calcru: runtime went backwards errors

[Bob Johnson]

On 6/30/08, David Allen <the.real.david.allen at gmail.com> wrote:
> I've been seeing errors like the following appearing:
>
> Jun 30 03:13:57 ford kernel: calcru: runtime went backwards from 261
> usec to 258 usec for pid 516 (devd)
[...]
> Jun 30 03:13:57 ford kernel: calcru: runtime went backwards from 486
> usec to 481 usec for pid 0 (swapper)
>
> and narrowed down the cause to openntpd.
>
> Do these errors fall into the Mostly Harmless category?

It's probably just an annoyance, unless it is happening so often it
causes other problems.

The FreeBSD FAQ used to have a nice explanation of this, but it has
been replaced by a discussion that simply assumes the problem is
caused by the Intel SpeedStep implementation on your motherboard:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/faq/troubleshoot.html#Q5.24.

But in general, this error can be caused by several things, including
a device that is slow to respond to interrupts. One thing that often
helps on SMP systems is to make sure your timecounter isn't using TSC:

$ sysctl kern.timecounter
kern.timecounter.tick: 1
kern.timecounter.choice: TSC(-100) HPET(900) ACPI-fast(1000) i8254(0)
dummy(-1000000)
kern.timecounter.hardware: ACPI-fast
[...]

- Bob