[Mpi3-rma] MPI-3.1 RMA planning

Pavan Balaji balaji at mcs.anl.gov
Thu Jul 26 08:52:27 CDT 2012

On 07/26/2012 05:05 AM, Torsten Hoefler wrote:
> Why would an MPI implementation ever want to return SEPARATE if UNIFIED
> would be supported? And why would a user want this?

An MPI implementation might want to put the public copy in a special 
memory region, such as a symmetric address space or shared memory 
region.  In this case, the public copy is different from the private copy.

For the user's perspective, SEPARATE might be sufficient if (s)he is 
only planning to do PUT/GET operations, for example, in which case 
UNIFIED and SEPARATE might not have much of a semantic difference.

>> I realize that the above recommendation is bad in many ways including:
>> (1) we are changing the semantics of "asserts" here, and (2) this
>> might force local PUT/GET operations to always go through the network
>> as if they are issued by a remote process because of cache coherence
>> issues and thus might lose performance.  But this is just to get some
>> discussion started.
> I'm not sure about this ... but I think this needs some face-to-face time.

I agree.  I only have the problem, but no solution currently.  Some 
group discussion (either telecon or face-to-face) will be good.

> I would like to add some info arguments to the list of things to
> consider. I don't have the full list at this point, but we have the
> "same_size" argument for create and allocate. However, we have no
> "same_displ_unit", which goes by the same rationale. We could also add
> some info arguments to dynamic windows to mitigate some of the
> implementation issues (allow optimized implementations on RMA systems).

Sounds good.

  -- Pavan

Pavan Balaji

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