Re: [Xen-devel] [PATCH RFC V4 0/5] kvm : Paravirt-spinlock support for KVM guests

[Alexander Graf <agraf@xxxxxxx>]

On 17.01.2012, at 18:27, Raghavendra K T wrote:

> On 01/17/2012 12:12 AM, Alexander Graf wrote:
>> 
>> On 16.01.2012, at 19:38, Raghavendra K T wrote:
>> 
>>> On 01/16/2012 07:53 PM, Alexander Graf wrote:
>>>> 
>>>> On 16.01.2012, at 15:20, Srivatsa Vaddagiri wrote:
>>>> 
>>>>> * Alexander Graf<agraf@xxxxxxx>   [2012-01-16 04:57:45]:
>>>>> 
>>>>>> Speaking of which - have you benchmarked performance degradation of pv 
>>>>>> ticket locks on bare metal?
>>>>> 
>>>>> You mean, run kernel on bare metal with CONFIG_PARAVIRT_SPINLOCKS
>>>>> enabled and compare how it performs with CONFIG_PARAVIRT_SPINLOCKS 
>>>>> disabled for
>>>>> some workload(s)?
>>>> 
>>>> Yup
>>>> 
>>>>> 
>>>>> In some sense, the 1x overcommitcase results posted does measure the 
>>>>> overhead
>>>>> of (pv-)spinlocks no? We don't see any overhead in that case for atleast
>>>>> kernbench ..
>>>>> 
>>>>>> Result for Non PLE machine :
>>>>>> ============================
>>>>> 
>>>>> [snip]
>>>>> 
>>>>>> Kernbench:
>>>>>>               BASE                    BASE+patch
>>>> 
>>>> What is BASE really? Is BASE already with the PV spinlocks enabled? I'm 
>>>> having a hard time understanding which tree you're working against, since 
>>>> the prerequisites aren't upstream yet.
>>>> 
>>>> 
>>>> Alex
>>> 
>>> Sorry for confusion, I think I was little imprecise on the BASE.
>>> 
>>> The BASE is pre 3.2.0 + Jeremy's following patches:
>>> xadd (https://lkml.org/lkml/2011/10/4/328)
>>> x86/ticketlocklock  (https://lkml.org/lkml/2011/10/12/496).
>>> So this would have ticketlock cleanups from Jeremy and
>>> CONFIG_PARAVIRT_SPINLOCKS=y
>>> 
>>> BASE+patch = pre 3.2.0 + Jeremy's above patches + above V5 PV spinlock
>>> series and CONFIG_PARAVIRT_SPINLOCKS=y
>>> 
>>> In both the cases  CONFIG_PARAVIRT_SPINLOCKS=y.
>>> 
>>> So let,
>>> A. pre-3.2.0 with CONFIG_PARAVIRT_SPINLOCKS = n
>>> B. pre-3.2.0 + Jeremy's above patches with CONFIG_PARAVIRT_SPINLOCKS = n
>>> C. pre-3.2.0 + Jeremy's above patches with CONFIG_PARAVIRT_SPINLOCKS = y
>>> D. pre-3.2.0 + Jeremy's above patches + V5 patches with 
>>> CONFIG_PARAVIRT_SPINLOCKS = n
>>> E. pre-3.2.0 + Jeremy's above patches + V5 patches with 
>>> CONFIG_PARAVIRT_SPINLOCKS = y
>>> 
>>> is it performance of A vs E ? (currently C vs E)
>> 
>> Since D and E only matter with KVM in use, yes, I'm mostly interested in A, 
>> B and C :).
>> 
>> 
>> Alex
>> 
>> 
> setup :
> Native: IBM xSeries with Intel(R) Xeon(R) x5570 2.93GHz CPU with 8 core , 
> 64GB RAM, (16 cpu online)
> 
> Guest : Single guest with 8 VCPU 4GB Ram.
> benchmark : kernbench -f -H -M -o 20
> 
> Here is the result :
> Native Run
> ============
> case A               case B             %improvement   case C  %improvement
> 56.1917 (2.57125)    56.035 (2.02439)   0.278867       56.27 (2.40401)   
> -0.139344    

This looks a lot like statistical derivation. How often did you execute the 
test case? Did you make sure to have a clean base state every time?

Maybe it'd be a good idea to create a small in-kernel microbenchmark with a 
couple threads that take spinlocks, then do work for a specified number of 
cycles, then release them again and start anew. At the end of it, we can check 
how long the whole thing took for n runs. That would enable us to measure the 
worst case scenario.

> 
> Guest Run
> ============
> case A               case B             %improvement   case C  %improvement
> 166.999 (15.7613)    161.876 (14.4874)        3.06768        161.24 (12.6497) 
>  3.44852

Is this the same machine? Why is the guest 3x slower?


Alex

> 
> We do not see much overhead in native run with CONFIG_PARAVIRT_SPINLOCKS = y
> 


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