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:	Sat, 23 Apr 2011 19:15:52 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	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

> 
> 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).

Unfortunately even PDIST doesn't completely fix the problem, but
it only makes it somewhat better. Also it's only statistical,
so you won't get a guaranteed answer for every sample.

-Andi
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