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

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Date:	Fri, 14 Aug 2009 17:51:06 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	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?

On Fri, Aug 14, 2009 at 05:10:55PM +0800, Johannes Weiner wrote:
> On Fri, Aug 14, 2009 at 10:16:26AM +0300, Avi Kivity wrote:
> > On 08/14/2009 12:16 AM, Johannes Weiner wrote:
> > >
> > >>- do not ignore the referenced bit
> > >>- if you see a run of N pages which all have the referenced bit set, do
> > >>swap one
> > >>
> > >>     
> > >
> > >But it also means destroying the LRU order.
> > >
> > >   
> > 
> > True, it would, but if we ignore the referenced bit, LRU order is really 
> > FIFO.
> 
> For the active list, yes.  But it's not that we degrade to First Fault
> First Out in a global scope, we still update the order from
> mark_page_accessed() and by activating referenced pages in
> shrink_page_list() etc.
> 
> So even with the active list being a FIFO, we keep usage information
> gathered from the inactive list.  If we deactivate pages in arbitrary
> list intervals, we throw this away.

We do have the danger of FIFO, if inactive list is small enough, so
that (unconditionally) deactivated pages quickly get reclaimed and
their life window in inactive list is too small to be useful.

> And while global FIFO-based reclaim does not work too well, initial
> fault order is a valuable hint in the aspect of referential locality
> as the pages get used in groups and thus move around the lists in
> groups.
> 
> Our granularity for regrouping decisions is pretty coarse, for
> non-filecache pages it's basically 'referenced or not refrenced in the
> last list round-trip', so it will take quite some time to regroup
> pages that are used in truly similar intervals.

Agreed.
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