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

[Mel Gorman <mgorman@suse.de>]

On Thu, Apr 18, 2013 at 08:01:05AM -0700, Johannes Weiner wrote:
> On Thu, Apr 11, 2013 at 08:57:50PM +0100, Mel Gorman wrote:
> > @@ -1841,17 +1848,58 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> >  							    lruvec, sc);
> >  			}
> >  		}
> > +
> > +		if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
> > +			continue;
> > +
> >  		/*
> > -		 * On large memory systems, scan >> priority can become
> > -		 * really large. This is fine for the starting priority;
> > -		 * we want to put equal scanning pressure on each zone.
> > -		 * However, if the VM has a harder time of freeing pages,
> > -		 * with multiple processes reclaiming pages, the total
> > -		 * freeing target can get unreasonably large.
> > +		 * For global direct reclaim, reclaim only the number of pages
> > +		 * requested. Less care is taken to scan proportionally as it
> > +		 * is more important to minimise direct reclaim stall latency
> > +		 * than it is to properly age the LRU lists.
> >  		 */
> > -		if (nr_reclaimed >= nr_to_reclaim &&
> > -		    sc->priority < DEF_PRIORITY)
> > +		if (global_reclaim(sc) && !current_is_kswapd())
> >  			break;
> > +
> > +		/*
> > +		 * For kswapd and memcg, reclaim at least the number of pages
> > +		 * requested. Ensure that the anon and file LRUs shrink
> > +		 * proportionally what was requested by get_scan_count(). We
> > +		 * stop reclaiming one LRU and reduce the amount scanning
> > +		 * proportional to the original scan target.
> > +		 */
> > +		nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
> > +		nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
> > +
> > +		if (nr_file > nr_anon) {
> > +			unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
> > +						targets[LRU_ACTIVE_ANON] + 1;
> > +			lru = LRU_BASE;
> > +			percentage = nr_anon * 100 / scan_target;
> > +		} else {
> > +			unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
> > +						targets[LRU_ACTIVE_FILE] + 1;
> > +			lru = LRU_FILE;
> > +			percentage = nr_file * 100 / scan_target;
> > +		}
> > +
> > +		/* Stop scanning the smaller of the LRU */
> > +		nr[lru] = 0;
> > +		nr[lru + LRU_ACTIVE] = 0;
> > +
> > +		/*
> > +		 * Recalculate the other LRU scan count based on its original
> > +		 * scan target and the percentage scanning already complete
> > +		 */
> > +		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
> > +		nr[lru] = targets[lru] * (100 - percentage) / 100;
> > +		nr[lru] -= min(nr[lru], (targets[lru] - nr[lru]));
> 
> This doesn't seem right.  Say percentage is 60, then
> 
>     nr[lru] = targets[lru] * (100 - percentage) / 100;
> 
> sets nr[lru] to 40% of targets[lru], and so in
> 
>     nr[lru] -= min(nr[lru], (targets[lru] - nr[lru]));
> 
> targets[lru] - nr[lru] is 60% of targets[lru], making it bigger than
> nr[lru], which is in turn subtracted from itself, i.e. it leaves the
> remaining type at 0 if >= 50% of the other type were scanned, and at
> half of the inverted scan percentage if less than 50% were scanned.
> 
> Would this be more sensible?
> 
>     already_scanned = targets[lru] - nr[lru];
>     nr[lru] = targets[lru] * percentage / 100; /* adjusted original target */
>     nr[lru] -= min(nr[lru], already_scanned);  /* minus work already done */

Bah, yes, that was the intent as I was writing it. It's not what came
out my fingers. Thanks for the bashing with a clue stick.

-- 
Mel Gorman
SUSE Labs

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