RE: Time keeps on slipping... on Hyper-V

From: Thomas Shao
Date: Fri Sep 26 2014 - 05:47:25 EST

> -----Original Message-----
> From: Mike Surcouf [mailto:mps.surcouf.lkml@xxxxxxxxx]
> Sent: Friday, September 26, 2014 5:24 PM
> To: Thomas Shao
> Cc: Sitsofe Wheeler; Olaf Hering; gregkh@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx;
> apw@xxxxxxxxxxxxx; jasowang@xxxxxxxxxx; KY Srinivasan; Haiyang Zhang
> Subject: Re: Time keeps on slipping... on Hyper-V
> > We still recommend user to configure NTP in the guest VM. With the new
> > time sync feature in this patch, you could have one more option to enable
> the guest-host sync, if the NTP didn't work in the environment.
> > For example the guest VM didn't have network connection.
> Microsoft used to use host time-samples in their older drivers but this was
> dropped (when I don't know).
> Now we must use NTP to correct hyperv_clocksource which suffers from the
> usual problems associated with virtual environment cpu loading.
> Host time-samples in conjunction with an effective clock stability algorithm
> with slews rather than steps should be the default.
> NTP is a workaround and should not be the primary solution.
> I have seen a lots of posts for RHEL CENTOS Linux that have a
> FAST.hyper_clocksource confirmed by myself on CENTOS 6.5 and 7.0 (also
> confirmed by Olaf on SLES).
> If this patch steps the clock once in a while without any form of slewing then
> it has the potential to break things as files and logs travel BACK in time.
> Apologies Thomas could you send me the patch via email as does
> not have it in the archives.
Sure. I'll send the package again. It's strange that this patch didn't in the archives. And in my patch, it will slew the time, if the time drift is less than 1 s. It only steps the clock if the drift is larger than 1 s.

> Regards Mike
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