32-bit Kernels

RHEL 6.x      No additional kernel parameters required.
RHEL 5.4      divider=10
RHEL 5.1      divider=10 clocksource=acpi_pm
RHEL 4.0      clock=pmtmr hpet=disable

Debian 5.0      No additional kernel parameters required.
Debian 4.x      divider=10c locksource=acpi_pm

64-bit Kernels

Debian 5.0    No additional kernel parameters required.
Debian 4.x    notsc nohpet

divider=10

Using it reduces the frequency of timer interrupts by 10x, which reduces the CPU overhead of processing timer interrupts. (granularity of wakeups provided by the kernel changes from 1 ms to 10 ms.)