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

[Raghavendra K T]

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?

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