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Subject: Re: [OMPI users] MPI one-sided passive synchronization.
From: James Dinan (dinan_at_[hidden])
Date: 2011-04-13 18:16:46


Sudheer,

Locks in MPI don't mean mutexes, they mark the beginning and end of a
passive mode communication epoch. All MPI operations within an epoch
logically occur concurrently and must be non-conflicting. So, what
you're written below is incorrect: the get is not guaranteed to complete
until the call to unlock. Because of this, it conflicts with the
ensuing call to MPI_Accumulate, which is an error.

I don't share your pessimism about MPI-2 RMA asynchronous progress. As
Brian hinted, the standard says you should get progress without making
MPI calls. I think you might be getting tripped up by the poorly named
MPI_Lock/Unlock calls. These aren't like mutexes and can't be used to
ensure exclusive data access for read-modify-write operations (like in
your example). In order to do that, you need an actual mutex, which can
be implemented on top of MPI-2 RMA (I can provide reference if you need
it, I'm sure the code is available somewhere in MPI tests/examples too).

Best,
  ~Jim.

On 04/13/2011 03:11 PM, Abhishek Kulkarni wrote:
> But given the existing behavior, all bets are off and it renders passive
> synchronization
> (MPI_Win_unlock) mostly similar to active synchronization (MPI_Win_fence).
> In trying to emulate a distributed shared memory model, I was hoping to
> do things
> like:
>
> MPI_Win_lock(MPI_LOCK_EXCLUSIVE, 0, 0, win);
> MPI_Get(&out, 1, MPI_INT, 0, 0, 1, MPI_INT, win);
> if (out % 2 == 0)
> out++;
> MPI_Accumulate(&out, 1, MPI_INT, 0, 0, 1, MPI_INT, MPI_REPLACE, win);
> MPI_Win_unlock(0, win);
>
> but it is impossible to implement such atomic sections given no semantic
> guarantees
> on ordering of the RMA calls.