[mpiwg-tools] MPI_T names of variables

[Michael Knobloch]

Hi all,

as being participant in the initial discussion with Marc-Andre I'd like
to comment on that.


On 01/10/2014 04:27 PM, Martin Schulz wrote:
> Hi Marc-Andre,
>
> I always expected that if a variable named "foo" exists in two processes, it collects the same metric with the same semantics. When we wrote the paragraph we (or at least I) had in mind that the set of variables can be different (i.e., if "foo" exists on A, it does not have to exist on B) and the order can be different. Not sure if we can formalize this or if people would go along with it, but it would certainly be advantageous for the tools.
>

I understood the wording in the same way as Martin, but unfortunately
the advice to users doesn't say anything about semantics, so in a bad
implementation this could actually happen. Although I would have no
problem with having two names for the same information, as long as it is
consistent across all processes.

> However, having said that, I can actually envision scenarios where this would be hard to achieve. If you have a heterogeneous system (two architectures/networks or MPI bridging GPU and CPU) it may be a combination of two separate MPIs (with separate name spaces) or the same metric may have different meanings (similar to Flop counts having separate meanings on different architectures). I'd be curious to hear what our implementors think about that. In the homogeneous case, though, I would fully expect some consistency between processes.
>

Indeed, but here Marc-Andre's proposal with the realms could help (see
his mail from November 14th). We need some deterministic way to
determine which sets of processes offer the same information, otherwise
the MPI_T interface will be pretty hard to use by tools (at least in a
portable way).

-Michael

> Martin
>
>
>
> On Jan 10, 2014, at 10:52 AM, Marc-Andre Hermanns <m.a.hermanns at grs-sim.de>
>  wrote:
>
>> Dear all,
>>
>> I hope all of you had a good start into the new year.
>>
>> Let me apologize in advance for the lengthy text following.
>>
>> I am currently in the discussion with the developers of the metric
>> modules for Scalasca and Score-P, and I think we need some
>> clarification. Please correct me in my understanding if anything of the
>> following is not correct.
>>
>> Here is the current quote of the advice to users in the ticket PDF:
>>
>> "[...]Further, there is no guarantee that number of variables, variable
>> indices, and variable names are the same across processes."
>>
>> I read the following out of this:
>> A variable indicating a metric X may be called "foo" on one process and
>> "bar" on another.
>>
>> Is that correct?
>>
>> a) How should a tool aggregate and correlate metrics if there is no way
>> on knowing which belong together (after all indices and names are
>> different)?
>>
>> b) Does this allow the namespaces of metrics to overlap?
>> If the variable for metric X is "foo" on one process and "bar" on
>> another. Is the other process allowed to have a variable "foo" that
>> actually reports some different metric? How will a tool get the difference?
>>
>> I know that the new "Advice to implementers" is supposed to clarify
>> this. But I am unsure whether making this a sole "quality of
>> implementation" issue will work for providers of portable tools.
>>
>> Also, with differences among runs, tools like the Cube Algebra (where
>> you can compute averages of multiple measurements) do not work anymore,
>> as Cube will not know whether "foo" of run one correlates to "foo" of
>> run two, rendering the whole tool moot.
>>
>> Cheers,
>> Marc-Andre
>> --
>> Marc-Andre Hermanns
>> German Research School for
>> Simulation Sciences GmbH
>> c/o Laboratory for Parallel Programming
>> 52062 Aachen | Germany
>>
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>>
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>>
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>
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--
Michael Knobloch
Institute for Advanced Simulation (IAS)
Jülich Supercomputing Centre (JSC)
Telefon: +49 2461 61-3546
Telefax: +49 2461 61-6656
E-Mail: m.knobloch at fz-juelich.de
Internet: http://www.fz-juelich.de/jsc


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