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

[Lloyd Dizon <thecarrionkind@xxxxxxxxx>]

On Mon, Apr 16, 2012 at 4:57 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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

I do not fully understand the difference yet between and Classic
XenoLinux and Paravirt Ops Kernel (will dig the info later) but here
is the resume:

       
-------------------------------------------------------------------------------------------------------------------
       |  Xen3: Classic XenoLinux Kern                     |
    Xen4: Paravirt Ops Linux Kern                 |
--------------------------------------------------------------------------------------------------------------------------
 PV    | independent_wallclock = 0 or 1                    |
independent_wallclock =  1                                  |
--------------------------------------------------------------------------------------------------------------------------
 HVM   | -> no PV timesource, no ind. wallclock, run NTP   | -> w/o PV
time PVHVM extensions, no ind. wallclock, run NTP |
       |                                                   | -> PV
time source, ind. wallclock always 1, run NTP         |
--------------------------------------------------------------------------------------------------------------------------


Should we put this somewhere (wiki) until somebody else proves this is
inaccurate ?

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