linux-kernel - Re: Free memory never fully used, swapping

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Message-Id: <20101126181604.B6E4.A69D9226@jp.fujitsu.com>
Date:	Fri, 26 Nov 2010 18:18:21 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Simon Kirby <sim@...tway.ca>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Free memory never fully used, swapping

> On Fri, 2010-11-26 at 10:31 +0800, KOSAKI Motohiro wrote:
> > > record the order seems not sufficient. in balance_pgdat(), the for look
> > > exit only when:
> > > priority <0 or sc.nr_reclaimed >= SWAP_CLUSTER_MAX.
> > > but we do if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > >                         order = sc.order = 0;
> > > this means before we set order to 0, we already reclaimed a lot of
> > > pages, so I thought we need set order to 0 earlier before there are
> > > enough free pages. below is a debug patch.
> > > 
> > > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index d31d7ce..ee5d2ed 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -2117,6 +2117,26 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> > >  }
> > >  #endif
> > >  
> > > +static int all_zone_enough_free_pages(pg_data_t *pgdat)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < pgdat->nr_zones; i++) {
> > > +		struct zone *zone = pgdat->node_zones + i;
> > > +
> > > +		if (!populated_zone(zone))
> > > +			continue;
> > > +
> > > +		if (zone->all_unreclaimable)
> > > +			continue;
> > > +
> > > +		if (!zone_watermark_ok(zone, 0, high_wmark_pages(zone) * 8,
> > > +								0, 0))
> > > +			return 0;
> > > +	}
> > > +	return 1;
> > > +}
> > > +
> > >  /* is kswapd sleeping prematurely? */
> > >  static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> > >  {
> > > @@ -2355,7 +2375,8 @@ out:
> > >  		 * back to sleep. High-order users can still perform direct
> > >  		 * reclaim if they wish.
> > >  		 */
> > > -		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > > +		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX ||
> > > +		    (order > 0 && all_zone_enough_free_pages(pgdat)))
> > >  			order = sc.order = 0;
> > 
> > Ummm. this doesn't work. this place is processed every 32 pages reclaimed.
> > (see below code and comment). Theresore your patch break high order reclaim
> > logic.
> Yes, this will break high order reclaim, but we need a compromise.
> wrongly reclaim pages is more worse. could increase the watermark in
> all_zone_enough_free_pages() better?
> 

Hmm..
I guess I haven't catch your mention. you wrote 

> > > but we do if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > >                         order = sc.order = 0;
> > > this means before we set order to 0, we already reclaimed a lot of
> > > pages

and I wrote it's not a lot. So, I don't understand why you are talking
about watermark increasing now. Personally you seems to talk unrelated
topic. Can you please elablate your point more?






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