linux-kernel - Re: [PATCH v3 10/11] perf/x86/intel: Perform rotation on Intel CQM RMIDs

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Message-ID: <20141110210821.GF10501@worktop.programming.kicks-ass.net>
Date:	Mon, 10 Nov 2014 22:08:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Kanaka Juvva <kanaka.d.juvva@...el.com>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v3 10/11] perf/x86/intel: Perform rotation on Intel CQM
 RMIDs

On Mon, Nov 10, 2014 at 08:56:53PM +0000, Matt Fleming wrote:
> > Should we initialize that to a finite value? Surely results are absolute
> > crap if we do indeed reach that max?
> 
> I don't think we'll ever reach that max, it'll bottom out once it
> reaches the size of the LLC, since the pathological case is that the
> RMID you're currently trying to stabilize is used to tag every line in
> the LLC.

Sure, LLC size is the 'same'. Makes the entire ordeal rather pointless.

> Not sure what a reasonable finite value would be here though? 10% of the
> LLC size?

Can you ask the same Oracle that provided the magic 250 ms? Both are
magic/random numbers. 10% sounds rather large, but who knows.
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