linux-kernel - Re: [PATCH v4 3/5] memcg : stop scanning if enough

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Date:	Mon, 1 Aug 2011 16:37:46 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH v4 3/5] memcg : stop scanning if enough

On Wed 27-07-11 14:49:00, KAMEZAWA Hiroyuki wrote:
> memcg :avoid node fallback scan if possible.
> 
> Now, try_to_free_pages() scans all zonelist because the page allocator
> should visit all zonelists...but that behavior is harmful for memcg.
> Memcg just scans memory because it hits limit...no memory shortage
> in pased zonelist.
> 
> For example, with following unbalanced nodes
> 
>      Node 0    Node 1
> File 1G        0
> Anon 200M      200M
> 
> memcg will cause swap-out from Node1 at every vmscan.
> 
> Another example, assume 1024 nodes system.
> With 1024 node system, memcg will visit 1024 nodes
> pages per vmscan... This is overkilling. 
> 
> This is why memcg's victim node selection logic doesn't work
> as expected.

Previous patch adds nodemask filled by
mem_cgroup_select_victim_node. Shouldn't we rather limit that nodemask
to a victim node?

Or am I missing something?

The patch as is doesn't look nice it makes shrink_zones even more
memcg-hacky:
	for_each_zone_zonelist_nodemask
		if (scanning_global_lru(sc))
			/.../

		shrink_zone(priority, zone, sc);

		if (!scanning_global_lru(sc))
			/.../

> 
> This patch is a help for stopping vmscan when we scanned enough.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  mm/vmscan.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> Index: mmotm-0710/mm/vmscan.c
> ===================================================================
> --- mmotm-0710.orig/mm/vmscan.c
> +++ mmotm-0710/mm/vmscan.c
> @@ -2058,6 +2058,16 @@ static void shrink_zones(int priority, s
>  		}
>  
>  		shrink_zone(priority, zone, sc);
> +		if (!scanning_global_lru(sc)) {
> +			/*
> +			 * When we do scan for memcg's limit, it's bad to do
> +			 * fallback into more node/zones because there is no
> +			 * memory shortage. We quit as much as possible when
> +			 * we reache target.
> +			 */
> +			if (sc->nr_to_reclaim <= sc->nr_reclaimed)
> +				break;
> +		}
>  	}
>  }
>  
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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