Re: time drift on dom0

[maney@two14.net (Martin Maney)]

On Sun, Jan 16, 2005 at 09:17:58AM +0000, Keir Fraser wrote:
> If you run NTPD in DOM0 then all other domains will be sync'ed to
> DOM0's accurate timebase. This is how we run our own machines.

Excellent!  I didn't recall noticing any timing problems when I was
playing with Xen (on a machine that has since moved on to other uses,
so I don't have a working Xen testbed right now), but it was already
running NTP in the system image that became dom0.

Is this syncing what gives rise under some circumstances to the clock
moving backwards that I've seen mentioned here?  Never saw that, but I
never got around to pushing that box hard.

> The two main places we get drift from when not using NTP are
> inaccuracy of the PIT oscillator (can easily be 100 or more ppm), and

Yep.  As I say, I've seen at least a couple different machines that run
right around 200 ppm (one fast and one slow, as it happens) under
non-Xen Linux.  The frequencey correction is quite stable over the half
year or so I've been logging the NTP stats.

> Not much we can do about the former -- we could deal with the latter
> by stretching/shrinking jiffies or by inserting leap jiffies. 

Seems like a waste of effort, given errors an order of magnitude larger
than the nominal setting error.  If you want accurate time, you need to
run NTP anyway.  :-)

Or... was it adjtimex that I used to use back in the dark ages of
dialup?  Would that work to trim the dom0 clock error (it was more or
less a manual correction for drift, as I recall)?

-- 
Vs lbh pna ernq guvf, lbh'er va ivbyngvba bs
gur Qvtvgny Zvyyraavhz Pbclevtug Npg.  -- anon.



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