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

[Gleb Natapov <gleb@xxxxxxxxxx>]

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.

--
                        Gleb.

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