[mpiwg-rma] same_op_no_op

[Jeff Hammond]

It is extremely difficult to see that this is what the MPI-3 standard says.

First we have this:

"The outcome of concurrent accumulate operations to the same location
with the same predefined datatype is as if the accumulates were done
at that location in some serial order. Additional restrictions on the
operation apply; see the info key accumulate_ops in Section 11.2.1.
Concurrent accumulate operations with different origin and target
pairs are not ordered. Thus, there is no guarantee that the entire
call to an accumulate operation is executed atomically. The effect of
this lack of atomicity is limited: The previous correctness conditions
imply that a location updated by a call to an accumulate operation
cannot be accessed by a load or an RMA call other than accumulate
until the accumulate operation has completed (at the target).
Different interleavings can lead to different results only to the
extent that computer arithmetics are not truly associative or
commutative. The outcome of accumulate operations with overlapping
types of different sizes or target displacements is undefined."
[11.7.1 Atomicity]

Then we have this:

"accumulate_ops — if set to same_op, the implementation will assume
that all concurrent accumulate calls to the same target address will
use the same operation. If set to same_op_no_op, then the
implementation will assume that all concurrent accumulate calls to the
same target address will use the same operation or MPI_NO_OP. This can
eliminate the need to protect access for certain operation types where
the hardware can guarantee atomicity. The default is same_op_no_op."
[11.2.1 Window Creation]

I was not aware that the definition of info keys was normative, given
that implementations are free to ignore them.  Even if info key text
is normative, one has to infer from the fact that same_op_no_op is the
default info behavior - and thus RMA semantic - that accumulate
atomicity is restricted to the case where one uses the same op or noop
but not replace.

The MPI-2.2 spec is unambiguous because it explicitly requires the
same operation in 11.7.1 Atomicity.  This text was removed in MPI-3.0
in favor of the info key text.

Best,

Jeff

On Tue, Mar 11, 2014 at 12:04 AM, Balaji, Pavan <balaji at anl.gov> wrote:
>
> MPI-2 defines atomicity only for the same operation, not any operation for MPI_ACCUMULATE.
>
>   — Pavan
>
> On Mar 10, 2014, at 11:22 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>
>> So MPI-2 denied compatibility between replace and not-replace?
>>
>> Jeff
>>
>> Sent from my iPhone
>>
>>> On Mar 11, 2014, at 12:06 AM, "Balaji, Pavan" <balaji at anl.gov> wrote:
>>>
>>>
>>> It doesn’t break backward compatibility.  The info argument is still useful when you don’t want to use replace.  I don’t see anything wrong with it.
>>>
>>>> On Mar 10, 2014, at 11:01 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>>>>
>>>> Does this or does this not break BW compatibility w.r.t. MPI-2.2 and
>>>> did we do it intentionally?  Unless we did so intentionally and
>>>> explicitly, I will argue that the WG screwed up and the info key+val
>>>> is invalid.
>>>>
>>>> Jeff
>>>>
>>>>> On Mon, Mar 10, 2014 at 11:03 PM, Balaji, Pavan <balaji at anl.gov> wrote:
>>>>>
>>>>> If a hardware can implement MPI_SUM, it should be able to implement MPI_SUM with 0 as well.
>>>>>
>>>>> But that’s not a generic solution.
>>>>>
>>>>> Jeff: at some point you were planning to bring in a ticket which does more combinations of operations than just same_op and no_op.  Maybe it’s worthwhile bringing that up again?
>>>>>
>>>>> — Pavan
>>>>>
>>>>>> On Mar 10, 2014, at 9:26 PM, Jim Dinan <james.dinan at gmail.com> wrote:
>>>>>>
>>>>>> Maybe there's a loophole that I'm forgetting?
>>>>>>
>>>>>>
>>>>>> On Mon, Mar 10, 2014 at 9:43 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>>>>>> How the hell can I do GA or SHMEM then? Roll my own mutexes and commit perf-suicide?
>>>>>>
>>>>>> Jeff
>>>>>>
>>>>>> Sent from my iPhone
>>>>>>
>>>>>>> On Mar 10, 2014, at 8:32 PM, Jim Dinan <james.dinan at gmail.com> wrote:
>>>>>>>
>>>>>>> You can't use replace and sum concurrently at a given target address.
>>>>>>>
>>>>>>> ~Jim.
>>>>>>>
>>>>>>> On Mon, Mar 10, 2014 at 4:30 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>>>>>>> Given the following, how do I use MPI_NO_OP, MPI_REPLACE and MPI_SUM
>>>>>>> in accumulate/atomic operations in a standard-compliant way?
>>>>>>>
>>>>>>> accumulate_ops — if set to same_op, the implementation will assume
>>>>>>> that all concurrent accumulate calls to the same target address will
>>>>>>> use the same operation. If set to same_op_no_op, then the
>>>>>>> implementation will assume that all concurrent accumulate calls to the
>>>>>>> same target address will use the same operation or MPI_NO_OP. This can
>>>>>>> eliminate the need to protect access for certain operation types where
>>>>>>> the hardware can guarantee atomicity. The default is same_op_no_op.
>>>>>>>
>>>>>>> We discuss this before and the resolution was not satisfying to me.
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> 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
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> 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
>>>>>>
>>>>>> _______________________________________________
>>>>>> 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
>>>>
>>>>
>>>>
>>>> --
>>>> 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
>>>
>>> _______________________________________________
>>> 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
>
> _______________________________________________
> mpiwg-rma mailing list
> mpiwg-rma at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma



-- 
Jeff Hammond
jeff.science at gmail.com