[CentOS] Xen clock drift

Tue Jan 8 13:20:34 UTC 2008
Johnny Hughes <johnny at centos.org>

Rick Barnes 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.
> 

The first note is to address the fact the "divider=" and not
"tick_divider=" should be used ... though the actual usage is detailed
in the upstream release notes:

http://www.centos.org/docs/5/html/release-notes/as-x86/RELEASE-NOTES-U1-x86-en.html

This does say (well sort of) that it can not be used on Xen:

"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."

Regardless ... divider= is not for xen kernels, only normal kernels.

Thanks,
Johnny Hughes

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