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

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Date:	Tue, 18 Aug 2009 10:26:09 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Avi Kivity <avi@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.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 Tue, Aug 18, 2009 at 02:04:46AM +0800, Dike, Jeffrey G wrote:
> > Jeff, can you confirm if the mem cgroup's inactive list is small?
> 
> Nope.  I have plenty on the inactive anon list, between 13K and 16K pages (i.e. 52M to 64M).
>
> The inactive mapped list is much smaller - 0 to ~700 pages.
> 
> The active lists are comparable in size, but larger - 16K - 19K pages for anon and 60 - 450 pages for mapped.

The anon inactive list is "over scanned".  Take 16k pages for example,
with DEF_PRIORITY=12, (16k >> 12) = 4.  So when shrink_zone() expects
to scan 4 pages in the active/inactive list, it will be scanned
SWAP_CLUSTER_MAX=32 pages in effect.

This triggers the background aging of active anon list because
inactive_anon_is_low() is found to be true, which keeps the
active:inactive ratio in balance.

So anon inactive list over scanned => anon active list over scanned =>
anon lists over scanned relative to file lists. (The inactive file list
may or may not be over scanned depending on its size <> (1<<prio) pages.)

Anyway this is not the expected way vmscan should work, and batching
up the cgroup vmscan could get rid of the mess.

Thanks,
Fengguang
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