linux-kernel - Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

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Date:	Wed, 29 Jun 2011 20:28:37 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

On 06/29/2011 08:18 PM, Avi Kivity wrote:
> On 06/29/2011 02:50 PM, Xiao Guangrong wrote:
>> >>  >
>> >>  >   I think we should do this unconditionally.  The cost of ping-ponging the shared cache line containing reader_counter will increase with large smp counts.  On the other hand, zap_page is very rare, so it can be a little slower.  Also, less code paths = easier to understand.
>> >>  >
>> >>
>> >>  On soft mmu, zap_page is very frequently, it can cause performance regression in my test.
>> >
>> >  Any idea what the cause of the regression is?  It seems to me that simply deferring freeing shouldn't have a large impact.
>> >
>>
>> I guess it is because the page is freed too frequently, i have done the test, it shows
>> about 3219 pages is freed per second
>>
>> Kernbench performance comparing:
>>
>> the origin way: 3m27.723
>> free all shadow page in rcu context: 3m30.519
> 
> I don't recall seeing such a high free rate.  Who is doing all this zapping?
> 
> You may be able to find out with the function tracer + call graph.
> 

I looked into it before, it is caused by "write flood" detected, i also noticed
some pages are zapped and allocation again and again, maybe we need to improve
the algorithm of detecting "write flood".
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