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

[Ingo Molnar <mingo@elte.hu>]

* Stephane Eranian <eranian@google.com> wrote:

> > This certainly does not match the results i'm seeing on real applications, 
> > using "-e instructions:pp" PEBS+LBR profiling. How do you explain that? 
> > Also, can you demonstrate your claim with a real example?
> 
> LBR removes the off-by-1 IP problem, it does not remove the shadow effect, 
> i.e., that blind spot of N cycles caused by the PEBS arming mechanism.

I really think you are grasping at straws here - unless you are able to 
demonstrate clear problems - which you have failed to do so far. The pure act
of profiling probably disturbs a typical workload statistically more than a
few cycles skew of the period.

I could imagine artifacts with realy short periods and artificially short and 
dominant hotpaths - but in those cases the skew does not matter much in 
practice: a short and dominant hotpath is pinpointed very easily ...

So i really think it's a non-issue in practice - but you can certainly prove me 
wrong by demonstrating whatever problems you suspect.

Thanks,

	Ingo