[mpiwg-rma] [EXTERNAL] Re: Synchronization on shared memory windows

[Jeff Hammond]

On Tue, Feb 4, 2014 at 11:39 AM, Rolf Rabenseifner <rabenseifner at hlrs.de> wrote:
> Brian and all,
>
> No wording in the MPI-3.0 tells anything about the compiler problems.
> They are addressed by MPI_F_SYNC_REG (or work around with MPI_Get_address)
> to be sure that the store instruction is issued before I do any
> synchronization calls.

See MPI-3 Section 11.7.4.  This is not new text.  It's been there for years.

> If I must do an additional MPI_WIN_SYNC, when must it be done?

Whenever you might have a race condition due to two threads or
processes accessing the same memory via:
- RMA + load/store
- load/store on shared memory windows
- RMA + non-RMA

This may not be an inclusive list.

Jeff

> My example is simple:
> X is part of a shared memory window and should mean the same
> memory location in both processes
>
> Process A         Process B
>
> x=13
> MPI_F_SYNC_REG(X)
> MPI_Barrier       MPI_Barrier
>                   MPI_F_SYNC_REG(X)
>                   print X
>
> Where exactly do I need in which process an additional MPI_WIN_SYNC?
> Which wording in the MPI-3.0 does tell this need?
>
> Best regards
> Rolf
>
>
>
> ----- Original Message -----
>> From: "Brian W Barrett" <bwbarre at sandia.gov>
>> To: "MPI WG Remote Memory Access working group" <mpiwg-rma at lists.mpi-forum.org>
>> Cc: "Stefan Andersson" <stefan at cray.com>, "Bill Long" <longb at cray.com>
>> Sent: Tuesday, February 4, 2014 6:06:02 PM
>> Subject: Re: [mpiwg-rma] [EXTERNAL] Re: Synchronization on shared memory windows
>>
>> On 2/4/14 9:59 AM, "Rolf Rabenseifner" <rabenseifner at hlrs.de> wrote:
>>
>> >Jeff,
>> >
>> >thank you for the MPI_FREE_MEM hint. Yes, I'll fix it in my
>> >examples.
>> >
>> >About the synchronization problems:
>> >If I use shared memory windows and direct remote load and store
>> >instead of the RMA functions PUT or GET,
>> >is it then correct, when I never use MPI_WIN-synchronization
>> >routines?
>> >
>> >I would expect this, because in the unified RMA modell,
>> >the load and store accesses to the neighbors memory are done
>> >directly and MPI is not involved.
>> >Because of the unified RMA modell (must be for shared memory
>> >windows),
>> >there should be no need for cache flush routines.
>> >Correct?
>>
>> No.  Maybe.  Sometimes.
>>
>> Basically, unified is saying that the memory has the same eventually
>> completeness that users have come to expect with shared memory
>> programming.  But what users have come to expect depends on the
>> architecture and the programming language.  So in some processor /
>> language / compiler combinations, you might not need any
>> synchronization.
>> In other combinations, you might need compiler or processor memory
>> barriers in your code.  MPI guarantees, however, that MPI_WIN_SYNC
>> will
>> implement whatever compiler / processor memory barriers you need, so
>> it
>> can be used as the portable hammer in your toolchest of shared memory
>> programming.
>>
>> If you're only using load/stores in shared memory windows, you should
>> not
>> need the other synchronization routines (FLUSH, etc.).
>>
>> Brian
>>
>>
>> --
>>   Brian W. Barrett
>>   Scalable System Software Group
>>   Sandia National Laboratories
>>
>>
>>
>>
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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)
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-- 
Jeff Hammond
jeff.science at gmail.com