Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 10/04/2012 08:11 PM, Andrew Theurer wrote:
> On Thu, 2012-10-04 at 14:41 +0200, Avi Kivity wrote:
>> On 10/04/2012 12:49 PM, Raghavendra K T wrote:
>>> On 10/03/2012 10:35 PM, Avi Kivity wrote:
>>>> On 10/03/2012 02:22 PM, Raghavendra K T wrote:
>>>>>> So I think it's worth trying again with ple_window of 20000-40000.
>>>>>>
>>>>>
>>>>> Hi Avi,
>>>>>
>>>>> I ran different benchmarks increasing ple_window, and results does not
>>>>> seem to be encouraging for increasing ple_window.
>>>>
>>>> Thanks for testing! Comments below.
>>>>
>>>>> Results:
>>>>> 16 core PLE machine with 16 vcpu guest.
>>>>>
>>>>> base kernel = 3.6-rc5 + ple handler optimization patch
>>>>> base_pleopt_8k = base kernel + ple window = 8k
>>>>> base_pleopt_16k = base kernel + ple window = 16k
>>>>> base_pleopt_32k = base kernel + ple window = 32k
>>>>>
>>>>>
>>>>> Percentage improvements of benchmarks w.r.t base_pleopt with
>>>>> ple_window = 4096
>>>>>
>>>>>          base_pleopt_8k    base_pleopt_16k    base_pleopt_32k
>>>>> -----------------------------------------------------------------
>>>>>
>>>>> kernbench_1x    -5.54915    -15.94529    -44.31562
>>>>> kernbench_2x    -7.89399    -17.75039    -37.73498
>>>>
>>>> So, 44% degradation even with no overcommit?  That's surprising.
>>>
>>> Yes. Kernbench was run with #threads = #vcpu * 2 as usual. Is it
>>> spending 8 times the original ple_window cycles for 16 vcpus
>>> significant?
>>
>> A PLE exit when not overcommitted cannot do any good, it is better to
>> spin in the guest rather that look for candidates on the host.  In fact
>> when we benchmark we often disable PLE completely.
>
> Agreed.  However, I really do not understand why the kernbench regressed
> with bigger ple_window.  It should stay the same or improve.  Raghu, do
> you have perf data for the kernbench runs?

Andrew, No. 'll get this with perf kvm.