Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

[Raghavendra K T]

On 05/07/2012 07:28 PM, Avi Kivity wrote:
> On 05/07/2012 04:53 PM, Raghavendra K T wrote:
> > > Is the improvement so low, because PLE is interfering with the patch, or
> > > because PLE already does a good job?
> >
> >
> > It is because PLE already does a good job (of not burning cpu). The
> > 1-3% improvement is because, patchset knows atleast who is next to hold
> > lock, which is lacking in PLE.
>
> Not good.  Solving a problem in software that is already solved by
> hardware?  It's okay if there are no costs involved, but here we're
> introducing a new ABI that we'll have to maintain for a long time.

Hmm agree that being a step ahead of mighty hardware (and just an
improvement of 1-3%) is no good for long term (where PLE is future).

Having said that, it is hard for me to resist saying :
 bottleneck is somewhere else on PLE m/c and IMHO answer would be
combination of paravirt-spinlock + pv-flush-tb.

But I need to come up with good number to argue in favour of the claim.

PS: Nikunj had experimented that pv-flush tlb + paravirt-spinlock is a 
win on PLE where only one of them alone could not prove the benefit.

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