Re: mmap vs fs cache

Howard Chu
2013-03-06T10:14:15Z

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/