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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 07/10/2013 05:11 PM, Gleb Natapov wrote:
> On Wed, Jul 10, 2013 at 04:54:12PM +0530, Raghavendra K T wrote:
> > > > Ingo, Gleb,
> > > >
> > > >  From the results perspective, Andrew Theurer, Vinod's test results are
> > > > pro-pvspinlock.
> > > > Could you please help me to know what will make it a mergeable
> > > > candidate?.
> > > I need to spend more time reviewing it :) The problem with PV interfaces
> > > is that they are easy to add but hard to get rid of if better solution
> > > (HW or otherwise) appears.
> > Infact Avi had acked the whole V8 series, but delayed for seeing how
> > PLE improvement would affect it.
> I see that Ingo was happy with it too.
>
> > The only addition from that series has been
> > 1. tuning the SPIN_THRESHOLD to 32k (from 2k)
> > and
> > 2. the halt handler now calls vcpu_on_spin to take the advantage of
> > PLE improvements. (this can also go as an independent patch into
> > kvm)
> >
> > The rationale for making SPIN_THERSHOLD 32k needs big explanation.
> > Before PLE improvements, as you know,
> > kvm undercommit scenario was very worse in ple enabled cases.
> > (compared to ple disabled cases).
> > pvspinlock patches behaved equally bad in undercommit. Both had
> > similar reason so at the end there was no degradation w.r.t base.
> >
> > The reason for bad performance in PLE case was unneeded vcpu
> > iteration in ple handler resulting in high yield_to calls and double
> > run queue locks.
> > With pvspinlock applied, same villain role was played by excessive
> > halt exits.
> >
> > But after ple handler improved, we needed to throttle unnecessary halts
> > in undercommit for pvspinlock to be on par with 1x result.
> Make sense. I will review it ASAP. BTW the latest version is V10 right?
Yes. Thank you.


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