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Re: [Xen-devel] Dom0 <> DomU clock sync

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Dom0 <> DomU clock sync
From: "Alex Tomkins" <tomkins@xxxxxxxxxxxx>
Date: Mon, 3 Sep 2007 15:54:50 +0100
Cc: Sébastien RICCIO <sr@xxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 03/09/07, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
> So long as ntpdate calls settimeofday(), this should be sufficient to update
> Xen's notion of time too. It seems that isn't happening for some reason.
> Do all guests (including dom0) have independent_wallclock equal to 0?
> What kernel are you using in dom0 (e.g., vendor kernel?).
> Ntpd in dom0 would be an interesting thing to try. That should cause dom0 to
> periodically (every few minutes) push its time down to Xen to be picked up
> by all domUs. But really ntpdate should be doing the right thing...
> In our kernels it should be *impossible* for dom0 and domU time to diverge
> when independent_wallclock is set to zero everywhere...
>  -- Keir

It occurs with ntpd in dom0 as well:

tomkins@host1 ~ $ date
Mon Sep  3 14:42:04 UTC 2007
testdomu:~# date
Mon Sep  3 14:43:11 UTC 2007

dom0 runs ntpd, the only thing which causes the domUs to correct
themselves is hwclock in dom0.  It's a fairly old machine though and
would need to be updated for testing:

Linux host1 #1 Sat Nov 25 19:57:50 Local time
zone must be set--see zic manu x86_64 AMD Athlon(tm) 64 Processor
3700+ AuthenticAMD GNU/Linux

However I've been moving all Xen servers away from Gentoo and
switching them to CentOS which doesn't seem to have the same problem.
I remember someone else having similar problems:


I've still got a couple of servers which haven't been updated to
CentOS yet.  I could upgrade them to a more recent Xen release as well
as a more recent kernel to test it.

Alex Tomkins <tomkins@xxxxxxxxxxxx>

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