[Mpi3-bwcompat] Ticket 265 ready for review

[Fab Tillier]

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 Sorry, I've resolved it as withdrawn, with a comment that it was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