linux-kernel - Re: [PATCH v2] fix get_scan_count for working well with small targets

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Date:	Wed, 27 Apr 2011 14:08:18 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"mgorman@...e.de" <mgorman@...e.de>, Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH v2] fix get_scan_count for working well with small targets

Hi Kame,

On Wed, Apr 27, 2011 at 10:50 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Tue, 26 Apr 2011 13:59:34 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
>
>> What about simply removing the nr_saved_scan logic and permitting small
>> scans?  That simplifies the code and I bet it makes no measurable
>> performance difference.
>>
>
> ok, v2 here. How this looks ?
> For memcg, I think I should add select_victim_node() for direct reclaim,
> then, we'll be tune big memcg using small memory on a zone case.
>
> ==
> At memory reclaim, we determine the number of pages to be scanned
> per zone as
>        (anon + file) >> priority.
> Assume
>        scan = (anon + file) >> priority.
>
> If scan < SWAP_CLUSTER_MAX, the scan will be skipped for this time
> and priority gets higher. This has some problems.
>
>  1. This increases priority as 1 without any scan.
>     To do scan in this priority, amount of pages should be larger than 512M.
>     If pages>>priority < SWAP_CLUSTER_MAX, it's recorded and scan will be
>     batched, later. (But we lose 1 priority.)

Nice catch!  It looks to be much enhance.

>     But if the amount of pages is smaller than 16M, no scan at priority==0
>     forever.

Before reviewing the code, I have a question about this.
Now, in case of (priority = 0), we don't do shift operation with priority.
So nr_saved_scan would be the number of lru list pages. ie, 16M.
Why no-scan happens in case of (priority == 0 and 16M lru pages)?
What am I missing now?

>
>  2. If zone->all_unreclaimabe==true, it's scanned only when priority==0.
>     So, x86's ZONE_DMA will never be recoverred until the user of pages
>     frees memory by itself.
>
>  3. With memcg, the limit of memory can be small. When using small memcg,
>     it gets priority < DEF_PRIORITY-2 very easily and need to call
>     wait_iff_congested().
>     For doing scan before priorty=9, 64MB of memory should be used.

It makes sense.



-- 
Kind regards,
Minchan Kim
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