linux-kernel - Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapd reclaims at each priority

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Message-ID: <20130321164737.GL1878@suse.de>
Date:	Thu, 21 Mar 2013 16:47:37 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
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 01/10] mm: vmscan: Limit the number of pages kswapd
 reclaims at each priority

On Thu, Mar 21, 2013 at 11:57:05AM -0400, Johannes Weiner wrote:
> On Sun, Mar 17, 2013 at 01:04:07PM +0000, Mel Gorman wrote:
> > The number of pages kswapd can reclaim is bound by the number of pages it
> > scans which is related to the size of the zone and the scanning priority. In
> > many cases the priority remains low because it's reset every SWAP_CLUSTER_MAX
> > reclaimed pages but in the event kswapd scans a large number of pages it
> > cannot reclaim, it will raise the priority and potentially discard a large
> > percentage of the zone as sc->nr_to_reclaim is ULONG_MAX. The user-visible
> > effect is a reclaim "spike" where a large percentage of memory is suddenly
> > freed. It would be bad enough if this was just unused memory but because
> > of how anon/file pages are balanced it is possible that applications get
> > pushed to swap unnecessarily.
> > 
> > This patch limits the number of pages kswapd will reclaim to the high
> > watermark. Reclaim will will overshoot due to it not being a hard limit as
> 
> will -> still?
> 
> > shrink_lruvec() will ignore the sc.nr_to_reclaim at DEF_PRIORITY but it
> > prevents kswapd reclaiming the world at higher priorities. The number of
> > pages it reclaims is not adjusted for high-order allocations as kswapd will
> > reclaim excessively if it is to balance zones for high-order allocations.
> 
> I don't really understand this last sentence.  Is the excessive
> reclaim a result of the patch, a description of what's happening
> now...?
> 

It's a very basic description of what happens now and with the patch
applied. Until patch 5 is applied, kswapd can still reclaim the world if
it reaches priority 0.

> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> 
> Nice, thank you.  Using the high watermark for larger zones is more
> reasonable than my hack that just always went with SWAP_CLUSTER_MAX,
> what with inter-zone LRU cycle time balancing and all.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Thanks.

-- 
Mel Gorman
SUSE Labs
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