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

[Mel Gorman <mgorman@suse.de>]

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.

-- 
Mel Gorman
SUSE Labs