linux-kernel - Re: [PATCH 2/4] vmscan: get_scan

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Message-Id: <20091230214955.1A10.A69D9226@jp.fujitsu.com>
Date:	Wed, 30 Dec 2009 22:06:46 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/4] vmscan: get_scan_ratio cleanup

> * KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> [2009-12-28 16:48:06]:
> 
> > The get_scan_ratio() should have all scan-ratio related calculations.
> > Thus, this patch move some calculation into get_scan_ratio.
> > 
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > ---
> >  mm/vmscan.c |   23 ++++++++++++++---------
> >  1 files changed, 14 insertions(+), 9 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2bbee91..640486b 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1501,6 +1501,13 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
> >  	unsigned long ap, fp;
> >  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> > 
> > +	/* If we have no swap space, do not bother scanning anon pages. */
> > +	if (!sc->may_swap || (nr_swap_pages <= 0)) {
> > +		percent[0] = 0;
> > +		percent[1] = 100;
> > +		return;
> > +	}
> > +
> 
> 
> >  	anon  = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_ANON) +
> >  		zone_nr_lru_pages(zone, sc, LRU_INACTIVE_ANON);
> >  	file  = zone_nr_lru_pages(zone, sc, LRU_ACTIVE_FILE) +
> > @@ -1598,22 +1605,20 @@ static void shrink_zone(int priority, struct zone *zone,
> >  	unsigned long nr_reclaimed = sc->nr_reclaimed;
> >  	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> >  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> > -	int noswap = 0;
> > 
> > -	/* If we have no swap space, do not bother scanning anon pages. */
> > -	if (!sc->may_swap || (nr_swap_pages <= 0)) {
> > -		noswap = 1;
> > -		percent[0] = 0;
> > -		percent[1] = 100;
> > -	} else
> > -		get_scan_ratio(zone, sc, percent);
> > +	get_scan_ratio(zone, sc, percent);
> 
> Where do we set noswap? Is percent[0] == 0 used to indicate noswap =
> 1?

Yes, I intended so. I guess your question can convert next sentence.

	following case makes different result, is it intentional?
	  - there are free swap
	  - sc->may_swap == 1
	  - priority == 0
	  - percent[0] == 0

My answer is, it isn't happen on practical workload. if priority reach to 0, vmscan always
scan and reclaim some anon (please recall, now vmscan automatically enable lumpy_reclaim
if priority < 10), then recent_rotated_anon isn't 0 always.. practically.

Do you think this is wrong assumption?


> >  	for_each_evictable_lru(l) {
> >  		int file = is_file_lru(l);
> >  		unsigned long scan;
> > 
> > +		if (percent[file] == 0) {
> > +			nr[l] = 0;
> > +			continue;
> > +		}
> > +
> 
> Is this really needed? Won't nr_scan_try_batch handle it correctly?

this two if branch is nicer than "priority || noswap", I think.
it clearly explain what do it.

> 
> >  		scan = zone_nr_lru_pages(zone, sc, l);
> > -		if (priority || noswap) {
> > +		if (priority) {
> >  			scan >>= priority;
> >  			scan = (scan * percent[file]) / 100;
> >  		}
> 
> -- 
> 	Balbir



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