Date: Wed, 27 Jun 2018 10:05:32 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> To: Alan Somers <asomers@freebsd.org> Cc: Jung-uk Kim <jkim@freebsd.org>, Andriy Gapon <avg@freebsd.org>, FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: TSC calibration in virtual machines Message-ID: <201806271705.w5RH5WZ7009269@pdx.rh.CN85.dnsmgr.net> In-Reply-To: <CAOtMX2gcUybMhPdEzBWX07-oPdmJdqn%2BvW7KkNZvs2sFmcHFNw@mail.gmail.com>
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> On Wed, Jun 27, 2018 at 10:36 AM, Jung-uk Kim <jkim@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 > > > > We already do that for VMware hosts since r221214. > > > > https://svnweb.freebsd.org/changeset/base/221214 > > > > We should do the same for each hypervisor. > > > > Jung-uk Kim > > > > > We probably should. But why does calibration fail in the first place? If > it can fail in a VM, then it can probably fail on bare metal too. It would > be worth investigating. No, the failure in a VM is unique to a VM, it has to do with the fact your have the hypervisor timeslicing a CPU that you believe to be 100% dedicated to you. There are several white papers, including one from VMWare about what they have done to help with the time keeping problems. What is suggested above would be a correct thing to do. Bhyve creates these issues as well, and use of certain timers in a bhyve guest can cause you nightmares with ntp. -- Rod Grimes rgrimes@freebsd.org
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