linux-kernel - Re: [PATCH v2 0/4] KVM: optimize the kvm_vcpu_on

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Date:   Fri, 11 Aug 2017 09:19:53 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Eric Farman <farman@...ux.vnet.ibm.com>
Cc:     "Longpeng (Mike)" <longpeng2@...wei.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, agraf@...e.com, borntraeger@...ibm.com,
        christoffer.dall@...aro.org, marc.zyngier@....com,
        james.hogan@...tec.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, weidong.huang@...wei.com,
        arei.gonglei@...wei.com, wangxinxin.wang@...wei.com,
        longpeng.mike@...il.com, david@...hat.com
Subject: Re: [PATCH v2 0/4] KVM: optimize the kvm_vcpu_on_spin

On Thu, 10 Aug 2017 09:18:09 -0400
Eric Farman <farman@...ux.vnet.ibm.com> wrote:

> On 08/08/2017 04:14 AM, Longpeng (Mike) wrote:
> > 
> > 
> > On 2017/8/8 15:41, Cornelia Huck wrote:
> >   
> >> On Tue, 8 Aug 2017 12:05:31 +0800
> >> "Longpeng(Mike)" <longpeng2@...wei.com> wrote:
> >>  
> >>> This is a simple optimization for kvm_vcpu_on_spin, the
> >>> main idea is described in patch-1's commit msg.  
> >>
> >> I think this generally looks good now.
> >>  
> >>>
> >>> I did some tests base on the RFC version, the result shows
> >>> that it can improves the performance slightly.  
> >>
> >> Did you re-run tests on this version?  
> > 
> > 
> > Hi Cornelia,
> > 
> > I didn't re-run tests on V2. But the major difference between RFC and V2
> > is that V2 only cache result for X86 (s390/arm needn't) and V2 saves a
> > expensive operation ( 440-1400 cycles on my test machine ) for X86/VMX.
> > 
> > So I think V2's performance is at least the same as RFC or even slightly
> > better. :)
> >   
> >>
> >> I would also like to see some s390 numbers; unfortunately I only have a
> >> z/VM environment and any performance numbers would be nearly useless
> >> there. Maybe somebody within IBM with a better setup can run a quick
> >> test?  
> 
> Won't swear I didn't screw something up, but here's some quick numbers. 
> Host was 4.12.0 with and without this series, running QEMU 2.10.0-rc0. 
> Created 4 guests, each with 4 CPU (unpinned) and 4GB RAM.  VM1 did full 
> kernel compiles with kernbench, which took averages of 5 runs of 
> different job sizes (I threw away the "-j 1" numbers). VM2-VM4 ran cpu 
> burners on 2 of their 4 cpus.
> 
> Numbers from VM1 kernbench output, and the delta between runs:
> 
> load -j 3		before		after		delta
> Elapsed Time		183.178		182.58		-0.598
> User Time		534.19		531.52		-2.67
> System Time		32.538		33.37		0.832
> Percent CPU		308.8		309		0.2
> Context Switches	98484.6		99001		516.4
> Sleeps			227347		228752		1405
> 
> load -j 16		before		after		delta
> Elapsed Time		153.352		147.59		-5.762
> User Time		545.829		533.41		-12.419
> System Time		34.289		34.85		0.561
> Percent CPU		347.6		348		0.4
> Context Switches	160518		159120		-1398
> Sleeps			240740		240536		-204

Thanks a lot, Eric!

The decreases in elapsed time look nice, and we probably should not
care about the increases reported.