[MPI3 Fortran] (j3.2006) (SC22WG5.3909) [ukfortran] [MPI3 Fortran] MPI non-blocking transfers

[Bill Long]

Van Snyder wrote:
> On Fri, 2009-01-23 at 01:56 -0800, N.M. Maclaren wrote:
>   
>> In theory, but not in practice.  Not merely is the identifier an integer
>> value (and may be the result of arbitrarily complicated extraction), the
>> WAIT statement may be inside an external procedure.
>>     
>
> That's what the ASYNCHRONOUS attribute for arguments is supposed to
> address.
>
>   

Since a WAIT statement could occur in a subprogram (at least ones the 
compiler does not know about, such as ones not intrinsic or inlined) 
then it is part of the required implementation that operations on a 
variable with the asynchronous attribute not be moved across such a 
call.  Included in the class of calls that block code motion would be 
those to the MPI routines.  Having the asynchronous attribute in the 
caller scope effectively solves the code motion problem.  Having it on 
the corresponding dummy probably solves it as well.


Jim pointed out that asynchronous may not be that useful in the end.  I 
think this is an important point.  As noted above, asynchronous can 
solve the code motion problem.  It is not clear what you mean by "solve" 
in regard to the other main problem - copy-in/copy-out of actual 
arguments.   What the asynchronous attribute will do is cause a program 
that would otherwise have to include copy-in/copy-out code at the call 
to the MPI_Ixxx routine to, instead, get a hard error at compile time 
based on a constraint violation.  It does not make the program "work", 
which is probably somewhere on the user's agenda.  To get a working 
program, there are two possible solutions that I see:

1) Require that the user always supply a contiguous actual argument 
corresponding to the buffer dummy  (in which case it seems to make more 
sense for THAT to be the requirement on the user in the first place), or

2) Provide a Fortran interface than can handle non-contiguous sections 
by having assumed-shape corresponding dummies.  This solution off-loads 
the bulk of the work to the MPI folks. But is does seem more robust.

It would be useful to hear feedback from the MPI group on these 
alternatives.

Cheers,
Bill




-- 
Bill Long                                   longb at cray.com
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