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

Chris Wright
2010-12-02T22:42:04Z

* 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).

> In other workloads a pause can also be counter-productive, with
> spinlock detection resulting in one guest giving up its CPU time
> to the others. Instead of spinning, it ends up simply not running
> much at all.
>
> This patch series aims to fix that, by having a VCPU that spins
> give the remainder of its timeslice to another VCPU in the same
> guest before yielding the CPU - one that is runnable but got
> preempted, hopefully the lock holder.