linux-kernel - Re: [RFC][PATCH] the proposal of improve page reclaim by throttle

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Date:	Thu, 21 Feb 2008 21:29:30 +0900
From:	"minchan Kim" <barrioskmc@...il.com>
To:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"Rik van Riel" <riel@...hat.com>,
	"Lee Schermerhorn" <Lee.Schermerhorn@...com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] the proposal of improve page reclaim by throttle

On Thu, Feb 21, 2008 at 7:55 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
> Hi Kim-san,
>
>  Thank you very much.
>  btw, what different between <test 1> and <test 2>?

<test 1> have no swap device with 200 tasks by hackbench.
But <test 2> have swap device(32M) with 240 tasks by hackbench.
If <test2> have no swap device without your patch, <test2> is killed by OOM.

<test 1> - NO SWAP
Running with 5*40 (== 200) tasks.
...
<test 2> - SWAP
Running with 6*40 (== 240) tasks.
...

>
>  >  It was a very interesting result.
>  >  In embedded system, your patch improve performance a little in case
>  >  without noswap(normal case in embedded system).
>  >  But, more important thing is OOM occured when I made 240 process
>  >  without swap device and vanilla kernel.
>  >  Then, I applied your patch, it worked very well without OOM.
>
>  Wow, it is very interesting result!
>  I am very happy.
>
>
>  >  I think that's why zone's page_scanned was six times greater than
>  >  number of lru pages.
>  >  At result, OOM happened.
>
>  please repost question with change subject.
>  i don't know reason of vanilla kernel behavior, sorry.

Normally, embedded linux have only one zone(DMA).

If your patch isn't applied, several processes can reclaim memory in parallel.
then, DMA zone's pages_scanned is suddenly increased largely. Because
embedded linux have no swap device,  kernel can't stop to scan lru
list until meeting page cache page. so if zone->pages_scanned is
greater six time than lru list pages, kernel make the zone with
unreclaimable state, As a result, OOM will kill it, too.

-- 
Thanks,
barrios
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