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

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Date:	Mon, 8 Aug 2011 16:51:35 +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:
> On Mon, Aug 8, 2011 at 3:18 PM, Konstantin Khlebnikov
> <khlebnikov@...allels.com>  wrote:
>> 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)
>
> 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@...nel.org>

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