On 12/03/2010 12:41 AM, Chris Wright wrote:
* Rik van Riel (riel@xxxxxxxxxx) 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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