[Mpi3-bwcompat] Ticket 265 ready for review

[Fab Tillier]

Hi Rajeev,

Rajeev Thakur wrote on Wed, 26 Jan 2011 at 14:31:06

> I wasn't at the last Forum meeting, so may have missed some of the
> background.
> 
> Is ticket #224 obsolete now? If so, you may want to indicate that in 224.

Sorry, I've resolved it as withdrawn, with a comment that it was supersceded by 265.

> Do MPI_Send/Recv etc. remain unchanged, i.e., no MPI_Count in them? If
> so, why do we need a new MPI_Get_count?

MPI_Send/Recv remain unchanged, and users are expected to create derived datatypes to express data structures that are larger than 2^31 basic elements.  Now that you point it out, though, I would think MPI_Get_elements is sufficient, as MPI_Get_count should be using the same datatype as used in the operation that transferred the data.  Hoping that Jeff, Quincy, or David can chime in here and clarify why we need it.

> Are the new "w" versions of the collectives specifically related to
> this ticket (i.e. to address the 64-bit count requirement) or are they
> a separate issue (i.e. a general need for array of datatypes instead of
> one datatype)?

By addressing the need for large counts via derived datatypes, we effectively encapsulate the 'count' in the 'datatype' parameters.  As an example, if you want to gather different 'counts' from different ranks where there is no common denominator, you would need to derive a datatype for each source rank, and specify those individual datatypes.  That can't be done today, we can only specify different counts, but are limited by the 2^31 range of the count fields.  So the missing 'w' functions allow the datatype to be used to encapsulate the count.

> Minor typos: Two of the prototypes for Scatterw say Scatterv instead.

Fixed, thanks.

-Fab

> 
> Rajeev
> 
> 
> 
> 
> On Jan 26, 2011, at 3:38 PM, Fab Tillier wrote:
> 
>> Ok folks, not a whole lot of time before the meeting, so it would be
> great if we could get everyone to read through the ticket and make sure
> I didn't miss something.  I'd like to have David Solt generate a PDF
> sometime next week, in time for me to read it at the forum the
> following week (our time slot for this is 'working lunch' on Tuesday.
>> 
>> https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/265
>> 
>> Thanks for your help,
>> -Fab
>> 
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