3 standards meeting where I
>> discussed with them the issues surrounding new Fortran MPI
>> bindings.  They were very receptive to our needs and instructed me
>> to write a J3 paper in response (attached).  In summary, J3 will try
>> to get changes made in the Fortran standard so that we won't need to
>> use CLOC(buffer) for a void* buffer argument.  J3 still hasn't
>> decided the best way to do this but the likely favorite is a new
>> type, TYPE(*), as an interoperable type with void*.
>>
>> J3 also decided that the way to limit copyin/copyout semantics and
>> code motion in asynchronous MPI calls is to use the volatile
>> attribute on both Fortran actual and dummy arguments.  The
>> performance effects of volatile could be limited with the use of the
>> new F2008 block construct.  For example,
>>
>>
>>   real, dimension(100) :: buffer
>>
>>   BEGIN BLOCK
>>       VOLATILE :: buf
>>       err = MPI_Irecv(buf, ..., req)
>>       .
>>       .
>>       err = MPI_Wait(req, ...)
>>   END BLOCK
>>
>> The interface for MPI_Irecv would have something like
>>
>>   TYPE(*), volatile :: buf
>>
>> Cheers,
>> Craig
>>
>> <08-185r1.txt>
>>
>>
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>
> -- 
> Cheers!
>
> Dan Nagle
>
>
>
>
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