[mpich-discuss] MPI RMA

[Jeff Hammond]

>From MPI-3 11.5.1:

- The MPI call MPI_WIN_FENCE(assert, win) synchronizes RMA calls on win.
- The call completes an RMA access epoch if it was preceded by another
fence call and the local process issued RMA communication calls on win
between these two calls.
- The call completes an RMA exposure epoch if it was preceded by
another fence call and the local window was the target of RMA accesses
between these two calls.
- The call starts an RMA access epoch if it is followed by another
fence call and by RMA communication calls issued between these two
fence calls.
- The call starts an exposure epoch if it is followed by another fence
call and the local window is the target of RMA accesses between these
two fence calls.

MPI_Win_fence may both start or complete an RMA epoch but it may also
complete-one-and-start-another RMA epoch depending on the context and
the assertions.  All of the tests below are valid in my opinion.

What the assertions below in the first example are saying is that the
first call to MPI_Win_fence need not complete any RMA calls, which is
true because it is the first sync call.  The last MPI_Win_fence
asserts the same thing in reverse.  The middle one asserts nothing
because it is both completing and starting an epoch.

I think that MPI_Win_fence is poorly designed but that ship has sailed
and I believe that the usage is well-defined in the standard despite
the confusing properties of this function.

Jeff

On Fri, Feb 8, 2013 at 11:36 AM, Nick Radcliffe <nradclif at cray.com> wrote:
> Hi,
>
> I have a question about MPI RMA, and the ANL regression tests for RMA in particular. The tests mixedsync.c and epochtest.c seem to have contradictory views of fence synchronization.
>
> epochtest.c seems to suggest that access/exposure epochs opened by a call to MPI_Win_fence are not closed until a call to MPI_Win_fence with assert==MPI_MODE_NOSUCCEED. The test looks roughly like this:
>
> MPI_Win_fence(MPI_MODE_NOPRECEDE, win)
> if (rank == src) MPI_Put
> MPI_Win_fence(0, win)
> if (rank == dest) MPI_Put
> etc...
> MPI_Win_fence(MPI_MODE_NOSUCCEED, win)
>
> Since there is a call to MPI_Put after the second call to MPI_Win_fence, it would seem that the second call could not have ended the access epoch for dest, or the exposure epoch for src (which is the target of the second Put).
>
> On the other hand, the test mixedsync.c looks roughly like this:
>
> if (rank == src) {
>     MPI_Win_lock(...,win)
>     MPI_Put
>     MPI_Win_unlock(...,win)
> }
>
> MPI_Win_fence(0, win)
> if (rank == src) MPI_Put
> MPI_Win_fence(0, win)
>
> if (rank == src) {
>     MPI_Win_lock(...,win)
>     MPI_Put
>     MPI_Win_unlock(...,win)
> }
>
> The problem is that it is erroneous to call MPI_Win_lock on a window while that window is exposed due to a call to MPI_Win_fence. If mixedsync.c is not erroneous, then the second call to MPI_Win_fence must end the exposure epoch on win, contradicting what's implied about fence synchronization by epochtest.c.
>
> Sorry for the long post, but if anyone can shed some light on this for me, I would greatly appreciate it.
>
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-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond