Re: [PATCH 1/2] vmscan: promote shared file mapped pages

[Konstantin Khlebnikov]

Pekka Enberg wrote:
> On Mon, Aug 8, 2011 at 3:18 PM, Konstantin Khlebnikov
> <khlebnikov@xxxxxxxxxxxxx>  wrote:
> > Pekka Enberg wrote:
> > > Hi Konstantin,
> > >
> > > On Mon, Aug 8, 2011 at 2:06 PM, Konstantin Khlebnikov
> > > <khlebnikov@xxxxxxxxxx>    wrote:
> > > > Commit v2.6.33-5448-g6457474 (vmscan: detect mapped file pages used only
> > > > once)
> > > > greatly decreases lifetime of single-used mapped file pages.
> > > > Unfortunately it also decreases life time of all shared mapped file
> > > > pages.
> > > > Because after commit v2.6.28-6130-gbf3f3bc (mm: don't mark_page_accessed
> > > > in fault path)
> > > > page-fault handler does not mark page active or even referenced.
> > > >
> > > > Thus page_check_references() activates file page only if it was used
> > > > twice while
> > > > it stays in inactive list, meanwhile it activates anon pages after first
> > > > access.
> > > > Inactive list can be small enough, this way reclaimer can accidentally
> > > > throw away any widely used page if it wasn't used twice in short period.
> > > >
> > > > After this patch page_check_references() also activate file mapped page
> > > > at first
> > > > inactive list scan if this page is already used multiple times via
> > > > several ptes.
> > > >
> > > > Signed-off-by: Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx>
> > >
> > > Both patches seem reasonable but the changelogs don't really explain
> > > why you're doing the changes. How did you find out about the problem?
> > > Is there some workload that's affected? How did you test your changes?
> >
> > I found this while trying to fix degragation in rhel6 (~2.6.32) from rhel5
> > (~2.6.18).
> > There a complete mess with>100 web/mail/spam/ftp containers,
> > they share all their files but there a lot of anonymous pages:
> > ~500mb shared file mapped memory and 15-20Gb non-shared anonymous memory.
> > In this situation major-pagefaults are very costly, because all containers
> > share the same page.
> > In my load kernel created a disproportionate pressure on the file memory,
> > compared with the anonymous,
> > they equaled only if I raise swappiness up to 150 =)
> >
> > These patches actually wasn't helped a lot in my problem,
> > but I saw noticable (10-20 times) reduce in count and average time of
> > major-pagefault in file-mapped areas.
> >
> > Actually both patches are fixes for commit v2.6.33-5448-g6457474,
> > because it was aimed at one scenario (singly used pages),
> > but it breaks the logic in other scenarios (shared and/or executable pages)
>
> It'd be nice to have such details in the changelogs. FWIW,

It's not quite honest, I did not do any measurements in the mainline kernel,
I only boot 3.1-rc1 with them on my laptop.
It would be nice to repeat the measurements from v2.6.30-5507-g8cab475,
but I do not have time for that right now.

> Acked-by: Pekka Enberg<penberg@xxxxxxxxxx>

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