[mpiwg-rma] [EXTERNAL] Re: same_op_no_op and SHMEM

[Jeff Hammond]

Honestly, I think REPLACE+NO_OP is a useful option, I just think
REPLACE+NO_OP+<SUM or ...> is _also_ a useful option.  Why don't we
just turn our frowns upside down and add this to the standard?

Does anyone object to info = same_op_no_op_replace?

I would appreciate any feedback on my type-homogeneity suggestion as
well.  Do people agree that is worth adding?  I imagine that there is
hardware that can do e.g. sum+long but not sum+double, especially if
it has to be atomic.

Jeff

On Thu, Oct 24, 2013 at 1:46 PM, Barrett, Brian W <bwbarre at sandia.gov> wrote:
> Or "I will only do gets and compare-and-swap", which is something a couple
> of graph codes I've looked at do.  But I agree, we probably should have
> made it REPLACE, NO_OP, and 1 other op or not added it at all.  Sigh :).
>
> Brian
>
> On 10/24/13 11:39 AM, "Jeff Hammond" <jeff.science at gmail.com> wrote:
>
>>I read same_op_no_op as "I will use only MPI_REPLACE and MPI_NO_OP"
>>i.e. give me nothing more than atomic Put/Get, I do not want to
>>actually accumulate anything.
>>
>>Jeff
>>
>>On Thu, Oct 24, 2013 at 12:34 PM, Underwood, Keith D
>><keith.d.underwood at intel.com> wrote:
>>> Yes, that's the motivation, but I'm not sure if anybody does atomics
>>>without puts....  It seems to me like we should have included
>>>MPI_REPLACE in that list
>>>
>>>> -----Original Message-----
>>>> From: mpiwg-rma [mailto:mpiwg-rma-bounces at lists.mpi-forum.org] On
>>>> Behalf Of Pavan Balaji
>>>> Sent: Thursday, October 24, 2013 1:33 PM
>>>> To: MPI WG Remote Memory Access working group
>>>> Subject: Re: [mpiwg-rma] same_op_no_op and SHMEM
>>>>
>>>>
>>>> The motivation was that it's hard to maintain atomicity when different
>>>> operations are done.  For example, if the hardware only supports some
>>>> atomic operations, but not all, this might result in some operations
>>>> happening in hardware and some in software making atomicity hard.  In
>>>>such
>>>> cases, the MPI implementation might need to fall back to software-only
>>>> implementations.
>>>>
>>>>   -- Pavan
>>>>
>>>> On Oct 24, 2013, at 12:23 PM, Jeff Hammond <jeff.science at gmail.com>
>>>> wrote:
>>>>
>>>> > I recall that Brian and/or Keith wanted same_op_no_op because of
>>>> > SHMEM.  However, SHMEM requires the use of MPI_NO_OP (for atomic
>>>> Get
>>>> > via Get_accumulate), MPI_REPLACE (for atomic Put via Accumulate) and
>>>> > MPI_SUM (for add, fadd, inc and finc).  So what is the benefit of
>>>> > same_op_no_op to SHMEM?  Perhaps I remember completely wrong and
>>>> the
>>>> > motivation was something that does not use the latter atomics.  Or
>>>> > perhaps it is common for SHMEM codes to not use these and thus the
>>>> > assumption is MPI_SUM can be ignored.
>>>> >
>>>> > Jeff
>>>> >
>>>> > --
>>>> > Jeff Hammond
>>>> > jeff.science at gmail.com
>>>> > _______________________________________________
>>>> > mpiwg-rma mailing list
>>>> > mpiwg-rma at lists.mpi-forum.org
>>>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
>>>>
>>>> --
>>>> Pavan Balaji
>>>> http://www.mcs.anl.gov/~balaji
>>>>
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>>
>>
>>--
>>Jeff Hammond
>>jeff.science at gmail.com
>>_______________________________________________
>>mpiwg-rma mailing list
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>>http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
>>
>
>
> --
>   Brian W. Barrett
>   Scalable System Software Group
>   Sandia National Laboratories
>
>
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com