[Xen-users] Re: [Xen-devel] Domain-Virtual time

[Priya <pbhat@xxxxxxxxxxxx>]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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