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

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Message-ID: <4A82D24D.6020402@redhat.com>
Date:	Wed, 12 Aug 2009 10:31:41 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	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>, Avi Kivity <avi@...hat.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?

Wu Fengguang wrote:
> On Fri, Aug 07, 2009 at 11:17:22AM +0800, KOSAKI Motohiro wrote:
>>> Andrea Arcangeli wrote:
>>>
>>>> Likely we need a cut-off point, if we detect it takes more than X
>>>> seconds to scan the whole active list, we start ignoring young bits,
>>> We could just make this depend on the calculated inactive_ratio,
>>> which depends on the size of the list.
>>>
>>> For small systems, it may make sense to make every accessed bit
>>> count, because the working set will often approach the size of
>>> memory.
>>>
>>> On very large systems, the working set may also approach the
>>> size of memory, but the inactive list only contains a small
>>> percentage of the pages, so there is enough space for everything.
>>>
>>> Say, if the inactive_ratio is 3 or less, make the accessed bit
>>> on the active lists count.
>> Sound reasonable.
> 
> Yes, such kind of global measurements would be much better.
> 
>> How do we confirm the idea correctness?
> 
> In general the active list tends to grow large on under-scanned LRU.
> I guess Rik is pretty familiar with typical inactive_ratio values of
> the large memory systems and may even have some real numbers :)
> 
>> Wu, your X focus switching benchmark is sufficient test?
> 
> It is a major test case for memory tight desktop.  Jeff presents
> another interesting one for KVM, hehe.
> 
> Anyway I collected the active/inactive list sizes, and the numbers
> show that the inactive_ratio is roughly 1 when the LRU is scanned
> actively and may go very high when it is under-scanned.

inactive_ratio is based on the zone (or cgroup) size.

For zones it is a fixed value, which is available in
/proc/zoneinfo

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