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

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Date:	Fri, 20 Nov 2015 13:37:07 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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, 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 :-)
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