Re: [Xen-devel] System time monotonicity

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

On Mon, Mar 26, 2007 at 07:47:49PM +0100, Keir Fraser wrote:

> >> From what I can work out, time is supposed to be monotonic but I admit I
> > can't really understand the time code yet at least. I couldn't find any
> > documentation on what to expect from system time. Any suggestions?
> > 
> > This seems to happen across all the hardware we've tried but this
> > particular case is a Sun V20Z with two CPUs:
> > 
> >   x86 (AuthenticAMD family 15 model 5 step 10 clock 2392 MHz)
> >         AMD Opteron(tm) Processor 250
> 
> Small backwards time deltas are possible from the current time code. You'll
> have to filter them out yourself if you can't deal with them. We could add
> extra code in Xen to stop this happening for any individual VCPU

Some instrumentation indicated that we had cross-VCPU jitter of
significant deltas, ~18us at worst. Though the instrumentation wasn't
completely reliable so that might not be accurate.

On real hardware we deal with any jitter via comparing against a regular
clock on CPU0. This obviously won't work for Xen since we have no
control over where VCPU0 actually lives. Presumably the Linux code
(monotonic_clock() especially) must have something to handle it, but I
can't see where...

regards
john

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