linux-kernel - Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning requirements for kswapd

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Date:	Fri, 22 Mar 2013 11:04:49 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
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>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] mm: vmscan: Obey proportional scanning
 requirements for kswapd

On Fri 22-03-13 08:37:04, Mel Gorman wrote:
> On Fri, Mar 22, 2013 at 08:54:27AM +0100, Michal Hocko wrote:
> > On Thu 21-03-13 15:34:42, Mel Gorman wrote:
> > > On Thu, Mar 21, 2013 at 04:07:55PM +0100, Michal Hocko wrote:
> > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > > > index 4835a7a..182ff15 100644
> > > > > > > --- a/mm/vmscan.c
> > > > > > > +++ b/mm/vmscan.c
> > > > > > > @@ -1815,6 +1815,45 @@ out:
> > > > > > >  	}
> > > > > > >  }
> > > > > > >  
> > > > > > > +static void recalculate_scan_count(unsigned long nr_reclaimed,
> > > > > > > +		unsigned long nr_to_reclaim,
> > > > > > > +		unsigned long nr[NR_LRU_LISTS])
> > > > > > > +{
> > > > > > > +	enum lru_list l;
> > > > > > > +
> > > > > > > +	/*
> > > > > > > +	 * For direct reclaim, reclaim the number of pages requested. Less
> > > > > > > +	 * care is taken to ensure that scanning for each LRU is properly
> > > > > > > +	 * proportional. This is unfortunate and is improper aging but
> > > > > > > +	 * minimises the amount of time a process is stalled.
> > > > > > > +	 */
> > > > > > > +	if (!current_is_kswapd()) {
> > > > > > > +		if (nr_reclaimed >= nr_to_reclaim) {
> > > > > > > +			for_each_evictable_lru(l)
> > > > > > > +				nr[l] = 0;
> > > > > > > +		}
> > > > > > > +		return;
> > > > > > 
> > > > > > Heh, this is nicely cryptically said what could be done in shrink_lruvec
> > > > > > as
> > > > > > 	if (!current_is_kswapd()) {
> > > > > > 		if (nr_reclaimed >= nr_to_reclaim)
> > > > > > 			break;
> > > > > > 	}
> > > > > > 
> > > > > 
> > > > > Pretty much. At one point during development, this function was more
> > > > > complex and it evolved into this without me rechecking if splitting it
> > > > > out still made sense.
> > > > > 
> > > > > > Besides that this is not memcg aware which I think it would break
> > > > > > targeted reclaim which is kind of direct reclaim but it still would be
> > > > > > good to stay proportional because it starts with DEF_PRIORITY.
> > > > > > 
> > > > > 
> > > > > This does break memcg because it's a special sort of direct reclaim.
> > > > > 
> > > > > > I would suggest moving this back to shrink_lruvec and update the test as
> > > > > > follows:
> > > > > 
> > > > > I also noticed that we check whether the scan counts need to be
> > > > > normalised more than once
> > > > 
> > > > I didn't mind this because it "disqualified" at least one LRU every
> > > > round which sounds reasonable to me because all LRUs would be scanned
> > > > proportionally.
> > > 
> > > Once the scan count for one LRU is 0 then min will always be 0 and no
> > > further adjustment is made. It's just redundant to check again.
> > 
> > Hmm, I was almost sure I wrote that min should be adjusted only if it is >0
> > in the first loop but it is not there...
> > 
> > So for real this time.
> > 			for_each_evictable_lru(l)
> > 				if (nr[l] && nr[l] < min)
> > 					min = nr[l];
> > 
> > This should work, no? Everytime you shrink all LRUs you and you have
> > reclaimed enough already you get the smallest LRU out of game. This
> > should keep proportions evenly.
> 
> Lets say we started like this
> 
> LRU_INACTIVE_ANON	  60
> LRU_ACTIVE_FILE		1000
> LRU_INACTIVE_FILE	3000
> 
> and we've reclaimed nr_to_reclaim pages then we recalculate the number
> of pages to scan from each list as;
> 
> LRU_INACTIVE_ANON	  0
> LRU_ACTIVE_FILE		940
> LRU_INACTIVE_FILE      2940
> 
> We then shrink SWAP_CLUSTER_MAX from each LRU giving us this.
> 
> LRU_INACTIVE_ANON	  0
> LRU_ACTIVE_FILE		908
> LRU_INACTIVE_FILE      2908
> 
> Then under your suggestion this would be recalculated as
> 
> LRU_INACTIVE_ANON	  0
> LRU_ACTIVE_FILE		  0
> LRU_INACTIVE_FILE      2000
> 
> another SWAP_CLUSTER_MAX reclaims and then it stops we stop reclaiming. I
> might still be missing the point of your suggestion but I do not think it
> would preserve the proportion of pages we reclaim from the anon or file LRUs.

It wouldn't preserve proportion precisely because each reclaim round is
in SWAP_CLUSTER_MAX units but it would reclaim bigger lists more than
smaller ones which I thought was the whole point. So yes using word
"proportionally" is unfortunate but I didn't find out better one.
-- 
Michal Hocko
SUSE Labs
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