Re: [question] call mark_page_accessed() in minor fault

[Zheng Liu <gnehzuil.liu@gmail.com>]

Hi Konstantin,

On Tue, Apr 23, 2013 at 05:02:34PM +0400, Konstantin Khlebnikov wrote:
> Zheng Liu wrote:
> >Hi all,
> >
> >Recently we meet a performance regression about mmaped page.  When we upgrade
> >our product system from 2.6.18 kernel to a latest kernel, such as 2.6.32 kernel,
> >we will find that mmaped pages are reclaimed very quickly.  We found that when
> >we hit a minor fault mark_page_accessed() is called in 2.6.18 kernel, but in
> >2.6.32 kernel we don't call mark_page_accesed().  This means that mmaped pages
> >in 2.6.18 kernel are activated and moved into active list.  While in 2.6.32
> >kernel mmaped pages are still kept in inactive list.
> >
> >So my question is why we call mark_page_accessed() in 2.6.18 kernel, but don't
> >call it in 2.6.32 kernel.  Has any reason here?
> 
> Behavior was changed in commit
> v2.6.28-6130-gbf3f3bc "mm: don't mark_page_accessed in fault path"

Thanks for pointing it out.

> 
> Please see also commits
> v3.2-4876-g34dbc67 "vmscan: promote shared file mapped pages" and

Yes, I will give it try.  If I understand correctly, this commit is
useful for multi-processes program that access a shared mmaped page,
but that could not be useful for us because our program is multi-thread.

> v3.2-4877-gc909e99 "vmscan: activate executable pages after first usage".

We have backported this patch, but it is useless.  This commit only
tries to activate a executable page, but our mmaped pages aren't with
this flag.

Additional question is that currently mmaped page is reclaimed too
quickly.  I think maybe we need to adjust our page reclaim strategy to
balance number of pages between mmaped page and file page cache.  Now
every time we access a page using read(2)/write(2), this page will be
touched.  But after first time we touch a mmaped page, we never touch it
again except that this page is evicted.

Regards,
                                                - Zheng

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