linux-kernel - Re: [PATCH v2] vmscan: reverse lru scanning order

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Date:	Mon, 8 Aug 2011 14:22:55 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH v2] vmscan: reverse lru scanning order

On Mon 08-08-11 15:02:07, Konstantin Khlebnikov wrote:
> LRU scanning order was accidentially changed in commit v2.6.27-5584-gb69408e:
> "vmscan: Use an indexed array for LRU variables".
> Before that commit reclaimer always scan active lists first.
> 
> This patch just reverse it back.

I am still not sure I see why the ordering matters that much.
One thing that might matter is that shrink_list moves some pages from
active to inactive list if inactive is low so it makes sense to try to
shrink active before inactive. It would be a problem if inactive was
almost empty. Then we would just waste time by shrinking inactive first.
I am not sure how real problem is that, though. 

Whatever is the reason, I think it should be documented in the
changelog.
The change makes sense to me.

> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> ---
>  include/linux/mmzone.h |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index be1ac8d..0094389 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -141,7 +141,8 @@ enum lru_list {
>  
>  #define for_each_lru(l) for (l = 0; l < NR_LRU_LISTS; l++)
>  
> -#define for_each_evictable_lru(l) for (l = 0; l <= LRU_ACTIVE_FILE; l++)
> +#define for_each_evictable_lru(l) \
> +	for (l = LRU_ACTIVE_FILE; (int)l >= LRU_INACTIVE_ANON; l--)
>  
>  static inline int is_file_lru(enum lru_list l)
>  {
> 
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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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