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

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Message-ID: <20130411100115.GJ3710@suse.de>
Date:	Thu, 11 Apr 2013 11:01:15 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
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>,
	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 Wed, Apr 10, 2013 at 02:21:42PM +0900, Joonsoo Kim wrote:
> > > > @@ -2673,9 +2674,15 @@ static bool kswapd_shrink_zone(struct zone *zone,
> > > >  	sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
> > > >  	shrink_zone(zone, sc);
> > > >  
> > > > -	reclaim_state->reclaimed_slab = 0;
> > > > -	nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
> > > > -	sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> > > > +	/*
> > > > +	 * Slabs are shrunk for each zone once per priority or if the zone
> > > > +	 * being balanced is otherwise unreclaimable
> > > > +	 */
> > > > +	if (shrinking_slab || !zone_reclaimable(zone)) {
> > > > +		reclaim_state->reclaimed_slab = 0;
> > > > +		nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
> > > > +		sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> > > > +	}
> > > >  
> > > >  	if (nr_slab == 0 && !zone_reclaimable(zone))
> > > >  		zone->all_unreclaimable = 1;
> > > 
> > > Why shrink_slab() is called here?
> > 
> > Preserves existing behaviour.
> 
> Yes, but, with this patch, existing behaviour is changed, that is, we call
> shrink_slab() once per priority. For now, there is no reason this function
> is called here. How about separating it and executing it outside of
> zone loop?
> 

We are calling it fewer times but it's still receiving the same information
from sc->nr_scanned it received before. With the change you are suggesting
it would be necessary to accumulating sc->nr_scanned for each zone shrunk
and then pass the sum to shrink_slab() once per priority. While this is not
necessarily wrong, there is little or no motivation to alter the shrinkers
in this manner in this series.

-- 
Mel Gorman
SUSE Labs
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