linux-kernel - Re: [PATCH -v2 changelog updated] mm: do batched scans for mem

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Date:	Fri, 21 Aug 2009 19:57:24 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"menage@...gle.com" <menage@...gle.com>
Subject: Re: [PATCH -v2 changelog updated] mm: do batched scans for mem_cgroup

2009/8/21 Wu Fengguang <fengguang.wu@...el.com>:
> For mem_cgroup, shrink_zone() may call shrink_list() with nr_to_scan=1,
> in which case shrink_list() _still_ calls isolate_pages() with the much
> larger SWAP_CLUSTER_MAX.  It effectively scales up the inactive list
> scan rate by up to 32 times.
>
> For example, with 16k inactive pages and DEF_PRIORITY=12, (16k >> 12)=4.
> So when shrink_zone() expects to scan 4 pages in the active/inactive
> list, the active list will be scanned 4 pages, while the inactive list
> will be (over) scanned SWAP_CLUSTER_MAX=32 pages in effect. And that
> could break the balance between the two lists.
>
> It can further impact the scan of anon active list, due to the anon
> active/inactive ratio rebalance logic in balance_pgdat()/shrink_zone():
>
> inactive anon list over scanned => inactive_anon_is_low() == TRUE
>                                => shrink_active_list()
>                                => active anon list over scanned
>
> So the end result may be
>
> - anon inactive  => over scanned
> - anon active    => over scanned (maybe not as much)
> - file inactive  => over scanned
> - file active    => under scanned (relatively)
>
> The accesses to nr_saved_scan are not lock protected and so not 100%
> accurate, however we can tolerate small errors and the resulted small
> imbalanced scan rates between zones.
>

Looks good to me.
  Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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