[Mpi3-rma] RMA WG discussion

[Jeff Hammond]

Are we talking about the same thing?  I am asserting that it is not
required that the size of the input and output buffers - regardless of
how they are represented with datatypes - do not have to be the same
size, but only need to satisfy the requirement that no buffer overrun
occurs on either end.

Are you disagreeing with this or are you saying something else?

Jeff

On Fri, Dec 7, 2012 at 1:29 PM, Jim Dinan <dinan at mcs.anl.gov> wrote:
> This restriction on derived datatypes does not apply to get or put.  It only
> applies to accumulate operations, and is stated in the 3.0 spec in the first
> paragraph of page 426 for accumulate, and again in the second paragraph on
> page 428 for get_accumulate:
>
> "Each datatype argument must be a prede fined datatype or a derived
> datatype, where all basic components are of the same predefined datatype.
> Both datatype arguments must be constructed from the same predefined
> datatype."
>
>  ~Jim.
>
>
> On 12/7/12 1:13 PM, Jeff Hammond wrote:
>>>>>
>>>>> Which count variable defines the number of basic elements to be
>>>>> returned?
>>>>>     (result_count, result_type) or (target_count, target_type) ?
>>>>
>>>>
>>>> result_addr is the buffer that is written at the origin so I believe
>>>> the answer is (result_count, result_type).
>>>
>>>
>>> The number of basic elements needs to be the same across all <count,
>>> dtype>
>>> pairs.  The count and datatype for the result buffer determine where
>>> these
>>> elements will be in the origin process' memory.
>>
>>
>> This is not what the MPI standard says for MPI_GET and MPI_ACCUMULATE,
>> as I previously noted, or MPI_PUT, as noted here.  If we're going to
>> change the semantics in this respect, it needs to be considered
>> carefully, as it will break backwards compatibility with codes that
>> assume the same behavior as MPI_RECV, as is stipulated in 11.3.1
>> (MPI_PUT):
>>
>> "The communication must satisfy the same constraints as for a similar
>> message-passing communication. The target_datatype may not specify
>> overlapping entries in the target buffer. The message sent must fit,
>> without truncation, in the target buffer. Furthermore, the target
>> buffer must fit in the target window or in attached memory in a
>> dynamic window."
>>
>> "Fitting without truncation" is weaker than "needs to be the same".
>>
>> Best,
>>
>> Jeff
>>
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-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond