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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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" <[email protected]> To: "MPI WG Remote Memory Access working group" <[email protected]> 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 < [email protected] > 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" < [email protected] > To: "MPI WG Remote Memory Access working group" < [email protected] > 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 < [email protected] > 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 [email protected] 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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-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] 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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-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] 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 [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma

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-- Jeff Hammond [email protected] http://jeffhammond.github.io/