[Mpi3-bwcompat] MPI Backward Compatibility Working Group

[Fab Tillier]

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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