Re: Xen 3.1.2 domU clock goes out of sync

[Ben Guthro <bguthro@virtualiron.com>]

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Clock skew is a known problem in virtualized systems.

VMWare wrote a whitepaper a while back which goes into some of the 
problems that they faced while dealing with this same issue...
http://www.vmware.com/pdf/vmware_timekeeping.pdf

The Xen project is not immune to these problems - however, some efforts 
have been made recently on the list to address some of these issues

See these threads:
http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01010.html
http://lists.xensource.com/archives/html/xen-devel/2007-11/msg00554.html

What it basically comes down to is that it is necessary to keep the 
clock skew under 0.05% for NTP to work - so that was the design goal.

The code introduced by some recent patches claims to be accurate to 
0.02% (for at least HVM guests)


Hope this helps.

Ben


Tõnu Raitviir wrote:
>
>
> Hello!
> I have understood that when /proc/sys/xen/independent_wallclock is 0, 
> then domU clock stays in sync with dom0 clock. Now I have a case with 
> Xen 3.1.2 (also occured with 3.1.1, but I haven't documented it) where 
> domU clock runs significantly faster - after 65 hours domU clock is 20 
> seconds ahead of dom0 clock.
> In this case dom0 is not running ntpd. When ntpd is running on dom0, 
> domU clock also stays correct. Is this normal behaviour?
>
> Some examples to show clock differences:
>
> domU# uptime
>  14:54:09 up 2 days, 17:05,  1 user,  load average: 0.75, 0.49, 0.43
>
> domU# ntpdate -q ntp.eenet.ee
> server 193.40.133.142, stratum 1, offset -20.856732, delay 0.02579
> 19 Nov 14:54:13 ntpdate[8467]: step time server 193.40.133.142 offset 
> -20.856732 sec
>
> dom0# uptime
>  14:55:02 up 2 days, 17:06,  1 user,  load average: 0.07, 0.02, 0.00
>
> dom0# ntpdate -q ntp.eenet.ee
> server 193.40.133.142, stratum 1, offset -0.438055, delay 0.02589
> 19 Nov 14:55:17 ntpdate[32284]: adjust time server 193.40.133.142 
> offset -0.438055 sec
>
>
> Xen version used:
>
> Xen version 3.1.2 (jussuf@localdomain) (gcc version 4.1.2 20061115 
> (prerelease) (Debian 4.1.1-21)) Fri Nov 16 17:52:59 EET 2007
> Latest ChangeSet: Wed Nov 14 23:35:43 2007 +0000 15502:c6776b6da8ee
>
> Linux xen-2 2.6.18.8-xen0 #1 SMP Fri Nov 16 17:39:31 EET 2007 x86_64 
> GNU/Linux
>
> ------------------------------------------------------------------------
>
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