[Mpi3-bwcompat] MPI Backward Compatibility Working Group

[Jeff Squyres]

Thanks Dave; we'll finish this up by Monday so that we can get the PDF out in time for a formal reading.

Have a great vacation!

On Mar 13, 2011, at 1:19 AM, Solt, David George wrote:

> I am on vacation this week, so I need to finish up my involvement as I can't say for sure if I will be on e-mail or not during the week.  
> 
> I checked in the changes I made here:
> 
> https://svn.mpi-forum.org/svn/mpi-forum-docs/trunk/working-groups/mpi-3/bw-compat
> 
> I uploaded new versions of the pdf's to both tickets with the changes we discussed on Friday.   For the issue we are discussing here, I added 
> 
> "and any value that may be stored in an MPI_Aint type."
> 
> but did not add the rationale.  
> 
> Thanks,
> Dave
> 
> -----Original Message-----
> From: mpi3-bwcompat-bounces_at_[hidden] [mailto:mpi3-bwcompat-bounces_at_[hidden]] On Behalf Of Rajeev Thakur
> Sent: Friday, March 11, 2011 11:21 PM
> To: MPI-3 backwards compatability WG
> Subject: Re: [Mpi3-bwcompat] MPI Backward Compatibility Working Group
> 
> Could we say that MPI_Count should be the larger of MPI_Aint and MPI_Offset? (Anything bigger than that is not really useful.) That might help the I/O problem better.
> 
> Rajeev
> 
> 
> On Mar 11, 2011, at 11:07 PM, Fab Tillier wrote:
> 
>> Hi Rajeev,
>> 
>> A data buffer can't be larger than MPI_Aint in memory, but an MPI Datatype can be larger than what can fit in memory, and thus larger than MPI_Aint.  MPI_Count allows such large datatypes to be represented and used in MPI, even if they can't be held in memory all at once.
>> 
>> This proposal is intended to (minimally) solve the I/O problem, not just large network transfers.
>> 
>> Does that make sense?
>> -Fab
>> 
>> -----Original Message-----
>> From: mpi3-bwcompat-bounces_at_[hidden] [mailto:mpi3-bwcompat-bounces_at_[hidden]] On Behalf Of Rajeev Thakur
>> Sent: Friday, March 11, 2011 8:06 PM
>> To: MPI-3 backwards compatability WG
>> Subject: Re: [Mpi3-bwcompat] MPI Backward Compatibility Working Group
>> 
>> What was the motivation for the new MPI_Type_get_extent_x and MPI_Type_get_true_extent_x functions?
>> 
>> If we say MPI_Count should be at least as large as MPI_Aint, why can't we just use MPI_Aint instead? The data buffer can't be larger than Aint size. I know that for I/O MPI_Offset can be larger than Aint, but this proposal does not solve the I/O problem as far as I know.
>> 
>> https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/265
>> 
>> Rajeev
>> 
>> 
>> 
>> On Mar 11, 2011, at 5:06 PM, Solt, David George wrote:
>> 
>>> During our meeting today we decided that MPI_Count should be at least as large as an MPI_Aint.    We already had the restriction that it had to be as large as a C/Fortran integer.   We want both restrictions (they cannot be combined because there is no guarantee that MPI_Aint > integer).
>>> 
>>> I'll throw this out as a starting point for discussion:
>>> 
>>> 
>>> 
>>> 
>>> 2.5.8 Last sentence change to:
>>> 
>>> The size of the MPI_Count type is determined by the MPI implementation 
>>> with the restriction that it must be minimally capable of encoding a C 
>>> int and Fortran INTEGER and any value that may be stored in an MPI_Aint type.
>>> 
>>> Rational thingy:
>>> 
>>> MPI_Count is used to communicate both the number of elements in a datatype and the bounds of a datatype.
>>> The number of elements in a datatype is specified at creation time using a C int or Fortran INTEGER.   The bounds
>>> may be specified using an MPI_Aint. 
>>> 
>>> 
>>> So, I was especially brief here.   I did not want to get into the "new" routines vs. "old routines", but if you attempt to go into more depth than what I said, you will inevitably end up going down that path.  I can think of a thousand ways not to say it, but its hard to come up with a good way.
>>> Dave
>>> 
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-- 
Jeff Squyres
jsquyres_at_[hidden]
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