[Mpi3-rma] MPI-3 UNIFIED model clarification

[Pavan Balaji]

Sayantan,

Yes, doing a remote memory barrier using an active message is an option. 
  But I think the point is that any of these approaches adds more 
unnecessary overhead.

Separating into three memory models provides better flexibility to 
implementations (and to applications, once we allow them to ask for the 
"required" value through an info argument).

  -- Pavan

On 07/29/2013 12:00 PM, Sur, Sayantan wrote:
> Hi Pavan,
>
>> Specifically, the concern was that some members of the WG believed that in
>> the UNIFIED model, data is usable by the remote process after a PUT without
>> an additional WIN_SYNC, while some members believed that it is not.  Here's
>> the example in question:
>>
>> P0:
>> Win_lock_all
>> Put(a, P1)
>> Flush
>> MPI_Send(P1)
>>
>> P1:
>> Win_lock_all
>> MPI_Recv(P0)
>> read a
>>
>> The question was whether the above program was valid without a
>> WIN_SYNC on P1 between the Recv(P0) and "read a".  If we want this to be
>> valid in the UNIFIED model, only x86-like architectures can provide UNIFIED
>> efficiently.  Other architectures, such as PPC or ARM, that require an
>> additional read barrier on P1 will not be able to provide UNIFIED even if they
>> are cache-coherent, unless they add a memory barrier in every other MPI call
>> (e.g., MPI_Recv in this case).
>>
>
> Adding a memory barrier in MPI_Recv is one of the implementation options, and probably not the best one. For relaxed memory architectures, one may want to shift the burden onto Flush to do a memory barrier after the data has been written (through an active message for example).
>
> The description of Flush in the spec is: "MPI_WIN_FLUSH completes all outstanding RMA operations initiated by the calling process to the target rank on the specified window. The operations are completed both at the origin and at the target."
>
> The way I read it, no further action should be required to view contents of the memory attached to the window after MPI_Win_flush. Therefore, an implementation of MPI_Win_flush needs to do whatever is required by the underlying platform and the model the MPI is supposed to provide.
>
>> One possible solution we discussed was to clarify that this is not allowed in
>> UNIFIED, but provide a third memory model called UBER_SUPER_UNIFIED,
>> that will allow this.  (or say that it is allowed in UNIFIED and provide a third
>> model called KIND_OF_UNIFIED, which is in between UNIFIED and
>> SEPARATE).
>>
>> Other solutions are welcome.
>>
>> Irrespective of when we make the change of possibly adding an additional
>> memory model (MPI-3.1, MPI-4, whatever), we should clarify the standard
>> on what is allowed in MPI-3 and what is not, as an errata item.  Without that,
>> it's confusing for implementors.
>>
>>    -- Pavan
>>
>> --
>> Pavan Balaji
>> http://www.mcs.anl.gov/~balaji
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>
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-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji