[Xen-devel] domU time drifting

[Carl Jones <carl@xxxxxxxxxxxxx>]

Hi,

I am trying to track down the cause of time drifting on our domUs.
We're using various versions of Xen, and various domU kernels.

dom0s all run ntpd and are in sync. No issues there.

dom0s are all running CentOS 5 64bit. domUs run various distributions
(the examples below happen to be Debian and Ubuntu only).

domUs that do not run ntpd tend to drift (both forwards and
backwards). domUs are using clocksource=xen. dom0s are using
clocksource=jiffies.

Some example setups:

Xen: 3.4.3
dom0: 2.6.18.8 x86_64 based on el5 sources
domU: 2.6.30.5 i386
domU uptime: 68 days

dom0: Sun Dec 12 23:45:59 UTC 2010
domU: Sun Dec 12 23:44:48 UTC 2010

Xen: 3.3.1
dom0 2.6.18.8 x86_64 from xen.org
domU: 2.6.33.3 i386
domU uptime: 34 days

dom0: Sun Dec 12 23:51:20 UTC 2010
domU: Sun Dec 12 23:50:39 UTC 2010

Xen: 3.4.3
dom0: 2.6.18.8 x86_64 based on el5 sources
domU: 2.6.30.5 i386
domU uptime: #1 28 days #2 26 days

dom0: Sun Dec 12 23:58:23 UTC 2010
domU #1: Sun Dec 12 23:58:03 UTC 2010
domU #2: Sun Dec 12 23:58:04 UTC 2010

The last example is interesting. Two domUs on the same host have the
same (incorrect) time. Same domU kernel. Different uptime.

I've tried suggestions in bug reports like these:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=534978
http://wiki.debian.org/Xen#A.27clocksource.2BAC8-0.3ATimewentbackwards.27
http://my.opera.com/marcomarongiu/blog/2010/08/18/debugging-ntp-again-part-4-and-last

But so far the problem persists.

Is there anything we can do on the host side of things to prevent
domUs from drifting? With clocksource=xen shouldn't the domUs stay in
sync provided the host keeps the correct time?

Please let me know if I can provide further details.

Regards,
Carl

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