linux-kernel - Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

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Date:	Tue, 14 Jan 2014 09:58:26 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
CC:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Li Zefan <lizefan@...wei.com>,
	"containers@...ts.linux-foundation.org" 
	<containers@...ts.linux-foundation.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

On 01/12/2014 11:55 PM, Peter Zijlstra wrote:
> 
> The problem is, since there's a limited number of RMIDs we have to
> rotate at some point, but since changing RMIDs is nondeterministic we
> can't.
> 

This is fundamentally the crux here.  RMIDs are quite expensive for the
hardware to implement, so they are limited - but recycling them is
*very* expensive because you literally have to touch every line in the
cache.

	-hpa


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