[mpiwg-rma] Problems with RMA synchronization in combination with load/store shared memory accesses

Sat May 31 15:31:50 CDT 2014

I think before ticket 429 (https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/429) is put up for a vote as errata, the RMA working group needs to decide whether remote loads/stores to shared memory windows are treated as local loads and stores or as put/get operations (for the purpose of the assert definitions). The text will be different depending on that. 

If remote loads/stores to shared memory windows are considered as local loads/stores they will be covered under MPI_MODE_NOSTORE; if considered as put/get operations, they will be covered under MPI_MODE_NOPRECEDE, MPI_MODE_NOSUCCEED, and MPI_MODE_NOPUT.

Ticket 429 says they should be considered as local loads/stores.

Rajeev

On May 27, 2014, at 1:25 PM, Jim Dinan <james.dinan at gmail.com> wrote:

> Hi Rolf,
> 
> MPI_MODE_NOSTORE applies to local updates that should be made visible to other processes following the end of the access epoch.  I believe that visibility of updates made by other processes were intended to be incorporated into the NOPRECEDE/NOSUCCEED assertions.  I think that Hubert's proposal may be the right approach -- that remote load/store accesses to the shared memory window should be treated as "RMA" (e.g. analogous to get/put) operations.
> 
>  ~Jim.
> 
> 
> On Mon, May 19, 2014 at 1:16 PM, Rolf Rabenseifner <rabenseifner at hlrs.de> wrote:
> Jim and RMA WG,
> 
> There are now two questions:
> 
> Jim asked:
> > Question to WG: Do we need to update the fence assertions to better
> > define interaction with local load/store accesses and remote stores?
> >
> 
> Rolf asked:
> > Additionally, I would recommend that we add after MPI-3.0 p451:33
> >
> >   Note that in shared memory windows (allocated with
> >   MPI_WIN_ALLOCATE_SHARED), there is no difference
> >   between remote store accesses and local store accesses
> >   to the window.
> >
> > This would help to understand that "the local window
> > was not updated by stores" does not mean "by local stores",
> > see p452:1 and p452:9.
> 
> For me, it is important to understand the meaning of the
> current assertions if they are used in a shared memory window.
> Therefore my proposal above as erratum to MPI-3.0.
> 
> In MPI-3.1 and 4.0, you may want to add additional assertions.
> 
> Your analysis below, will also show that mpich implements
> Post-Start-Complete-Wait synchronization in a wrong way,
> if there are no calls to RMA routines.
> 
> Best regards
> Rolf
> 
> ----- Original Message -----
> > From: "Jim Dinan" <james.dinan at gmail.com>
> > To: "MPI WG Remote Memory Access working group" <mpiwg-rma at lists.mpi-forum.org>
> > Sent: Thursday, May 15, 2014 4:06:08 PM
> > Subject: Re: [mpiwg-rma] Problems with RMA synchronization in combination with load/store shared memory accesses
> >
> >
> >
> > Rolf,
> >
> >
> > Here is an attempt to simplify your example for discussion.  Given a
> > shared memory window, shr_mem_win, with buffer, shr_mem_buf:
> >
> > MPI_Win_fence(MPI_MODE_NOSTORE | MPI_MODE_NOPUT | MPI_MODE_NOPRECEDE
> > | MPI_MODE_NOSUCCEED, shr_mem_win );
> >
> > shr_mem_buf[...] = ...;
> >
> > MPI_Win_fence(MPI_MODE_NOPUT | MPI_MODE_NOPRECEDE |
> > MPI_MODE_NOSUCCEED, shr_mem_win);
> >
> >
> > Right now, Fence assertions don't say anything special about shared
> > memory windows:
> >
> >
> > Inline image 1
> >
> >
> > NOPRECEDE/SUCCEED are defined in terms of MPI RMA function calls, and
> > do not cover load/store.  Thus, Rolf's usage appears to be correct
> > per the current text.  In the MPICH fence implementation,
> > src/mpid/ch3/src/ch3u_rma_sync.c:935 we have:
> >
> > if (!(assert & MPI_MODE_NOSUCCEED)) win_ptr->fence_issued = 1;
> >
> > Because of this check, we don't actually start an active target epoch
> > on the first fence in the example above.  On the second fence, we
> > therefore don't perform the necessary synchronization, leading to
> > incorrect output in Rolf's example.
> >
> >
> > Question to WG: Do we need to update the fence assertions to better
> > define interaction with local load/store accesses and remote stores?
> >
> >
> > If not, then Rolf's code is correct and we need to modify the check
> > above in MPICH to something like:
> >
> >
> > if (!(assert & MPI_MODE_NOSUCCEED) || win_ptr->create_ flavor  ==
> > MPI_WIN_FLAVOR_SHARED )
> >   win_ptr->fence_issued = 1;
> >
> >
> >  ~Jim.
> >
> >
> >
> >
> > On Tue, Apr 8, 2014 at 12:02 PM, Rolf Rabenseifner <
> > rabenseifner at hlrs.de > wrote:
> >
> >
> > Jim,
> >
> > I'm now sure, that mpich has a bug with assertions on shared memory
> > windows.
> >
> > in the example, rcv_buf_left and rcv_buf_right are the windows.
> > the only accesses to these rcv_buf_... are stores from remote
> > and fully local loads.
> > Both accesses are done in different epochs surrounded by
> > MPI_Win_fence.
> >
> > According to your interpretation (which is really okay)
> > all fences can use all possible assertions (!!!),
> > except after the remote stores, MPI_MODE_NOSTORE cannot be used.
> >
> > I updated the example and mpich is executing it wrong.
> >
> > Please check it yourself on your installation:
> > halo_1sided_store_win_alloc_shared_w-a-2-cray.c
> >
> > Without the assertions, all works:
> > halo_1sided_store_win_alloc_shared_w-a-2NO-cray.c
> >
> > Could you verify that mpich has a bug?
> >
> > Additionally, I would recommend that we add after MPI-3.0 p451:33
> >
> >   Note that in shared memory windows (allocated with
> >   MPI_WIN_ALLOCATE_SHARED), there is no difference
> >   between remote store accesses and local store accesses
> >   to the window.
> >
> > This would help to understand that "the local window
> > was not updated by stores" does not mean "by local stores",
> > see p452:1 and p452:9.
> >
> > Is it a good idea?
> >
> > Best regards
> > Rolf
> >
> >
> >
> > ----- Original Message -----
> > > From: "Jim Dinan" < james.dinan at gmail.com >
> > > To: "MPI WG Remote Memory Access working group" <
> > > mpiwg-rma at lists.mpi-forum.org >
> > > Sent: Friday, March 21, 2014 8:14:22 PM
> > > Subject: Re: [mpiwg-rma] Problems with RMA synchronization in
> > > combination with load/store shared memory accesses
> > >
> > >
> > >
> >
> > > Rolf,
> > >
> > >
> > > This line is incorrect: MPI_Win_fence(MPI_MODE_NOSTORE +
> > > MPI_MODE_NOPRECEDE, win_ rcv_buf _left );
> >
> >
> > >
> > >
> > > You need to do a bitwise OR of the assertions (MPI_MODE_NOSTORE |
> > > MPI_MODE_NOPRECEDE).
> > >
> > > In halo_1sided_store_win_alloc_shared.c, you are doing stores
> > > within
> > > the epoch, so MPI_MODE_NOSTORE looks like an incorrect assertion on
> > > the closing fence.
> > >
> > > Following the Fence epoch, you are reading from the left/right recv
> > > buffers.  That also needs to be done within an RMA epoch, if you
> > > are
> > > reading non-local data.
> > >
> > >  ~Jim.
> > >
> > >
> > >
> > > On Fri, Feb 21, 2014 at 6:07 AM, Rolf Rabenseifner <
> > > rabenseifner at hlrs.de > wrote:
> > >
> > >
> > > Dear member of the RMA group and especially the mpich developers,
> > >
> > > I have real problems with the new shared memory in MPI-3.0,
> > > i.e., the load/stores together with the RMA synchronization
> > > causes wrong execution results.
> > >
> > > The attached
> > >     1sided_halo_C_mpich_problems_rabenseifner.tar.gz or .zip
> > > contains
> > >
> > > - 1sided/halo_1sided_put_win_alloc.c
> > >
> > >   The basis that works. It uses MPI_Put and MPI_Win_fence for
> > >   duplex left/right halo communication.
> > >
> > > - 1sided/halo_1sided_store_win_alloc_shared.c
> > >
> > >    This is the same, but a shared memory window is used and
> > >    the MPU_Put is substituted by storing the data in the
> > >    neighbors window. Same MPI_Win_fence with same assertions.
> > >
> > >    This does not work, although I'm sure that my assertions are
> > > correct.
> > >
> > >    Known possibilities:
> > >    - I'm wrong and was not able to understand the assertions
> > >      on MPI-3.0 p452:8-19.
> > >    - I'm wrong because it is invalid to use the MPI_Win_fence
> > >      together with the shared memory windows.
> > >    - mpich has a bug.
> > >    (The first two possibilities are the reason, why I use this
> > >     Forum email list)
> > >
> > > - 1sided/halo_1sided_store_win_alloc_shared_w-a-cray.c
> > >
> > >    This is a work-around-for Cray that works on our Cray
> > >    and does not use MPI_MODE_NOPRECEDE and MPI_MODE_NOSUCCEED.
> > >    It also runs on another mpich installation.
> > >
> > > - 1sided/halo_1sided_store_win_alloc_shared_pscw.c
> > >
> > >    Here, MPI_Win_fence is substituted by Post-Start-Complete-Wait
> > >    and it does not work for any assertions.
> > >
> > >    Same possibilities as above.
> > >
> > > - 1sided/halo_1sided_store_win_alloc_shared_query.c
> > > - 1sided/halo_1sided_store_win_alloc_shared_query_w-a-cray.c
> > >
> > >    Same as halo_1sided_store_win_alloc_shared.c
> > >    but non-contigues windows are used.
> > >    Same problems as above.
> > >
> > > - 1sided/halo_1sided_store_win_alloc_shared_othersync.c
> > >
> > >    This version uses the synchronization according to
> > >    #413 and it is tested and works on two platforms.
> > >
> > > Best regards
> > > Rolf
> > >
> > > --
> > > Dr. Rolf Rabenseifner . . . . . . . . . .. email
> > > rabenseifner at hlrs.de
> > > High Performance Computing Center (HLRS) . phone
> > > ++49(0)711/685-65530
> > > University of Stuttgart . . . . . . . . .. fax ++49(0)711 /
> > > 685-65832
> > > Head of Dpmt Parallel Computing . . .
> > > www.hlrs.de/people/rabenseifner
> > > Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room
> > > 1.307)
> > >
> > > _______________________________________________
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> > > mpiwg-rma at lists.mpi-forum.org
> > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
> > >
> > >
> > > _______________________________________________
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> > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
> >
> > --
> > Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
> > High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
> > University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
> > Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
> > Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)
> >
> > _______________________________________________
> > mpiwg-rma mailing list
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> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
> >
> >
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> 
> --
> Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
> High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
> University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
> Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
> Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)
> _______________________________________________
> mpiwg-rma mailing list
> mpiwg-rma at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
> 
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