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

Eric Farman
2017-08-10T13:18:09Z

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

- Eric