Re: [PATCH v2 0/4] KVM: optimize the kvm_vcpu_on_spin

Cornelia Huck
2017-08-11T07:19:53Z

On Thu, 10 Aug 2017 09:18:09 -0400
Eric Farman <farman@linux.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@huawei.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.