linux-kernel - Re: [PATCH] perf x86: Change SandyBridge and IvyBrige instructions event to be precise

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Message-ID: <20130220214816.GC2928@two.firstfloor.org>
Date:	Wed, 20 Feb 2013 22:48:16 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Ahern <dsahern@...il.com>
Subject: Re: [PATCH] perf x86: Change SandyBridge and IvyBrige instructions
 event to be precise

> I don't think it buys you much. People read too much into what
> PEBS gives you. I can show you examples where PEBS
> is worse than no PEBS.

Nothing is perfect in sampling, but the precise instructions event
is definitely better in a range of situations. It avoids
a lot of shadow effects.

I think it's a good choice for a lot of situations, although
on Ivy Bridge without EventingRip the BB walking is still
fairly expensive.

It would be a good default for the "user only asks for instructions
for sampling and nothing else" situation, especially on Haswell
which has EventingRip. But that's not straight forward to detect.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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