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

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Message-ID: <4A7ADED1.5030408@redhat.com>
Date:	Thu, 06 Aug 2009 16:46:57 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.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?

On 08/06/2009 04:06 PM, Wu Fengguang wrote:
> On Thu, Aug 06, 2009 at 07:44:01PM +0800, Avi Kivity wrote:
>    
>> On 08/06/2009 01:59 PM, Wu Fengguang wrote:
>>      
>
> scheme KEEP_MOST:
>
>    
>>> How about, for every N pages that you scan, evict at least 1 page,
>>> regardless of young bit status?  That limits overscanning to a N:1
>>> ratio.  With N=250 we'll spend at most 25 usec in order to locate one
>>> page to evict.
>>>        
>
> scheme DROP_CONTINUOUS:
>
>    
>>> This is a quick hack to materialize the idea. It remembers roughly
>>> the last 32*SWAP_CLUSTER_MAX=1024 active (mapped) pages scanned,
>>> and if _all of them_ are referenced, then the referenced bit is
>>> probably meaningless and should not be taken seriously.
>>>        
>
>    

Or one scheme, with N=parameter.

>> I don't think we should ignore the referenced bit. There could still be
>> a large batch of unreferenced pages later on that we should
>> preferentially swap. If we swap at least 1 page for every 250 scanned,
>> after 4K swaps we will have traversed 1M pages, enough to find them.
>>      
>
> I guess both schemes have unacceptable flaws.
>
> For JVM/BIGMEM workload, most pages would be found referenced _all the time_.
> So the KEEP_MOST scheme could increase reclaim overheads by N=250 times;
> while the DROP_CONTINUOUS scheme is effectively zero cost.
>    

Maybe 250 is an exaggeration.  But even with 250, the cost is still 
pretty low compared to the cpu cost of evicting a page (with IPIs and 
tlb flushes).

-- 
error compiling committee.c: too many arguments to function

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