[Mpi3-rma] RMA WG discussion

[Jim Dinan]

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
>