brbarret at open-mpi.org
Fri Mar 14 11:25:05 CDT 2014
I agree with Rajeev; concurrent access with different ops wasn't undefined in MPI-2.2, it was erroneous.
During the Forum meetings on RMA, there was strong opposition to relaxing that semantic, which is how we ended up with same_op_no_op. It sucked, but it sucked less than the semantics of MPI-2.2.
I'm not opposed to loosening the restrictions (and, in fact, wanted them looser during MPI-3), but it would be good to understand why we as a group felt that wasn't ok 2 years ago.
On Mar 14, 2014, at 6:12 AM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
> Where does it say in MPI 2.2 that concurrent accumulates with different operations are allowed?
> On pg 365, ln 29-33, it says two concurrent accumulates that use the same operation are allowed.
> • A location in a window must not be accessed as a target of an RMA operation once an update to that location has started, until the update becomes visible in the public window copy. There is one exception to this rule, in the case where the same variable is updated by two concurrent accumulates that use the same operation, with the same predefined datatype, on the same window.
> On Mar 14, 2014, at 12:25 AM, "Balaji, Pavan" <balaji at anl.gov> wrote:
>> I’ve written up these thoughts in the below ticket:
>> Comments are welcome on the ticket.
>> — Pavan
>> On Mar 13, 2014, at 4:13 PM, Balaji, Pavan <balaji at anl.gov> wrote:
>>> On Mar 13, 2014, at 4:08 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>>>>> It was not disallowed in MPI-2. I meant that we made a mistake in MPI-3 to disallow it since that is not backward compatible.
>>>> You told me two days ago it was at best undefined, which is no more
>>>> useful than disallowed:
>>> I’m not sure what you are referring to, but that was no my intention. Perhaps I misunderstood what you were asking.
>>> It think it’s pretty clear that it’s disallowed in MPI-3, not undefined.
>>>>> IMO, we should have kept the same semantics as MPI-2, but allowed the user to relax it with info arguments.
>>>> Well we broke backwards compatibility but made it almost impossible
>>>> for anyone to notice and certainly didn't add an advice to users so we
>>>> are clearly all jerks.
>>> I like MPI-2’s approach. The data content is undefined, but not an error.
>>> — Pavan
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