| Re: [chrony-users] Chrony offset and stability adjustments? (fwd) |
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On Wed, Jan 09, 2019 at 10:20:23AM -0800, Bill Unruh wrote:
> chrony and ntp assume that the
> local clock is reasonable in that it counts some internal process wich is
> reasonably regular, it is just that its rate fluctuates. chrony estimates that
> and corrects it. But a VM can simply forget to count that internal process for
> huge swathes of time, randomly.
This used to be a big problem in older VM implementations, but in
modern VMs it shouldn't be, at least when configured properly. The VM
can use the same hardware counter in the CPU as the host - the
time-stamp counter (TSC), which is always running, even when the VM is
not. In some VMs this counter can be virtualized in order to allow
migrations between different machines (with CPUs running at a
different frequency), which can cause the problem you describe, but
there should be an option to use the real TSC.
--
Miroslav Lichvar
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