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

[Mel Gorman <mgorman@suse.de>]

On Wed, Mar 20, 2013 at 05:18:47PM +0100, Michal Hocko wrote:
> On Sun 17-03-13 13:04:07, Mel Gorman wrote:
> [...]
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 88c5fed..4835a7a 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2593,6 +2593,32 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
> >  }
> >  
> >  /*
> > + * kswapd shrinks the zone by the number of pages required to reach
> > + * the high watermark.
> > + */
> > +static void kswapd_shrink_zone(struct zone *zone,
> > +			       struct scan_control *sc,
> > +			       unsigned long lru_pages)
> > +{
> > +	unsigned long nr_slab;
> > +	struct reclaim_state *reclaim_state = current->reclaim_state;
> > +	struct shrink_control shrink = {
> > +		.gfp_mask = sc->gfp_mask,
> > +	};
> > +
> > +	/* Reclaim above the high watermark. */
> > +	sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
> 
> OK, so the cap is at high watermark which sounds OK to me, although I
> would expect balance_gap being considered here. Is it not used
> intentionally or you just wanted to have a reasonable upper bound?
> 

It's intentional. The balance_gap is taken into account before the
decision to shrink but not afterwards. As the watermark check after
shrinking is based on just the high watermark, I decided to have
shrink_zone reclaim on that basis.

-- 
Mel Gorman
SUSE Labs