* 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). > 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
