Re: [Xen-devel] System time monotonicity

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

On Mon, Mar 26, 2007 at 09:04:47PM +0100, John Levon 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.

I've done some more testing, the problem appears to be significantly
worse than you're claiming. I store the previously read value in the
thread structure and breakpoint whenever it fails:

[2]> fffffffece9e6040::print kthread_t t_dhrtime t_dhrtime_tsc t_dhrtime_cpu 
t_dhrtime_vers
t_dhrtime = 0x3de7923989c
t_dhrtime_tsc = 0x9414981ce0e
t_dhrtime_cpu = 0x2
t_dhrtime_vers = 0x2616

And the new values:
t_dhrtime = 0x3de7923088b
t_dhrtime_tsc = 0x9414982169b
t_dhrtime_cpu = 0x2
t_dhrtime_vers = 0x2618

[2]> 0x3de7923989c - 0x000003de7923088b=D
                36881

That's 36us. It doesn't even seem that we migrated across physical CPUs
given the version field. So something seems significantly awry to me?

(I must admit I still don't quite understand why monotonic_clock() is
called monotonic_clock() ...)

regards
john

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