Re: [Xen-users] Xen time drift issues

[Priya <pbhat@xxxxxxxxxxxx>]

Hi.

My tests involve recording the Time Stamp Counter (tsc) reading and
the time returned by timeofday at the beginning of a long computation
over and over again.

In the post processing stage, I divide TSC readings with the processor
frequency (obtained from /proc/cpuinfo) to get the actual (real time)
and then measure the drift between the actual and time returned by
time of day (apparent time).

My machines are PVMs.

My kernel version is vmlinuz-2.6.24-16-xen

Thanks!
Priya



On Wed, Jul 8, 2009 at 3:10 PM, Peter Booth<peter_booth@xxxxxxx> wrote:
> Priya,
>
> What test are you using to  measure  the drift?
> Are your VMs HVM or PVM?
> What kernel version is your DomUs?
>
> Do you have acpi enabled on any HVM DomUs?
>
>
> On Jul 8, 2009, at 9:53 AM, Priya wrote:
>
>> @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.
>>
>> Thanks!
>>
>> Priya
>>
>>
>> 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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>>>> http://lists.xensource.com/xen-users
>>>>
>>>>
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>>>
>>>
>>
>>
>>
>> --
>>
>> Calvin Trillin  - "Health food makes me sick." -
>> http://www.brainyquote.com/quotes/authors/c/calvin_trillin.html
>>
>> _______________________________________________
>> Xen-users mailing list
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>> http://lists.xensource.com/xen-users
>
>
>



-- 

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