linux-kernel - Re: [PATCH 2/9] ksm: let shared pages be swappable

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Date:	Wed, 02 Dec 2009 00:08:18 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Chris Wright <chrisw@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable

On 12/01/2009 04:59 AM, Andrea Arcangeli wrote:
> On Tue, Dec 01, 2009 at 06:46:06PM +0900, KOSAKI Motohiro wrote:
>    
>> Ah, well. please wait a bit. I'm under reviewing Larry's patch. I don't
>> dislike your idea. last mail only pointed out implementation thing.
>>      
> Yep thanks for pointing it out. It's an implementation thing I don't
> like. The VM should not ever touch ptes when there's light VM pressure
> and plenty of unmapped clean cache available, but I'm ok if others
> disagree and want to keep it that way.
>    
The VM needs to touch a few (but only a few) PTEs in
that situation, to make sure that anonymous pages get
moved to the inactive anon list and get to a real chance
at being referenced before we try to evict anonymous
pages.

Without a small amount of pre-aging, we would end up
essentially doing FIFO replacement of anonymous memory,
which has been known to be disastrous to performance
for over 40 years now.

A two-handed clock mechanism needs to put some distance
between the front and the back hands of the clock.

Having said that - it may be beneficial to keep very heavily
shared pages on the active list, without ever trying to scan
the ptes associated with them.
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