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

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Subject: [Xen-devel] timekeepeing in XEN - request fo comments
From: Bartosz Lis <bartoszl@xxxxxxxxxxxxx>
Date: Mon, 9 Nov 2009 13:21:29 +0100
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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 

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 

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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