[ntp:questions] Flash 400 on all peers; can't get ntpd to be happy

Miroslav Lichvar mlichvar at redhat.com
Wed Mar 9 11:36:56 UTC 2011

On Tue, Mar 08, 2011 at 03:26:34PM -0800, Chuck Swiger wrote:
> >> You are better off running ntpdate (or sntp) periodically via cron in
> >> the DomUs.
> > 
> > Perhaps in certain cases, but not across the board.
> I'd be happy to review counterexamples to my generalization....

I'd say it depends on the VM.

For instance, Fedora 14 running in kvm on Fedora 14. There are four
clocksources available in the guest system: kvm-clock tsc hpet
acpi_pm. With each of them the frequency seems to be stable, even when
the host or guest CPU is heavily loaded. The kvm-clock and hpet
clocks seem to be running at same rate as the host's system clock, tsc
at the real CPU's rate and acpi_pm is off by few tens of ppm.

Here is a rv output from ntpd running in the guest with the tsc clock,
the host is not synchronized:

associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,
version="ntpd 4.2.6p2 at 1.2194-o Mon Aug 23 12:18:41 UTC 2010 (1)",
processor="x86_64", system="Linux/", leap=00,
stratum=3, precision=-23, rootdelay=128.742, rootdisp=41.165,
reftime=d121ddab.0ab5b995  Wed, Mar  9 2011  6:06:19.041,
clock=d121ddab.5dd3a440  Wed, Mar  9 2011  6:06:19.366, peer=47730,
mintc=3, offset=-0.013, frequency=22.454, sys_jitter=0.011,
clk_jitter=0.016, clk_wander=0.028

Miroslav Lichvar

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