Re: [Xen-devel] wall clock drift on Coffee Lake / C24x mainboard (HPET broken?), best practices

[Andreas Kinzler <hfp@xxxxxxxxx>]

On 19.11.2019 10:29, Jan Beulich wrote:
> On 18.11.2019 20:35, Andreas Kinzler wrote:
> > On 15.11.2019 12:01, Andreas Kinzler wrote:
> > > On 14.11.2019 12:29, Jan Beulich wrote:
> > > > On 14.11.2019 00:10, Andreas Kinzler wrote:
> > > > > I came across the following: https://lkml.org/lkml/2019/8/29/536
> > > > > Could that be the reason for the problem mentioned below? Xen is using
> > > > > HPET as clocksource on the platform/mainboard. Is there an (easy) way to
> > > > > verify if Xen uses PC10?
> > > > Hence I can only suggest that you try again with limited or no
> > > > use of C states, to at least get a hint as to a possible
> > > I changed the BIOS setting to a limit of PC7 and it is now running. I
> > > have to wait for the result. Thanks.
> > Previously the drift after 4 days uptime was 60 sec. Now after 4 days
> > uptime drift is 9 sec. So setting the package c-state limit to PC7 was a
> > success.
> 9s still seems quite a lot to me, but yes, it's an improvement.
It seems it is even better than some other platforms now. Some snapshot 
measurements from running systems:
Xeon E3-1230v5 (Skylake): drift of 4 sec per day (23.999MHz HPET)
Xeon E3-1240v6 (Kaby Lake): drift of 1.9 sec per day (23.999MHz HPET)
Xeon E3-1240v5 (Skylake): drift of 4.85 sec per day (23.999MHz HPET)
Xeon E5-1620v4 (Broadwell): drift of 2.7 sec per day (14.318MHz HPET)

All these values are not great, but it is OK for me.

> Now would you be up to checking whether, rather than via BIOS
> settings (which not all BIOSes may offer) the same can be
> achieved by using Xen's command line option "max_cstate="?
> Also did you check whether further limiting C state use would
I cannot try on production machines. I may have a slot on lab machines 
but I cannot promise.
> further improve the situation? And did you possibly also check
> whether telling Xen not to use the HPET would make a difference?

Which other clocksource do you prefer? Is Xen tested (field-proven) on 
that other clocksource?
Regards Andreas

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