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RE: [Xen-devel] timekeepeing in XEN - request fo comments

To: Bartosz Lis <bartoszl@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] timekeepeing in XEN - request fo comments
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 9 Nov 2009 08:50:52 -0800 (PST)
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What kernels are running in your domUs?  Are they
also (all)  If your domU's are older kernels
(e.g. SLES10 or older, RHEL5 or older) there are
many issues that cannot be entirely solved by
Xen or dom0.

> -----Original Message-----
> From: Bartosz Lis [mailto:bartoszl@xxxxxxxxxxxxx]
> Sent: Monday, November 09, 2009 5:21 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] timekeepeing in XEN - request fo comments
> Dear developers,
> I have some issues with time synchronization among dom0 and 
> domUs. In my case 
> helped reconfiguring kernel with HZ=1000 for dom0 and HZ=100 
> for domUs and 
> running ntpd also on domUs. I think, running ntpd on domU is 
> not a good idea, 
> but it works acceptably good as a workaround for my 
> configuration: XEN 3.4.1 + 
> kernel with pv_ops (taken from git last week). When 
> I don't run ntpd 
> on domU clock slowly drifts. I have choosen xen clocksource 
> in both dom0 and 
> domU.
> I'm not the only one to have time synchronization issues. 
> Please write some 
> document specifying how time synchronization between dom0 and 
> domU should be 
> achieved. I have a number of special questions, but probably 
> there are more 
> questions that should be aswered:
> 1. What is the mechanics behind the clocksource named xen? 
> How does it depend 
> on phisical time sources available on different platforms?
> 2. How gets domU's clock initialized when domU boots? (I 
> observe that just 
> after domU has started its clock differs from dom0. The 
> difference is about 
> 10s.)
> 3. What are the consequences of not selecting xen clocksource 
> (by selecting 
> one of hpet, acpi_pm, etc.) for dom0 and for domU?
> 4. What are the best practices to achieve clock synchronization?
> 5. What has changed recently in timekeeping? What is going to 
> change? Are 
> there any improvements/bugfixes added to redhat or suse 
> kernels? Are these 
> bugfixes planned to be added upstream?
> Thank you in advance,
> -- 
> Bartosz Lis @ Inst. of Information Technology, Technical 
> Univ. of Lodz, Poland
>    bartoszl @ ics.p.lodz.pl
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