linux-kernel - Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning requirements for kswapd

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Message-ID: <20130321162518.GB27848@cmpxchg.org>
Date:	Thu, 21 Mar 2013 12:25:18 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning
 requirements for kswapd

On Sun, Mar 17, 2013 at 01:04:08PM +0000, Mel Gorman wrote:
> Simplistically, the anon and file LRU lists are scanned proportionally
> depending on the value of vm.swappiness although there are other factors
> taken into account by get_scan_count().  The patch "mm: vmscan: Limit
> the number of pages kswapd reclaims" limits the number of pages kswapd
> reclaims but it breaks this proportional scanning and may evenly shrink
> anon/file LRUs regardless of vm.swappiness.
> 
> This patch preserves the proportional scanning and reclaim. It does mean
> that kswapd will reclaim more than requested but the number of pages will
> be related to the high watermark.

Swappiness is about page types, but this implementation compares all
LRUs against each other, and I'm not convinced that this makes sense
as there is no guaranteed balance between the inactive and active
lists.  For example, the active file LRU could get knocked out when
it's almost empty while the inactive file LRU has more easy cache than
the anon lists combined.

Would it be better to compare the sum of file pages with the sum of
anon pages and then knock out the smaller pair?
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