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

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Date:	Mon, 25 Apr 2011 21:45:42 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Dehao Chen <danielcdh@...il.com>, Andi Kleen <ak@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>, arun@...rma-home.net,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, eranian@...il.com,
	Arun Sharma <asharma@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add
 missing user space support for config1/config2

* Stephane Eranian <eranian@...gle.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
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