Subject: [RFC -v2 PATCH 0/3] directed yield for Pause Loop Exiting
Date: Tuesday 14th December 2010 03:44:34 UTC (over 7 years ago)
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. 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. v2: - make lots of cleanups and improvements suggested - do not implement timeslice scheduling or fairness stuff yet, since it is not entirely clear how to do that right (suggestions welcome) -- All rights reversed.