Re: [Xen-devel] [PATCH] linux-2.6.18-xen.hg: fix interaction of NTP and dom0->xen time updates

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 14:31 +0000 on 18 Mar (1237386684), Dan Magenheimer wrote:
> Just checking... this provides improvement when:
> 
> 1) the patch is applied to dom0
> 2) dom0 is running NTP
> 3) pv domU has independent_wallclock=0
> 4) pv domU is NOT running NTP

Yes, exactly that.  Xen's wallclock tends to sawtooth, drifting with the
underlying stime and being pulled back whenever dom0 updates it.
Without this patch, it pulls back only when ntpd in dom0 calls adjtime
(allowing for up to 100ms of drift on my test machine).  With the patch
it updates every minute, as intended (still sawtoothing but not as
wildly).

> Correct?  Does it also help when:
> 
> 1) the patch is applied to pv domU
> 2) pv domU has independent_wallclock=1
> 3) pv domU IS running NTP

Not sure, but I would guess not -- with independent_wallclock==1, domU
shuold be ignoring the wc_sec and wc_nsec values provided by Xen, which
are the only things affected by this patch.

> Are there any other configurations where it will
> help?  Are there any configurations where it might
> cause problems?

I don't think so, to either question.  If there's some bad condition
that is triggered by the act of pulling time back in sync rather than
the size of the correction, it might be more frequent with the more
frequent updates.  Otherwise, it's just making an existing sawtooth a
bit less blatant.

Tim. 

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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