On 06/27/2018 12:47, Alan Somers wrote: > On Wed, Jun 27, 2018 at 10:36 AM, Jung-uk Kim <jkim_at_freebsd.org > <mailto:jkim_at_freebsd.org>> wrote: > > On 06/27/2018 03:14, Andriy Gapon wrote: > > > > It seems that TSC calibration in virtual machines sometimes can do more harm > > than good. Should we default to trusting the information provided by a hypervisor? > > > > Specifically, I am observing a problem on GCE instances where calibrated TSC > > frequency is about 10% lower than advertised frequency. And apparently the > > advertised frequency is the right one. > > > > I found this thread with similar reports and a variety of workarounds from > > administratively disabling the calibration to switching to a different timecounter: > > https://lists.freebsd.org/pipermail/freebsd-cloud/2017-January/000080.html > <https://lists.freebsd.org/pipermail/freebsd-cloud/2017-January/000080.html> > > We already do that for VMware hosts since r221214. > > https://svnweb.freebsd.org/changeset/base/221214 > <https://svnweb.freebsd.org/changeset/base/221214> > > We should do the same for each hypervisor. > > We probably should. But why does calibration fail in the first place? Because multiple guests are sharing same physical CPUs and guest OS has no control, timing cannot be 100% accurate. > If it can fail in a VM, then it can probably fail on bare metal too. It > would be worth investigating. It does not "fail" in bare metal because we have almost complete control. Jung-uk Kim
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