Some notes about pause loop exiting
1. background
Spin-locking code typically uses PAUSE instructions in a loop.
For more details, please refer to pause.
2. motivation
为什么需要pause loop exiting(ple)这一feature呢?
Lock-Holder Preemption(LHP)
Solution for LHP issue: Detect that vCPU is busy acquiring lock for long time and schedule out the vCPU.
3. details
Basic Exit Reason 40
PLE_Gap - upper bound on the amount of time between two successive executions of PAUSE in a loop.
PLE_Window - upper bound on the amount of time a guest is allowed to execute in a PAUSE loop
If the time, between this execution of PAUSE and previous one, exceeds the PLE_Gap, processor consider this PAUSE belongs to a new loop. Otherwise, processor determins the total execution time of this loop(since 1st PAUSE in this loop), and triggers a VM exit if total time exceeds the PLE_Window.
Pause-Loop Exiting can be used to detect Lock-Holder Preemption, where one VCPU is sched-out after hold a spinlock, then other VCPUs for same lock are sched-in to waste the CPU time.
个人感觉PLE_Gap与PLE_Window的设置，就是调参吧。
参考资料：