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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

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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