[mpi3-coll] Nonblocking collectives standard draft

[Torsten Hoefler]

On Mon, Nov 24, 2008 at 09:34:09PM -0800, Greg Bronevetsky wrote:
> 
> >- another argument _for_ MPI_REQUEST_FREE is the existence of 
> >intercommunicators where all processes (i.e., in group A and in 
> >group B) have to call the collective but only part of them might be 
> >involved (e.g., for MPI_IBCAST all but the root process in group A 
> >don't really participate in the operation - they pass MPI_PROC_NULL 
> >as root argument). So having a lightweight MPI_Wait alternative 
> >might not be too bad ...
> 
> I would argue against MPI_Request_free in general for both 
> point-to-point and collective operations. For one thing, if there is 
> an error during the operation, MPI has no logical place to deliver 
> this notification. Another reason is that on single-threaded 
> architectures (BG/L CNK, Cray Catamount) it is completely possible 
> for the non-blocking to not complete until MPI_Finalize because the 
> application is not required to issue any more MPI calls during which 
> progress may be made on the communication. At least if we have no 
> MPI_Request_free the user is eventually required to call MPI_Test or 
> MPI_Wait to ensure progress. I can go on but MPI_Request_free is bad.
yes, we should probably bring this up again in the discussion of
MPI-2.2. Let's talk about this on the telecon (I just put it on the
agenda).

> Having said that, I object to the text that explains the reasoning 
> for removing MPI_Request_free on page 50 for exactly the reason 
> above. We shouldn't have MPI_Request_free because its generally bad.
yes, I agree. I just wanted to stay consistent with the current standard
with the text, but I don't like it either. I think we can't say that
Request_free is generally bad, thu I chose this phrasing. However, I'm
very open to changes. Can you propose a better wording?

Thanks,
  Torsten

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Torsten Hoefler       | Postdoctoral Researcher
Open Systems Lab      | Indiana University    
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