RHEL and VMWare time skew problems

Well, I've gotten quite the number of questions over the last few days about time skew problems in RHEL4 and VMWare, so figured that I would write this.

In RHEL4.7, there will be a kernel feature that allows you to pass 'divider=' on the command line in grub. The number must be a divisor of 1000, and the most common value is 10 (yielding an effective timer interrupt rate of 100Hz). I'm expecting the 4.7 beta to become available Real Soon Now(TM), though I have no information as to when that will occur. In the meantime, this change is available as a hotfix from Red Hat's Global Support Services.

What this does is take the kernel tick rate (which defaults to 1000Hz) and divides it by the value of divider, to come up with an effective interrupt rate. You can verify that this is working by:

Before making the change, use 'watch --interval=1 cat /proc/interrupts' and you'll notice that the timer interrupt is incrementing by 1000 every second. After you make the change, you you notice with the same command that it is incrementing via 1000/ every second.

The vlaue of HZ that is exposed to modules is still 1000, thus not breaking module ABI.

This same change is already present in RHEL5.1 today. In recent Fedora's, this is unnecessary due to the advent of the tickless kernel (which is useful for other reasons, including reduced power consumption on laptops). It is likely that RHEL6 will also be based on a tickless kernel.