Avi Kivity wrote: > On 04/14/2010 06:24 AM, Zhang, Xiantao wrote: >> >>>>> Spin loops need to be addressed first, they are known to kill >>>>> performance in overcommit situations. >>>>> >>>>> >>>> Even in overcommit case, if vcpu threads of one qemu are not >>>> scheduled or pulled to the same logical processor, the performance >>>> drop is tolerant like Xen's case today. But for KVM, it has to >>>> suffer from additional performance loss, since host's scheduler >>>> actively pulls these vcpu threads together. >>>> >>>> >>>> >>> Can you quantify this loss? Give examples of what happens? >>> >> For example, one machine is configured with 2 pCPUs and there are >> two Windows guests running on the machine, and each guest is >> cconfigured with 2 vcpus and one webbench server runs in it. >> If use host's default scheduler, webbench's performance is very bad, >> but if pin each geust's vCPU0 to pCPU0 and vCPU1 to pCPU1, we can >> see 5-10X performance improvement with same CPU utilization. >> In addition, we also see kvm's perf scalability is also impacted in >> large systems, for some performance experiments, kvm's perf begins >> to drop when vCPU is overcommitted and pCPU are saturated, but once >> the wake_up_affine feature is switched off in scheduler, kvm's perf >> can keep rising in this case. >> > > Ok. This is probably due to spinlock contention. Yes, exactly. > 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. Xiantao -- 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
