Messages in this thread | | | Date | Sun, 05 Dec 2010 15:02:08 +0200 | From | Avi Kivity <> | Subject | Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting |
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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.
-- error compiling committee.c: too many arguments to function
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