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

Jeff Hammond jeff.science at gmail.com
Sat May 31 15:44:53 CDT 2014 

Remote load-store cannot be treated like local load-store from a
sequential consistency perspective.  If a process does local
load-store, it is likely that no memory barrier will be required to
see a consistent view of memory.  When another process does
load-store, this changes dramatically.

Jeff

On Sat, May 31, 2014 at 3:31 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
> 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)
>> > >
>> > > _______________________________________________
>> > > mpiwg-rma mailing list
>> > > mpiwg-rma at lists.mpi-forum.org
>> > > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
>> > >
>> > >
>> > > _______________________________________________
>> > > mpiwg-rma mailing list
>> > > mpiwg-rma at lists.mpi-forum.org
>> > > 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
>> > mpiwg-rma at lists.mpi-forum.org
>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
>> >
>> >
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>> > mpiwg-rma at lists.mpi-forum.org
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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
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>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/

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