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

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Date:	Mon, 8 Aug 2011 16:18:11 +0400
From:	Konstantin Khlebnikov <khlebnikov@...allels.com>
To:	Pekka Enberg <penberg@...nel.org>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/2] vmscan: promote shared file mapped pages

Pekka Enberg wrote:
> Hi Konstantin,
>
> On Mon, Aug 8, 2011 at 2:06 PM, Konstantin Khlebnikov
> <khlebnikov@...nvz.org>  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@...nvz.org>
>
> 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)

>                         Pekka

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