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RE: [perfmon] Re: perfmon2 TODO list (4/4)


Frank Ch. Eigler wrote:
>> I would hate to require SystemTap to know all of the possible
>> variations that should be included in the fat binary - that is the
>> point of having libpfm in the first place.
> We would still use libpfm itself, but instead of asking it to generate
> PMC data for one particular CPU only, we would ask it to generate
> several candidates.

Can libpfm enumerate the possible variations, so we know which
candidates to ask for?

>> A statement like "all within an architecture" is still painful when
>> dealing with the P4.
> 
> How painful do you mean?  A few dozen variants would still take only a
> couple of hundred bytes of "fat" PMC data.

I mean painful from a maintenance perspective.  Somehow you have to know
which variations to ask for, and suddenly SystemTap is forced to track
all new CPU releases.

Libpfm has to do this already, so if we can get this from libpfm then
it's not so bad.

> Downsides of having stpd resolve this stuff include the loss of
> proximity to script code for purposes such as error localization,
> advice heuristics; reliance on a smarter stpd precludes operation
> without it (such as in the boot-time probing scenario of bug #2035);
> it could preclude representation of CPU flavours to tapsets for
> purposes of event name abstraction.

All good points.

I guess the only concern I have left is making sure that the PMC data we
have match the processor we're loading on.  This needs to be addressed
regardless of whether we allow multiple CPU targets.  I don't think this
is necessarily a difficult task, but it's definitely required.


Josh


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