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Re: [Xen-users] Xen time drift issues

To: Peter Booth <peter_booth@xxxxxx>
Subject: Re: [Xen-users] Xen time drift issues
From: Priya <pbhat@xxxxxxxxxxxx>
Date: Wed, 8 Jul 2009 09:53:59 -0400
Cc: "bansal.tanuj83@xxxxxxxxx" <bansal.tanuj83@xxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>
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@tanuj: Thanks! I am not using NTP for syncing. The reason for that is
that I want to get an idea of the fidelity of the time returned by
domU and dom0 w.r.t. the actual real time. I expected to see a
difference, but what is puzzling me is that the difference increases
in such a linear fashion.

@peter: As I mentioned in my previous email, the drift is ~50
microseconds/second on domU and slightly less on dom0

I have been trying to find some documentation pertaining to how Xen
virtualizes the different hardware timers. If anyone has a link to
such a technical report, please send me a link.



On Wed, Jul 8, 2009 at 9:37 AM, Peter Booth<peter_booth@xxxxxx> wrote:
> How are you measuring this drift?
> Sent from my iPhone
> On Jul 8, 2009, at 7:59 AM, Tanuj Bansal <bansal.tanuj83@xxxxxxxxx> wrote:
>> Hi Priya:
>> If you are using NTP, then shutdown the NTP service on all DomU and
>> restart
>> it on Dom0 only.
>> Regards,
>> Tanuj Bansal
>> -----Original Message-----
>> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
>> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Priya
>> Sent: Wednesday, July 01, 2009 8:55 PM
>> To: xen-users@xxxxxxxxxxxxxxxxxxx
>> Subject: [Xen-users] Xen time drift issues
>> Hello everyone!
>> I been observing some peculiar time drift on my Xen domains (dom0 and
>> domUs).  The time (as returned by timeofday) on my domains drifts at
>> an almost steady rate from the real time ( as read from the time stamp
>> counter ).
>> The drift is so constant that it makes me think there should be some
>> simple explanation for it in the way Xen keeps time.
>> Some info about my set up:
>> guest OS: ubuntu
>> independent_wallclock is set to default 0 on all domains
>> no NTP synchronization
>> The drift is around 50 microseconds / second on all the domains. Has
>> anyone else observed this? If anyone can point me to some
>> documentation that explains this behavior, that would be great!
>> Good day!
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