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Re: [Xen-users] Time Synchronization

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: Stephan Austermühle <au@xxxxxxx>
  • Date: Tue, 11 Apr 2006 07:54:03 +0200
  • Delivery-date: Mon, 10 Apr 2006 22:54:47 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>


Urs Thuermann schrieb:

> How does that work?  I thought every domU has its own software system
> clock updated by regular interrupts e.g. at 100Hz or 1000Hz.  This
> would require running ntpd to synchronize the clock to some external
> reference clock.  Or is the gettimeofday system call modified in domU
> so that it communicates to dom0 to get the time, i.e. no more
> interrupt driven software clock in the domU?

Although I have not taken a look into the sources it seems that the
hardware clock (RTC) is present only once (in dom0) and is reported to
domU's when they are created. Once a domU is running the guest kernel
maintains its own system clock. To prevent it drifting away time
synchronization is necessary in every domU. The independent wallclock
switch just seems to allow a domU to write a time to the virtual RTC
that is different from the real RTC. Maybe the Xen people can clarify this?

Kind regards,


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