[mpiwg-rma] same_op_no_op

[Zhao, Xin]

Suppose I am a user that is going to write an MPI program, according to your opinion, I must know default values of all MPI infos beforehand to make sure that I will write a correct program, even though I will not use them, is that true? If so, I think it is not obvious in the standard, and easily to be ignored by the user.

Of course I agree with Dave that some strengthen text need to be added into future standard to remind users to know the default behavior of all MPI infos. 

BTW, I didn't see any text in the MPI-3 standard about set info to "" or "ignoreme". Where can I find the usage about them?

Xin
________________________________________
From: mpiwg-rma [mpiwg-rma-bounces at lists.mpi-forum.org] on behalf of Balaji, Pavan [balaji at anl.gov]
Sent: Thursday, March 13, 2014 1:49 PM
To: MPI WG Remote Memory Access working group
Subject: Re: [mpiwg-rma] same_op_no_op

I’m not sure what “it is indirect to the user since the user needs to infer by themselves” means.

As Dave suggested, the user is making an implicit promise that same_op_no_op is used, by default.  With this promise set (either explicitly or implicitly), it is an errorneous program to have accumulate with different ops on the same location.

In MPI-2, it was not erroneous to do so, though the atomicity was not guaranteed so the data might be corrupted.

  — Pavan

On Mar 13, 2014, at 1:07 PM, Jeff Hammond <jeff.science at gmail.com> wrote:

> Indeed Xin is capturing things well.
>
> My point is that the user promised nothing when not setting this info key and thus it cannot be erroneous to not abide by it.
>
> I've posted my question about whether or not info text is normative on the forum list...
>
> Jeff
>
> Sent from my iPhone
>
>> On Mar 13, 2014, at 1:00 PM, "Zhao, Xin" <xinzhao3 at illinois.edu> wrote:
>>
>> For my understanding, the difference between Pavan and Jeff is that, Pavan thinks the default behavior of an info value is actually a standard behavior ("same_op_no_op"), and user should follow that, whereas Jeff thinks it is indirect to the user since user needs to infer by themselves. It should be clearly defined in MPI-3 standard. Sorry if I understand wrongly.
>>
>> If I understand correctly, I agree with Jeff.
>>
>> Xin
>> ________________________________________
>> From: mpiwg-rma [mpiwg-rma-bounces at lists.mpi-forum.org] on behalf of Balaji, Pavan [balaji at anl.gov]
>> Sent: Thursday, March 13, 2014 12:52 PM
>> To: MPI WG Remote Memory Access working group
>> Subject: Re: [mpiwg-rma] same_op_no_op
>>
>>> On Mar 13, 2014, at 12:37 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>>> How does MPI-3 indicate even indirectly that they are not allowed?  I
>>> do not believe the standard meets the minimum burden of telling the
>>> user this and therefore it cannot be inferred.
>>
>> I’m still referring to the same sentence we have been discussing:
>>
>> "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 means that the user promised that (s)he will only issue the same op or no_op.  If the user breaks this promise, that’s an erroneous program and the MPI implementation is allowed to set the machine room on fire.
>>
>> This is the same model we have been using for other info arguments, e.g.,
>>
>> "no_locks — if set to true, then the implementation may assume that passive target syn- chronization (i.e., MPI_WIN_LOCK, MPI_LOCK_ALL) will not be used on the given window."
>>
>> If the user sets no_locks and uses MPI_WIN_LOCK, it’s an error.
>>
>> — Pavan
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