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

[Gleb Natapov <gleb@xxxxxxxxxx>]

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?

--
                        Gleb.

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