Re: [Xen-devel] Domain-Virtual time

[Priya <pbhat@acis.ufl.edu>]

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Thanks Tim!

I have some follow-up questions:

   1. How can I check/change the timer_mode on a *XenServer* HVM and on a
   Xen HVM?
   2. How can I check/change the time-source on my machines?
   3.
>
>    No; they are showing system time modified according to the timer_mode
>    setting, then extrapolated into a collection of virtual timers and
>    reconstituted by the linux kernel.

   Can you please elaborate a little bit. Or please point me to a reference.


Thanks again !
Pr



On Fri, Feb 26, 2010 at 12:01 PM, Tim Deegan <Tim.Deegan@citrix.com> wrote:

> Hi,
>
> At 16:46 +0000 on 26 Feb (1267202798), Priya wrote:
> > The funny thing is that NTP is measuring a very different drift on my
> > three machines (-189.206, -108.373 and -71.321 parts per million). The
> > drift reported on Domain-0 is -11.393. So I don't think my machines
> > are showing the system time.
>
> No; they are showing system time modified according to the timer_mode
> setting, then extrapolated into a collection of virtual timers and
> reconstituted by the linux kernel.
>
> I don't know a lot about linux HVM because I always run Linux with PV
> kernels, but on Windows I've found that the (virtual) ACPI PM timer is a
> better time-source than the HPET or RTC.
>
> > In addition, the negative sign on the drift means that my machines are
> > running faster that the real time, which is again puzzling.
>
> Xen system time generally does drift forward, IIRC that's because xen
> always tends to catch up to the fastest CPU, so cross-CPU jitter turns
> into forward drift.  Linux may be doing the same thing inside the HVM
> VM, where the jitter is higher.
>
> Tim.
>
> --
> Tim Deegan <Tim.Deegan@citrix.com>
> Principal Software Engineer, XenServer Engineering
> Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)
>
>

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