[mpich-devel] Win questions

[Jim Dinan]

Hi Bob,

Active target separates exposure and access epochs.  It is erroneous for a
process to engage in more than one exposure epoch or more than one access
epoch per window.  So it's ok for many processes to expose (post/wait) to
me concurrently, but for a given window I can access only one
(start/complete) at a time.

Your example code looks ok to me.  Are you getting an error message?

 ~Jim.
On Jun 7, 2013 9:11 AM, "Bob Cernohous" <bobc at us.ibm.com> wrote:

> I haven't worked on rma before but was working on a problem and ran into
> this **comment in MPI_Win_post:
>
> "Starts an RMA exposure epoch for the local window associated with win.
> **Only the processes belonging to group should access the window with RMA
> calls on win during this epoch.   Each process in group must issue a
> matching call to MPI_Win_start. MPI_Win_post does not block."
>
> Would overlapping epochs be violating the ** line?  I decided I probably
> need to support this but I wondered if it's bending or breaking the 'rules'?
>
> The problem (code at the bottom of this email) is using a cartesian
> communicator and alternating "left/right' accumulates with 'up/down'
> accumulates on a single win.  So:
>
> - Ranks 0,1,2,3 are doing a left/right accumulate.
> - Ranks 4,5,6,7 are doing a left/right accumulate.
> - ...
>
> and then sometimes...
>
> - Ranks 0,1,2,3 complete and enter the 'up/down' accumulate epoch
> -- Rank 0 does MPI_Win_post to ranks 4,12
> -- Rank 1 doesn MPI_Win_post to ranks 5,13
> ...
>
> So is Rank 0 posting to Rank 4 while 4 is still in the epoch with 5/6/7 a
> violation of "Only the processes belonging to group should access the
> window with RMA calls on win during this epoch"?   From Rank 4's point of
> view, rank 0 isn't in the group for the current win/epoch.
>
> Putting a barrier (or something) in between or using two different win's
> fixes it.  I like using two win's since it separates the epochs and clearly
> doesn't use the wrong group/rank on the win.
>
>     /* RMA transfers in left-right direction */
>     MPI_Win_post(grp_lr, 0, win);
>     MPI_Win_start(grp_lr, 0, win);
>     MPI_Accumulate(&i, 1, MPI_INT, ranks_lr[LEFT] , 0, 1, MPI_INT,
> MPI_SUM, win);
>     MPI_Accumulate(&i, 1, MPI_INT, ranks_lr[RIGHT], 0, 1, MPI_INT,
> MPI_SUM, win);
>     MPI_Win_complete(win);
>     MPI_Win_wait(win);
>
>     /* RMA transfers in up-down direction */
>     MPI_Win_post(grp_ud, 0, win);
>     MPI_Win_start(grp_ud, 0, win);
>     MPI_Accumulate(&i, 1, MPI_INT, ranks_ud[UP]  , 0, 1, MPI_INT, MPI_SUM,
> win);
>     MPI_Accumulate(&i, 1, MPI_INT, ranks_ud[DOWN], 0, 1, MPI_INT, MPI_SUM,
> win);
>     MPI_Win_complete(win);
>     MPI_Win_wait(win);
>
>
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