linux-kernel - Re: mmap vs fs cache

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Message-ID: <513716F3.4070100@symas.com>
Date:	Wed, 06 Mar 2013 02:14:11 -0800
From:	Howard Chu <hyc@...as.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: mmap vs fs cache

Howard Chu wrote:
> Howard Chu wrote:
>> 2 questions:
>>      why is there data in the FS cache that isn't owned by (the mmap of) the
>> process that caused it to be paged in in the first place?
>>      is there a tunable knob to discourage the page cache from stealing from the
>> process?
>
> This Unmapped page cache control http://lwn.net/Articles/436010/ sounds like
> it might have been helpful here. I.e., having a way to prioritize so that
> unmapped cache pages get reclaimed in preference to mapped pages could help.
> Though I still don't understand why these pages in the cache aren't mapped in
> the first place.
>
As implied by this post 
http://lkml.indiana.edu/hypermail/linux/kernel/0701.3/0354.html setting 
swappiness to 0 seems to give the desired effect of preventing mapped pages 
from being reclaimed. If this is an intended effect, it would be nice to have 
this documented in Documentation/sysctl/vm.txt. If this is not the intended 
effect, please don't "fix" this without providing a supported means of doing 
the same.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/
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