linux-kernel - Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch

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Message-ID: <20130528001802.GB1359@amt.cnet>
Date:	Mon, 27 May 2013 21:18:02 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch

On Mon, May 27, 2013 at 10:20:12AM +0800, Xiao Guangrong wrote:
> On 05/25/2013 04:34 AM, Marcelo Tosatti wrote:
> > On Thu, May 23, 2013 at 03:55:53AM +0800, Xiao Guangrong wrote:
> >> Zap at lease 10 pages before releasing mmu-lock to reduce the overload
> >> caused by requiring lock
> >>
> >> After the patch, kvm_zap_obsolete_pages can forward progress anyway,
> >> so update the comments
> >>
> >> [ It improves kernel building 0.6% ~ 1% ]
> > 
> > Can you please describe the overload in more detail? Under what scenario
> > is kernel building improved?
> 
> Yes.
> 
> The scenario is we do kernel building, meanwhile, repeatedly read PCI rom
> every one second.
> 
> [
>    echo 1 > /sys/bus/pci/devices/0000\:00\:03.0/rom
>    cat /sys/bus/pci/devices/0000\:00\:03.0/rom > /dev/null
> ]

Can't see why it reflects real world scenario (or a real world
scenario with same characteristics regarding kvm_mmu_zap_all vs faults)?

Point is, it would be good to understand why this change 
is improving performance? What are these cases where breaking out of
kvm_mmu_zap_all due to either (need_resched || spin_needbreak) on zapped
< 10 ?



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