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

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <ak@linux.intel.com> wrote:

> > 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?
> 
> It's one instruction off the instruction that was retired when the PEBS 
> interrupt was ready, but not one instruction off the instruction that caused 
> the event. There's still skid in triggering the interrupt.

Peter answered this in the other mail:

 |
 | 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.
 |

... and the resulting low level of noise in the average period length is what 
matters. The instruction itself will still be one of the hotspot instructions, 
statistically.

Thanks,

	Ingo