linux-kernel - Re: [PATCH 08/19] perf c2c: Shared data analyser

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Date:	Mon, 3 Mar 2014 19:58:19 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Joe Mario <jmario@...hat.com>, Andi Kleen <andi@...stfloor.org>,
	Don Zickus <dzickus@...hat.com>,
	Davidlohr Bueso <davidlohr@...com>, acme@...stprotocols.net,
	LKML <linux-kernel@...r.kernel.org>, jolsa@...hat.com,
	fowles@...each.com, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Richard Fowles <rfowles@...hat.com>
Subject: Re: [PATCH 08/19] perf c2c: Shared data analyser

On Mon, Mar 03, 2014 at 07:41:17PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 03, 2014 at 01:07:00PM -0500, Joe Mario wrote:
> > If you only sample on the HITMs then you don't get the store misses.
> > That means you'll not be able to detect who is simultaneously tugging
> > on the same cache lines.  That gives up much of the value of "perf
> > c2c".
> 
> As long as you know which lines are hurting bringing in (loads) you can
> often figure out who is doing the stores on them.

Yes, especially since every store is a load too (unless you're talking
WC)

The method c2c uses is more exact, but keep in mind it's a sampling
heuristic in any cases, with some potential bias. load-latency tags
the loads randomly and there's no guarantee that tagging is fully
uniform. Also you only see a subset in any case.

> 
> > As we developed this, we ended up settling on Ivy Bridge to get the
> > behavior we wanted.
> 
> Wouldn't SNB also work?

Yes.

Haswell is best however because it can report addresses on far more
events.

-Andi

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