linux-kernel - Re: [PATCH 08/10] mm: vmscan: Have kswapd shrink slab only once per priority

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Message-ID: <20130410010734.GR17758@dastard>
Date:	Wed, 10 Apr 2013 11:07:34 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	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>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/10] mm: vmscan: Have kswapd shrink slab only once per
 priority

On Tue, Apr 09, 2013 at 12:13:59PM +0100, Mel Gorman wrote:
> On Tue, Apr 09, 2013 at 03:53:25PM +0900, Joonsoo Kim wrote:
> 
> > I think that outside of zone loop is better place to run shrink_slab(),
> > because shrink_slab() is not directly related to a specific zone.
> > 
> 
> This is true and has been the case for a long time. The slab shrinkers
> are not zone aware and it is complicated by the fact that slab usage can
> indirectly pin memory on other zones.
......
> > And this is a question not related to this patch.
> > Why nr_slab is used here to decide zone->all_unreclaimable?
> 
> Slab is not directly associated with a slab but as reclaiming slab can
> free memory from unpredictable zones we do not consider a zone to be
> fully unreclaimable until we cannot shrink slab any more.

This is something the numa aware shrinkers will greatly help with -
instead of being a global shrink it becomes a
node-the-zone-belongs-to shrink, and so....

> You may be thinking that this is extremely heavy handed and you're
> right, it is.

... it is much less heavy handed than the current code...

> > nr_slab is not directly related whether a specific zone is reclaimable
> > or not, and, moreover, nr_slab is not directly related to number of
> > reclaimed pages. It just say some objects in the system are freed.
> > 
> 
> All true, it's the indirect relation between slab objects and the memory
> that is freed when slab objects are reclaimed that has to be taken into
> account.

Node awareness within the shrinker infrastructure and LRUs make the
relationship much more direct ;)

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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