[Mpi3-rma] Memory barriers in MPI_WIN_LOCK_ALL mode

[Pavan Balaji]

Keith pointed out offline that I forgot a FLUSH after the PUT on P0. 
Please do consider that added for your comments.

  -- Pavan

On 10/29/2012 11:00 PM, Pavan Balaji wrote:
>
> It's undefined without additional synchronization.  I'm doing additional
> synchronization in the example below.
>
>   -- Pavan
>
> On 10/29/2012 10:59 PM, Underwood, Keith D wrote:
>> Doesn't that specific usage have undefined results?  (put/get
>> targeting the same location during one epoch?)
>>
>>> -----Original Message-----
>>> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
>>> bounces at lists.mpi-forum.org] On Behalf Of Pavan Balaji
>>> Sent: Monday, October 29, 2012 11:39 PM
>>> To: mpi3-rma at lists.mpi-forum.org
>>> Subject: [Mpi3-rma] Memory barriers in MPI_WIN_LOCK_ALL mode
>>>
>>>
>>> Consider the following program:
>>>
>>> P0:
>>>     MPI_WIN_LOCK_ALL();
>>>     MPI_PUT(1, X, P1); /* Write 1 to variable X on P1 */
>>>     /* wave hand to P1 */
>>>
>>> P1:
>>>     MPI_WIN_LOCK_ALL();
>>>     /* wait for P0 to wave hand */
>>>     MPI_GET(X, P1);  /* local operation */
>>>
>>> If the MPI_GET on P1 does a local load operation internally, it is
>>> not memory
>>> consistent without a memory barrier.  Does this mean that I need to
>>> always
>>> do a memory barrier on all local GET operations to get the right value?
>>>
>>> Note that this inefficiency does not go away by replacing the waving
>>> of the
>>> hand with an MPI_BARRIER for example, as it does not know which window,
>>> if any, the synchronization is for.
>>>
>>> Note that this is likely only a theoretical exercise, since most
>>> (all?) compilers
>>> will do a memory barrier anyway if they see a function call (MPI_GET
>>> in this
>>> case).  But is MPI assuming that that's going to be the case for
>>> efficient
>>> execution?
>>>
>>>    -- Pavan
>>>
>>> --
>>> Pavan Balaji
>>> http://www.mcs.anl.gov/~balaji
>>> _______________________________________________
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>>> mpi3-rma at lists.mpi-forum.org
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>>
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>

-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji