Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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

On 07/11/2013 03:18 PM, Gleb Natapov wrote:
> On Thu, Jul 11, 2013 at 02:43:03PM +0530, Raghavendra K T wrote:
>> On 07/10/2013 04:03 PM, Gleb Natapov wrote:
>> [...] trimmed
>>
>>>>> Yes. you are right. dynamic ple window was an attempt to solve it.
>>>>>
>>>>> Probelm is, reducing the SPIN_THRESHOLD is resulting in excess halt
>>>>> exits in under-commits and increasing ple_window may be sometimes
>>>>> counter productive as it affects other busy-wait constructs such as
>>>>> flush_tlb AFAIK.
>>>>> So if we could have had a dynamically changing SPIN_THRESHOLD too, that
>>>>> would be nice.
>>>>>
>>>>
>>>> Gleb, Andrew,
>>>> I tested with the global ple window change (similar to what I posted
>>>> here https://lkml.org/lkml/2012/11/11/14 ),
>>> This does not look global. It changes PLE per vcpu.
>>
>> Okay. Got it. I was thinking it would change the global value. But IIRC
>>   It is changing global sysfs value and per vcpu ple_window.
>> Sorry. I missed this part yesterday.
>>
> Yes, it changes sysfs value but this does not affect already created
> vcpus.
>
>>>
>>>> But did not see good result. May be it is good to go with per VM
>>>> ple_window.
>>>>
>>>> Gleb,
>>>> Can you elaborate little more on what you have in mind regarding per
>>>> VM ple_window. (maintaining part of it as a per vm variable is clear
>>>> to
>>>>   me), but is it that we have to load that every time of guest entry?
>>>>
>>> Only when it changes, shouldn't be to often no?
>>
>> Ok. Thinking how to do. read the register and writeback if there need
>> to be a change during guest entry?
>>
> Why not do it like in the patch you've linked? When value changes write it
> to VMCS of the current vcpu.
>

Yes. can be done. So the running vcpu's ple_window gets updated only
after next pl-exit. right?