linux-kernel - Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapd reclaims at each priority

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Date:	Tue, 19 Mar 2013 18:16:50 +0800
From:	Simon Jeons <simon.jeons@...il.com>
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>,
	Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapd reclaims
 at each priority

Hi Mel,
On 03/19/2013 05:55 PM, Mel Gorman wrote:
> On Tue, Mar 19, 2013 at 07:53:16AM +0800, Simon Jeons wrote:
>> Hi Mel,
>> On 03/17/2013 09:04 PM, Mel Gorman wrote:
>>> The number of pages kswapd can reclaim is bound by the number of pages it
>>> scans which is related to the size of the zone and the scanning priority. In
>>> many cases the priority remains low because it's reset every SWAP_CLUSTER_MAX
>>> reclaimed pages but in the event kswapd scans a large number of pages it
>>> cannot reclaim, it will raise the priority and potentially discard a large
>>> percentage of the zone as sc->nr_to_reclaim is ULONG_MAX. The user-visible
>>> effect is a reclaim "spike" where a large percentage of memory is suddenly
>>> freed. It would be bad enough if this was just unused memory but because
>> Since there is nr_reclaimed >= nr_to_reclaim check if priority is
>> large than DEF_PRIORITY in shrink_lruvec, how can a large percentage
>> of memory is suddenly freed happen?
>>
> Because of the priority checks made in get_scan_count(). Patch 5 has
> more detail on why this happens.
>
But nr_reclaim >= nr_to_reclaim check in function shrink_lruvec is after 
scan each evictable lru, so if priority == 0, still scan the whole world.
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