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

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Message-ID: <4E0B182C.1090402@redhat.com>
Date:	Wed, 29 Jun 2011 15:18:52 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.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 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.

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

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