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

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Date:	Thu, 13 Aug 2009 09:03:56 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Rik van Riel <riel@...hat.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?

On Wed, Aug 12, 2009 at 10:31:41PM +0800, Rik van Riel wrote:
> 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.

Ah sorry, my word 'inactive_ratio' means runtime active:inactive ratio.

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

On my 64bit desktop with 4GB memory:

        DMA     inactive_ratio:    1
        DMA32   inactive_ratio:    4
        Normal  inactive_ratio:    1

The biggest zone DMA32 has inactive_ratio=4. But I guess the
referenced bit should not be ignored on this typical desktop
configuration?

Thanks,
Fengguang
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