[mpiwg-rma] [EXTERNAL] shared-like access within a node with non-shared windows

[Barrett, Brian W]

On 10/15/13 11:24 AM, "Jeff Hammond" <jeff.science at gmail.com> wrote:

>>>>>The non-scalable use of this feature would be to loop over all ranks
>>>>>in the group associated with the window and test for baseptr!=NULL,
>>>>>while the scalable use would presumably utilize MPI_Comm_split_type,
>>>>>MPI_Comm_group and MPI_Group_translate_ranks to determine the list of
>>>>>ranks corresponding to the node, hence the ones that might permit
>>>>>direct access.
>>>>
>>>> This brings up another questionŠ  0 is already a valid size.  What do
>>>>we
>>>> do with FORTRAN for your proposed case?
>>>
>>>I don't see what size has to do with this, but Pavan also pointed out
>>>that Fortran is a problem.  Thus, my second suggestion would become a
>>>requirement for usage, i.e. the user is only permitted to use
>>>win_shared_query on ranks in the communicator returned by
>>>MPI_Comm_split_type(type=SHARED).
>>
>> If size=0 wasn't valid, you could use that as a token to say there's
>> nothing useful at this process.  But size=0 is valid, so we need another
>> token.  And, just to be clear, Fortran sucks.
>
>Okay, I'll leave it up to e.g. Bill to decide what sort of magic value
>- new or old - is appropriate for this.  I really do not want to add a
>new function that adds a flag to indicate "yes, you can use this
>pointer".

Agreed.  I'm not sure if we could use MPI_BOTTOM as the magic token or
not; Fortan and pointers always hurts my brain :).

Brian

--
  Brian W. Barrett
  Scalable System Software Group
  Sandia National Laboratories