[Mpi3-rma] RMA WG discussion

[Jeff Hammond]

>>> 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

-- 
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