linux-kernel - Re: [PATCH] mm: vmscan: Obey indeed proportional scanning for kswapd and memcg

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Date:	Wed, 25 Nov 2015 11:28:51 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Yaowei Bai <baiyaowei@...s.chinamobile.com>
Cc:	akpm@...ux-foundation.org, riel@...hat.com, mhocko@...e.cz,
	hannes@...xchg.org, kamezawa.hiroyu@...fujitsu.com, jslaby@...e.cz,
	Valdis.Kletnieks@...edu, zcalusic@...sync.net, vbabka@...e.cz,
	vdavydov@...allels.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: Obey indeed proportional scanning for kswapd
 and memcg

On Wed, Nov 25, 2015 at 12:48:20PM +0800, Yaowei Bai wrote:
> Commit e82e0561dae9f3ae5 ("mm: vmscan: obey proportional scanning
> requirements for kswapd") intended to preserve the proportional scanning
> and reclaim what was requested by get_scan_count() for kswapd and memcg
> by stopping reclaiming one type(anon or file) LRU and reducing the other's
> amount of scanning proportional to the original scan target.
> 
> So the way to determine which LRU should be stopped reclaiming should be
> comparing scanned/unscanned percentages to the original scan target of two
> lru types instead of absolute values what implemented currently, because
> larger absolute value doesn't mean larger percentage, there shall be
> chance that larger absolute value with smaller percentage, for instance:
> 
> 	target_file = 1000
> 	target_anon = 500
> 	nr_file = 500
> 	nr_anon = 400
> 
> in this case, because nr_file > nr_anon, according to current implement,
> we will stop scanning anon lru and shrink file lru. This breaks
> proportional scanning intent and makes more unproportional.
> 
> This patch changes to compare percentage to the original scan target to
> determine which lru should be shrunk.
> 
> Signed-off-by: Yaowei Bai <baiyaowei@...s.chinamobile.com>

This one has gone back and forth a few times in the past. It really was
deliberate that the scanning was proportional to the scan target. While
I see what your concern is, it's unclear what the actual impact is. Have
you done any testing to check if the proposed new behaviour is actually
better?

-- 
Mel Gorman
SUSE Labs
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