linux-kernel - Re: [RFC] respect the referenced bit of KVM guest pages?

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Message-ID: <4A87829C.4090908@redhat.com>
Date:	Sat, 15 Aug 2009 23:53:00 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Jeff Dike <jdike@...toit.com>, Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?

Wu Fengguang wrote:
> On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote:
>> Side question -
>> 	Is there a good reason for this to be in shrink_active_list()
>> as opposed to __isolate_lru_page?
>>
>> 		if (unlikely(!page_evictable(page, NULL))) {
>> 			putback_lru_page(page);
>> 			continue;
>> 		}
>>
>> Maybe we want to minimize the amount of code under the lru lock or
>> avoid duplicate logic in the isolate_page functions.
> 
> I guess the quick test means to avoid the expensive page_referenced()
> call that follows it. But that should be mostly one shot cost - the
> unevictable pages are unlikely to cycle in active/inactive list again
> and again.

Please read what putback_lru_page does.

It moves the page onto the unevictable list, so that
it will not end up in this scan again.

>> But if there are important mlock-heavy workloads, this could make the
>> scan come up empty, or at least emptier than we might like.
> 
> Yes, if the above 'if' block is removed, the inactive lists might get
> more expensive to reclaim.

Why?

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