On Sun, May 05, 2019 at 08:43:24AM +0800, Wanpeng Li wrote: > On Wed, 1 May 2019 at 03:31, Sean Christopherson > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > On Sun, Apr 28, 2019 at 08:54:30AM +0800, Wanpeng Li wrote: > > > Hi Sean, > > > On Thu, 18 Apr 2019 at 01:18, Sean Christopherson > > > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > > > > > KVM's recently introduced adaptive tuning of lapic_timer_advance_ns has > > > > several critical flaws: > > > [.../...] > > > > > > > > - TSC scaling is done on a per-vCPU basis, while the advancement value > > > > is global. This issue is also present without adaptive tuning, but > > > > is now more pronounced. > > > > > > Did you test this against overcommit scenario? Your per-vCPU variable > > > can be a large number(yeah, below your 5000ns) when neighbour VMs on > > > the same host consume cpu heavily, however, kvm will wast a lot of > > > time to wait when the neighbour VMs are idle. My original patch > > > evaluate the conservative hypervisor overhead when the first VM is > > > deployed on the host. It doesn't matter whether or not the VMs on this > > > host alter their workload behaviors later. Unless you tune the > > > per-vCPU variable always, however, I think it will introduce more > > > overhead. So Liran's patch "Consider LAPIC TSC-Deadline Timer expired > > > if deadline too short" also can't depend on this. > > > > I didn't test it in overcommit scenarios. I wasn't aware of how the > > I think it should be considered. > > > automatic adjustments were being used in real deployments. > > > > The best option I can think of is to expose a vCPU's advance time to > > userspace (not sure what mechanism would be best). This would allow > > userspace to run a single vCPU VM with auto-tuning enabled, snapshot > > the final adjusted advancment, and then update KVM's parameter to set > > an explicit advancement and effectively disable auto-tuning. > > This step is too complex to deploy in real environment, the same as > w/o auto-tuning. My auto-tuning patch evaluates the conservative > hypervisor overhead when the first VM is deployed on the host, and > auto-tuning it only once for the whole machine. But even then the advancement could be corrupted or wildly inaccurate unless that first VM has a single vCPU. I thought of an idea that will hopefully fix the overcommit scenario and in general reduce the time spent auto-adjusting. Patch incoming...
