linux-kernel - Re: [RFC] respect the referenced bit of KVM guest pages?

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Date:	Sun, 16 Aug 2009 13:50:46 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Avi Kivity <avi@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?

On Sun, Aug 16, 2009 at 01:09:03PM +0800, Balbir Singh wrote:
> * Wu Fengguang <fengguang.wu@...el.com> [2009-08-15 13:45:24]:
> 
> > On Fri, Aug 14, 2009 at 09:19:35PM +0800, Rik van Riel wrote:
> > > Wu Fengguang wrote:
> > > > On Fri, Aug 14, 2009 at 05:10:55PM +0800, Johannes Weiner wrote:
> > > 
> > @@ -1541,11 +1542,11 @@ static void shrink_zone(int priority, st
> >  			scan = (scan * percent[file]) / 100;
> >  		}
> >  		if (scanning_global_lru(sc))
> > -			nr[l] = nr_scan_try_batch(scan,
> > -						  &zone->lru[l].nr_saved_scan,
> > -						  swap_cluster_max);
> > +			saved_scan = &zone->lru[l].nr_saved_scan;
> >  		else
> > -			nr[l] = scan;
> > +			saved_scan = mem_cgroup_get_saved_scan(sc->mem_cgroup,
> > +							       zone, l);
> > +		nr[l] = nr_scan_try_batch(scan, saved_scan, swap_cluster_max);
> >  	}
> >
> 
> This might be a concern (although not a big ATM), since we can't
> afford to miss limits by much. If a cgroup is near its limit and we
> drop scanning it. We'll have to work out what this means for the end
> user. May be more fundamental look through is required at the priority
> based logic of exposing how much to scan, I don't know.

I also had this worry at first. Then dismissed it because the page
reclaim should be driven by "pages reclaimed" rather than "pages
scanned". So when shrink_zone() decides to cancel one smallish scan,
it may well be called again and accumulate up nr_saved_scan.

So it should only be a problem for a very small mem_cgroup (which may
be _full_ scanned too much times in order to accumulate up nr_saved_scan).

Thanks,
Fengguang
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