linux-kernel - Re: [PATCH] vmscan: do not force-scan file lru if its absolute size is small

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Message-Id: <20151120134311.8ff0947215fc522f72f791fe@linux-foundation.org>
Date:	Fri, 20 Nov 2015 13:43:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Michal Hocko <mhocko@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vmscan: do not force-scan file lru if its absolute size
 is small

On Fri, 20 Nov 2015 13:37:07 -0500 Johannes Weiner <hannes@...xchg.org> wrote:

> On Fri, Nov 20, 2015 at 08:02:56PM +0300, Vladimir Davydov wrote:
> > We assume there is enough inactive page cache if the size of inactive
> > file lru is greater than the size of active file lru, in which case we
> > force-scan file lru ignoring anonymous pages. While this logic works
> > fine when there are plenty of page cache pages, it fails if the size of
> > file lru is small (several MB): in this case (lru_size >> prio) will be
> > 0 for normal scan priorities, as a result, if inactive file lru happens
> > to be larger than active file lru, anonymous pages of a cgroup will
> > never get evicted unless the system experiences severe memory pressure,
> > even if there are gigabytes of unused anonymous memory there, which is
> > unfair in respect to other cgroups, whose workloads might be page cache
> > oriented.
> > 
> > This patch attempts to fix this by elaborating the "enough inactive page
> > cache" check: it makes it not only check that inactive lru size > active
> > lru size, but also that we will scan something from the cgroup at the
> > current scan priority. If these conditions do not hold, we proceed to
> > SCAN_FRACT as usual.
> > 
> > Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>
> 
> This makes sense, the inactive:active ratio of the file list alone
> does not give the full picture to decide whether to skip anonymous.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> > @@ -2046,7 +2046,8 @@ static void get_scan_count(struct lruvec *lruvec, int swappiness,
> >  	 * There is enough inactive page cache, do not reclaim
> >  	 * anything from the anonymous working set right now.
> >  	 */
> > -	if (!inactive_file_is_low(lruvec)) {
> > +	if (!inactive_file_is_low(lruvec) &&
> > +	    get_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority > 0) {
> 
> The > 0 seems unnecessary, no? There are too many > in this line :-)

And an update to the code comment would be helpful.
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