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

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Date:	Sun, 16 Aug 2009 11:28:22 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Avi Kivity <avi@...hat.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>,
	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?

On Thu, Aug 06, 2009 at 09:16:14PM +0800, Rik van Riel wrote:
> Wu Fengguang wrote:
> 
> > I guess both schemes have unacceptable flaws.
> > 
> > For JVM/BIGMEM workload, most pages would be found referenced _all the time_.
> > So the KEEP_MOST scheme could increase reclaim overheads by N=250 times;
> > while the DROP_CONTINUOUS scheme is effectively zero cost.
> 
> The higher overhead may not be an issue on smaller systems,
> or inside smaller cgroups inside large systems, when doing
> cgroup reclaim.

Right.

> > However, the DROP_CONTINUOUS scheme does bring more _indeterminacy_.
> > It can behave vastly different on single active task and multi ones.
> > It is short sighted and can be cheated by bursty activities.
> 
> The split LRU VM tries to avoid the bursty page aging as
> much as possible, by doing background deactivating of
> anonymous pages whenever we reclaim page cache pages and
> the number of anonymous pages in the zone (or cgroup) is
> low.

Right, but I meant busty page allocations and accesses on them, which
can make a large continuous segment of referenced pages in LRU list,
say 50MB.  They may or may not be valuable as a whole, however a local
algorithm may keep the first 4MB and drop the remaining 46MB.

Thanks,
Fengguang
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