[Mpi3-bwcompat] Ticket 265 ready for review

[Rajeev Thakur]

Yes, it is not absolutely required, but becomes a convenience feature.

Rajeev

On Jan 27, 2011, at 11:33 AM, Fab Tillier wrote:

> Hi Rajeev,
> 
> Rajeev Thakur wrote on Wed, 26 Jan 2011 at 21:59:33
> 
>> OK, thanks for the explanation.
>> 
>> If count is encapsulated in derived datatypes, we might need new
>> datatype constructor functions that take MPI_Count, or at least a new
>> MPI_Type_contiguous. Let's say the user wants to send an array of size
>> X integers, where X is some weird number greater than 2G. If there is a
>> new Type_contiguous, we have to see how it affects Type_get_envelope
>> and Type_get_contents.
> 
> We shouldn't need new datatype creator functions for this to work - a user can nest types, for example by creating a struct type of contiguous types to achieve the length desired.  In this case, the MPI_Type_get_envelope/contents would still work as currently defined.
> 
> Does that make sense?
> 
> Do we want to capture this discussion as comments on the ticket?
> 
> Thanks for the feedback!
> -Fab
> 
>> Rajeev
>> 
>> 
>> On Jan 26, 2011, at 4:56 PM, Fab Tillier wrote:
>> 
>>> 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 Sorry, I've resolved it as withdrawn, with a comment that it was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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