Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2

[Peter Zijlstra <peterz@infradead.org>]

On Sat, 2011-04-23 at 14:06 +0200, Stephane Eranian wrote:
> On Sat, Apr 23, 2011 at 9:50 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, 2011-04-22 at 17:03 -0700, Andi Kleen wrote:
> >> > > Yes, and note that with instructions events we even have skid-less PEBS
> >> > > profiling so seeing the precise .
> >> >                                   - location of instructions is possible.
> >>
> >> It was better when it was eaten. PEBS does not actually eliminated
> >> skid unfortunately. The interrupt still occurs later, so the
> >> instruction location is off.
> >>
> >> PEBS merely gives you more information.
> >
> > You're so skilled at not actually saying anything useful. Are you
> > perchance referring to the fact that the IP reported in the PEBS data is
> > exactly _one_ instruction off? Something that is demonstrated to be
> > fixable?
> >
> > Or are you defining skid differently and not telling us your definition?
> >
> 
> PEBS is guaranteed to return an IP that is just after AN instruction that
> caused the event. However, that instruction is NOT the one at the end
> of your period. Let's take an example with INST_RETIRED, period=100000.
> Then, the IP you get is NOT after the 100,000th retired instruction. It's an
> instruction that is N cycles after that one. There is internal skid due to the
> way PEBS is implemented.
> 
> That is what Andi is referring to. The issue causes bias and thus impacts
> the quality of the samples. On SNB, there is a new INST_RETIRED:PREC_DIST
> event. PREC_DIST=precise distribution. It tries to correct for the skid
> on this event on INST_RETIRED with PEBS (look at Vol3b).

Sure, but who cares? So your period isn't exactly what you specified,
but the effective period will have an average and a fairly small stdev
(assuming the initial period is much larger than the relatively few
cycles it takes to arm the PEBS assist), therefore you still get a
fairly uniform spread.

I don't much get the obsession with precision here, its all a statistics
game anyway.

And while you keep saying the examples are too trivial and Andi keep
sprouting vague non-statements, neither of you actually provide anything
sensible to the discussion.

So stop f*cking whining and start talking sense or stop talking all
together.

I mean, you were in the room where Intel presented their research on
event correlations based on pathological micro-benches. That clearly
shows that exact event definitions simply don't matter.

Similarly all this precision wanking isn't _that_ important, the big
fish clearly stand out, its only when you start shaving off the last few
cycles that all that really comes in handy, before that its mostly: ooh
thinking is hard, lets go shopping.