[CentOS] Xen clock drift

Tue Jan 8 13:22:21 UTC 2008
Akemi Yagi <amyagi at gmail.com>

On Jan 8, 2008 5:05 AM, Rick Barnes <linux at sitevision.com> wrote:
>
> Akemi Yagi wrote:
> > On Jan 8, 2008 4:51 AM, Rick Barnes <linux at sitevision.com> wrote:
> >> Jack Bailey wrote:
> >>> These domains are fully virtualized and set up identically, except
> >>> "badclock" is allocated two processors versus one processor for
> >>> "goodclock".  DomU's clock is running normally.
> >>>
> >>> Anyone know what's going or know how to fix it?
> >> This is a known issue that has come up on this list a lot.
> >>
> >> For C5.1 see the first known issue:
> >>
> >> http://wiki.centos.org/Manuals/ReleaseNotes/CentOS5.1/
> >>
> >> Rick
> >
> > Please note that the clock rate issue in that description applies to
> > non-xen kernels.  xen kernels are set to 250Hz by default.
>
> "With this option you can reduce the clock rate from the default of
> 1000HZ to 100HZ which is desirable in a virtual machine."
>
> If it does not apply to xen then this should be made more clear.
>
> Rick

I think it is a good idea to add a note about xen kernels.  But this
is noted in the upstream Release Notes where the tick divider option
is mentioned:

"Note that the virtualized kernel does not support multiple timer
rates on guests. dom0 uses a fixed timing rate set across all guests;
this reduces the load that multiple tick rates could cause."

http://www.centos.org/docs/5/html/release-notes/as-x86/RELEASE-NOTES-U1-x86-en.html
(scroll down to tick_divider in the Feature Updates section)

Akemi