A few weeks ago my father pointed out that the date stamps on my blog posts were behind by a week. Upon investigation, I found that bonzo’s clock was a week behind. I updated it and declared victory.

Then, he pointed it out again a few days ago. Sure enough, it had slipped by several days. When I logged into the VMWare Console to check for options to sync the clock or whatever, I noticed a repeated error from the FreeBSD kernel that I’ve been getting on bonzo forever and always ignored:

calcru: runtime went backwards from [some big number] usec to [another] usec for pid [pid]

I googled this message, and found a whole community of FreeBSD users suffering under slipping clocks when running FreeBSD under VMWare. There’s something on the freebsd-current list, and VMWare’s own support forums.

There are a few proposed fixes, most involving the kern.timecounter.hardware sysctl. I tried changing it from its default of APIC to TSC and i8245, but none worked.

I then ran across a post on the VMWare forums suggesting:

In FreeBSD:

'tools.timeSync = "true"' added to .vmx file
 sysctl -w kern.timecounter.hardware=i8254
 kldload vmmemctl (from vmware-tools) and have vmware-guestd running
 add 'kern.hz="250"' to /boot/loader.conf

I don’t have APIC or ACPI disabled in my FreeBSD host either

Now, I don’t want to run the VMware tools just to keep the clock in sync, but I did put kern.hz="250" in /boot/loader.conf and kern.timecounter.hardware=i8254 in /etc/sysctl.conf, then rebooted.

It’s been several minutes now, and the clock seems to be holding. I’m afraid I don’t understand in detail why this helps, though a VMware knowledgebase article alludes to a problem of missed timer interrupts, with a fix being reducing the frequency of the timer interrupts requested by the OS. I think that’s kern.hz="250". The importance of switching the time counting method from APIC to i8254 is less clear, unless it’s just a more reliable source of ticks.

At any rate, this problem has caused me to notice that VMWare server is in RC-2. As it’s the free successor to GSX Server 3, I really need to upgrade. Perhaps over the coming long weekend…