Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

Avi Kivity
2010-12-05T13:02:08Z

On 12/03/2010 12:41 AM, Chris Wright wrote:
> * Rik van Riel (riel@redhat.com) wrote:
> > When running SMP virtual machines, it is possible for one VCPU to be
> > spinning on a spinlock, while the VCPU that holds the spinlock is not
> > currently running, because the host scheduler preempted it to run
> > something else.
> >
> > Both Intel and AMD CPUs have a feature that detects when a virtual
> > CPU is spinning on a lock and will trap to the host.
> >
> > The current KVM code sleeps for a bit whenever that happens, which
> > results in eg. a 64 VCPU Windows guest taking forever and a bit to
> > boot up. This is because the VCPU holding the lock is actually
> > running and not sleeping, so the pause is counter-productive.
>
> Seems like simply increasing the spin window help in that case? Or is
> it just too contended a lock (I think they use mcs locks, so I can see a
> single wrong sleep causing real contention problems).

It may, but that just pushes the problem to a more contended lock or to
a higher vcpu count. We want something that works after PLE threshold
tuning has failed.

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