Re: [Xen-devel] Domain-Virtual time

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

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Thanks Dan! That's a lot of useful information.

Since yesterday, I started NTP on my machines to correct and measure the
amount of drift. (I am running 3 HVMs with a Ubuntu 8.04 Linux (tick-less)
kernel which were all installed in an identical manner. At the time of
installing the HVMs I did not change the default timer_mode).

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.

In addition, the negative sign on the drift means that my machines are
running *faster* that the real time, which is again puzzling. I found out
that VMWare has issues with overcompensation on its linux kernels that cause
the VM time to run faster. Could Xen be having a similar problem ?

The fact that all three on my machines are showing different drifts makes me
doubt that they are showing the system time. I checked the CPU frequency
that the three machines are reporting (from /proc/cpuinfo) and they are
similar but not identical.

Any thoughts?

On Thu, Feb 25, 2010 at 3:36 PM, Dan Magenheimer <dan.magenheimer@oracle.com
> wrote:

> Should be "true" system time, i.e. should be very close to what
> you see on a "wallclock" (clock on the wall).
>
> HVM's are sadly very widely varied in the parameters needed
> to minimize time drift.  In general in the past, timer_mode=0
> (or timer_mode unspecified) would be best for 32-bit Linux
> domains, timer_mode=1 would be best for Windows domains,
> and timer_mode=2 would be best for 64-bit Linux domains.
> However, for best results on Linux, this must be combined with
> kernel boot parameters that properly select a clock -- and
> on some Linux kernel versions, the parameters needed are
> different between 32-bit and 64-bit versions of the same
> kernel version.  It is up to providers of HVM templates
> (aka "appliances") to choose parameters wisely.
>
> Also, you haven't specified your Xen version, but I believe
> Xen 4.0 switches the timer_mode default from 0 to 1 so, sadly,
> clock behavior may change when moving an unchanged HVM
> domain from pre-4.0 to 4.0.
>
> So for best results you should run ntpd in any Linux HVM
> domain (and I don't know what you do in Windows).  But
> even ntpd may be inadequate to avoid drift if poor parameters
> are chosen.
>
> ==========
>
> From: Priya [mailto:pbhat@acis.ufl.edu]
> Sent: Thursday, February 25, 2010 9:04 AM
> To: xen-devel@lists.xensource.com; xen-users@lists.xensource.com
> Subject: [Xen-devel] Domain-Virtual time
>
> Sorry for multiple emails. I sent the last one from the wrong address.
>
> Can anyone please tell me if the value returned by a time querying
> instruction like gettimeofday() on a Xen (Linux) HVM is the true (System)
> time or the Domain-virtual time?
>
> PS: Domain virtual time is defined as the time that progresses at the same
> pace as cycle counter, but only while a domain is executing. It stops while
> the domain is de-scheduled where as System time accurately reflects the
> passage of real time.
>
> I am facing issues because my HVMs show a time drift.
>
> Thanks
>
>

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