Re: [Xen-users] [Xen-devel] Xen timekeeping best practices

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-04-16 at 15:45 +0100, Lloyd Dizon wrote:
> On Mon, Apr 16, 2012 at 11:57 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
> wrote:
> >
> > On Fri, 2012-04-13 at 18:22 +0100, Todd Deshane wrote:
> > > Adding Xen-devel.
> > >
> > > What are the current timekeeping best practices now?
> >
> > pvops kernels behave as if independent_wallclock == 1, so the existing
> > advice:
> >        set /proc/sys/xen/independent_wallclock to 1 and run ntp on
> >        domU.
> > still applies.
> 
> This also applies to Xen 4.0 (4.0.0) if I understood correctly?

This is entirely a function of the guest kernel version and not the
hypervisor

> If so and based on previous posts can we resume that this is what is
> recommended:

If you s/Xen3/Classic XenoLinux Kernels/ and s/Xen4/Paravirt Ops Linux
Kernels/g then I think it is somewhat accurate, at least for the PV
line.

I must confess I don't really know for the HVM line. I suspect that it
is actually:

Classic XenoLinux -- no PV time source, independent_wallclock means
nothing, run NTP in the guest

Paravirt ops kernels _without_ PV time PVHVM extensions -- no PV time
source, independent_wallclock means nothing, run NTP in the guest

Paravirt ops kernels _with_ PV time PVHVM extensions -- PV time source,
independent_wallclock always == 1, run NTP.

So in short always run NTP in an HVM guest, regardless of the presence
or absence of PV time extensions.

>         |        Xen3                     |          Xen4               |
> ---------------------------------------------------------------------
> PV    | independent_wallclock = 0 or 1  | independent_wallclock =  1  |
> ---------------------------------------------------------------------
> HVM | independent_wallclock = 0       | independent_wallclock =  1  |
> ---------------------------------------------------------------------
> 
> With priority to independent_wallclock=1 and if activated, ntp required.



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