linux-kernel - Re: [RFC] respect the referenced bit of KVM guest pages?

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Message-ID: <4A79A1FB.6010406@redhat.com>
Date:	Wed, 05 Aug 2009 11:15:07 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Wu Fengguang <fengguang.wu@...el.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?

Avi Kivity wrote:

>> If so, we could unmap them when they get moved from the
>> active to the inactive list, and soft fault them back in
>> on access, emulating the referenced bit for EPT pages and
>> making page replacement on them work like it should.
> 
> It should be easy to implement via the mmu notifier callback: when the 
> mm calls clear_flush_young(), mark it as young, and unmap it from the 
> EPT pagetable.

You mean "mark it as old"?

>> Your approximation of pretending the page is accessed the
>> first time and pretending it's not the second time sounds
>> like it will just lead to less efficient FIFO replacement,
>> not to anything even vaguely approximating LRU.
> 
> Right, it's just a hack that gives EPT pages higher priority, like the 
> original patch suggested.  Note that LRU for VMs is not a good 
> algorithm, since the VM will also reference the least recently used 
> page, leading to thrashing.

That is one of the reasons we use a very coarse two
handed clock algorithm instead of true LRU.

LRU has more overhead and more artifacts :)

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