[Mpi3-bwcompat] Ticket 265 ready for review

[Fab Tillier]

Rajeev Thakur wrote on Fri, 28 Jan 2011 at 08:09:29

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

I believe Quincy will be bringing forth a proposal to address this, but we wanted to get the minimum functionality to provide full support for large datatypes captured in a single ticket without adding convenience features.

-Fab

> 
> 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 Sorry, I've resolved it as withdrawn, with a comment that it was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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>>>>> 
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