Negative ping times with FreeBSD 8.1-RELEASE on older Celeron system

[Dan Nelson]

In the last episode (Sep 13), Brett Glass said:
> Thank you! Since it's tunable at runtime I just tested it, and -- sure
> enough -- no negative ping times.
> 
> Ironically, it was the kernel that selected the ACPI timer, scoring it
> higher than the timestamp counter as a clock source.  Perhaps code should
> be added to ensure that the timer is not chosen if it rolls over in less
> than a second, since this clearly leads to imprecision and missed
> rollovers.
>
> kern.timecounter.tc.ACPI-safe.mask: 16777215                                    
> kern.timecounter.tc.ACPI-safe.counter: 7967112                                  
> kern.timecounter.tc.ACPI-safe.frequency: 3579545                                
> kern.timecounter.tc.ACPI-safe.quality: 850       

It doesn't roll over in less than a second; it rolls over in 16777215 /
3579545 = 4.6 seconds.  Your negative time delta problem isn't due to
rollover.  

-- 
	Dan Nelson
	dnelson at allantgroup.com