Re: [Xen-users] Time/clock issues with Xen 3.0.3?

["Ulrich Windl" <ulrich.windl@xxxxxxxxxxxxxxxxxxxx>]

On 27 Nov 2006 at 11:24, Harald Koenig wrote:

> I see the same problem here (xen-3.0.2 from SUSE 10.1): 
> dom0 is synched via ntpd, and the domUs drift away from dom0 and ntp server.

Harald,

use independent clocks everywhere, and it will work: "rkdvmso1" is a Dom0, 
every 
other is a DomU somewhere:
# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        LOCAL(0)        10 l   12   64  377    0.000    0.000   0.001
*rkdvmso1.dvm.kl 132.199.176.97   2 u  987 1024  377    0.207    0.035   0.742
+rksapas01.dvm.k 192.168.0.61     3 u  997 1024  377    0.120    0.020   0.772
+rksapas02.dvm.k 192.168.0.61     3 u  992 1024  377    0.101    0.049   0.769
+rksapas03.dvm.k 192.168.0.62     3 u  995 1024  377    0.282    0.019   4.211
+rksapas04.dvm.k 192.168.0.62     3 u  997 1024  377    0.274    0.001   0.775
+rksapas05.dvm.k 192.168.0.63     3 u   78 1024  377    0.281   -0.093   0.800
+rksapas06.dvm.k 192.168.0.63     3 u  999 1024  377    0.230   -0.213   0.825
 rksapas07.dvm.k .INIT.          16 u    - 1024    0    0.000    0.000 4000.00
+rksapas08.dvm.k 192.168.0.41     4 u   78 1024  377    0.243   -0.168   0.796

Ulrich


> 
> for an example how to reproduce this problem, see below... 
> 
> 
> right now I use a cron job on dom0 which re-sets the dom0 clock
> via date -s `date` (ntpdate doesn't work here).
> 
> 
> On Nov 25, Tim Post wrote:
> 
> > What are the values of /proc/sys/xen/independent_wallclock
> > and /proc/sys/xen/permitted_clock_jitter respectively?
> 
>       os2 koenig > cat /proc/sys/xen/independent_wallclock
>       0
> 
>       os2 koenig > cat /proc/sys/xen/permitted_clock_jitter
>       10000000
> 
> maybe I should just set /proc/sys/xen/independent_wallclock to 1 
> and run ntpd on all domUs ?
> 
> 
> now, how I was able to reproduce the domU clock due to ntp clock drift in dom0
> using SUSE 10.1 xen stuff.  in this examle both xen, dom0 and domU are SUSE 
> 10.1 
> and use the SUSE xen-kernel, but on my real XEN server I run may different
> distributions and kernels -- all alike...
> 
> 
> step 1: perfect clock sync with drift==0 :
> 
> run ntpd on dom0 with the following config file /etc/ntp.conf which uses
> only the dom0 system clock as "source", so it doesn't adjust the clock 
> ever.
> 
> ----------------------------- /etc/ntp.conf 
> -----------------------------------
> restrict default noquery notrust nomodify
> restrict 127.0.0.1
> restrict 192.168.8.0 mask 255.255.255.0
> server 127.127.1.1
> driftfile /var/lib/ntp/drift/ntp.drift
> logfile /var/log/ntp
> -------------------------------------------------------------------------------
> 
> before starting ntpd, make sure the clock drift is set to zero with
> 
>       echo 0 > /var/lib/ntp/drift/ntp.drift
> 
> now start ntpd, start domU (don't run ntpd in domU) and check the domU
> clock drift with
> 
>       ntpdate -d dom0
> 
> that's how it should always work (in theory;).  but in real world
> the real clock drift of a PC clock is not zero. prrtty often the clock
> shows a frequency error of 100 ppm and more (which is 8.64 secs per day!).  
> 
> 
> now, let's add some drift to dom0:
> 
>       /etc/init.d/ntpd stop
>       echo 100 > /var/lib/ntp/drift/ntp.drift
>       /etc/init.d/ntpd start
> 
> 
> now you check the domU clock by running ntpdate on domU:
> 
>       ntpdate -d dom0 ; sleep 60 ; ntpdate -d dom0
> 
> and there will be a domU clock drift relative to dom0 or any other ntpd server
> of ~6 msec per minute == 100 ppm.  qed.
> 
> 
> hope this helps to track and fix this clock problem!
> 
> Harald Koenig
> -- 
> "I hope to die                                      ___       _____
> before I *have* to use Microsoft Word.",           0--,|    /OOOOOOO\
> Donald E. Knuth, 02-Oct-2001 in Tuebingen.        <_/  /  /OOOOOOOOOOO\
>                                                     \  \/OOOOOOOOOOOOOOO\
>                                                       \ OOOOOOOOOOOOOOOOO|//
> Harald Koenig                                          \/\/\/\/\/\/\/\/\/
> science+computing ag                                    //  /     \\  \
> koenig@xxxxxxxxxxxxxxxxxxxx                            ^^^^^       ^^^^^
> 
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