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[Xen-users] Clock accuracy

  • To: xen-users@xxxxxxxxxxxxx
  • From: Iain Buchanan <iainbuc@xxxxxxxxx>
  • Date: Fri, 10 Feb 2017 15:18:36 +0000
  • Delivery-date: Fri, 10 Feb 2017 15:19:54 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>


I'm trying to determine how accurate the clock is on a Xen guest (running on AWS).  I hadn't realised how complex virtualising clocks were until I started looking into this!

I have a few questions around clocksource settings I hope someone can help me with:
- I get the impression that the "tsc" clocksource in Xen is relatively recent, and should bring the guest and host clocks in line, but I could not determine how often this synchronisation takes place or if there are any bounds on clock drift?

- The default (using an Ubuntu 14.04 image at least) under AWS seems to be the "xen" clocksource.  From messages online this appears to be able to go backwards, which suggests that it is also forcing the guest to be in line with the host.  Is this the case?

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