Messages in this thread | | | Date | Sat, 17 Apr 2010 22:02:46 +0300 | From | Avi Kivity <> | Subject | Re: VM performance issue in KVM guests. |
| |
On 04/16/2010 05:27 AM, Zhang, Xiantao wrote: > > >> When vcpus are pinned to pcpus, there is a 50% chance that a guest's >> vcpus will be co-scheduled and spinlocks will perform will. >> >> When vcpus are not pinned, but affine wakeups are disabled, there is a >> 33% chance that vcpus will be co-scheduled. >> >> When vcpus are not pinned and affine wakeups are enabled there is a 0% >> chance that vcpus will be co-scheduled. >> >> Keeping both vcpus on the same core actually makes sense since they >> can communicate through the local cache faster than across cores. >> What we need is to make sure that they don't spin. >> >> Windows 2008 can report spinlock spinning through a hypercall. Can >> you hook to that interface and see if it happens regularly? >> Altenatively use a PLE capable host and trace the kvm_vcpu_on_spin() >> function. >> > We only tried windows 2003 for the experiments, and have no data related to windows 2008. > But maybe we can have a try later. Anyway, the key point is we have to enhance the scheduler to let it > Know which threads are vcpu threads to avoid perf loss in this case. >
I have two worries about this approach:
1. Affine wakeups were introduced for a reason; if we disable them (even just for vcpus), we lost something. Maybe we can tune the mechanism not to fail, instead of disabling it.
2. Affine wakeups are a scheduler internal detail. How do we explain what it does? the scheduler may not have affine wakeups in a few years, yet we'll have an ABI to disable them.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
| |