Timekeeping in stable/9

[Ian Lepore]

On Tue, 2012-01-17 at 20:12 +0000, Joe Holden wrote:
> Hi guys,
> 
> Has anyone else noticed the tendency for 9.0-R to be unable to 
> accurately keep time?  I've got a couple of machines that have been 
> upgraded from 8.2 that are struggling, in particular a Virtual box guest 
> that was fine on 8.2, but now that's its been upgraded to 9.0 counts at 
> anything from 2 to 20 seconds per 5 second sample, the result is similar 
> with HPET, ACPI-fast and TSC.
> 
> I also have physical boxes which new seem to drift quite substantially, 
> ntpd cannot keep up and as these boxes need to be able to report the 
> time relatively accurately, it is causing problems with log times and 
> such...
> 
> Any suggestions most welcome!
> 
> Thanks,
> J

I finally got a 9.0 generic build done today and I've been watching the
timekeeping on 3 systems and they're all doing just fine.  Two of the
systems are performing pretty much identically to how they did on 8.2;
the clock frequency correction calculated by ntpd differs by less than
1ppm.  On the other system the kernel timekeeping routines are now
choosing to use a different clock so I don't get a direct comparison of
the old vs new drift rate, but the drift is still reasonable  (100ppm
now, used to be around 88, on an old 300mhz MediaGx-based system).

I haven't had time yet to learn about the new eventtimer stuff in 9.0,
but I know you can get some info on the choices it made via sysctl
kern.eventtimer.  Before 9.0 I'd check sysctl kern.clockrate and vmstat
-i and make sure the chosen clock is interrupting at the right rate, but
now with the eventtimer stuff there's not an obvious correlation between
hz and profhz and stathz and any particular device's interrupt rate, at
least for some clock choices (on the old MediaGx system without ACPI or
HPET it seems to work more like it used to).

-- Ian