For servers that do not have a High Precision Event Timer (HPET), what clock source should I use for my SMP or AMD system that uses PowerNow!?

Red Hat Enterprise Linux 4 Update 2 introduced the pmtmr boot time option. The pmtimer is part of the ACPI hardware and is implemented using pmtmr at boot time or in grub.conf.

kernel /vmlinuz-2.6.9-22.EL ro  root=/LABEL=/ rhgb quiet pmtmr

Most SMP Powernow! systems have pmtimer and pick it as the default. On systems that do not use pmtimer by default, use notsc as a boot line parameter so it does not use the Time Stamp Counter (TSC). TSC is unreliable on SMP systems and those running PowerNow! capable CPUs. For these systems, use pmtimer instead. Otherwise you will see clock skew.

If you can not use pmtimer and notsc is used the, Programmable Interval Timer (PIT) is used in its place.

The kernel should pick some other clock source. In general, if a clock source is known to be broken on a system, the appropriate command line argument should be added so the system does not accidentally pick it as a timesource. This includes nopmtimer, nohpet, and notsc.

The kernel should choose the proper timer for your system. Review the dmesg output to make sure.

Consult with your hardware manufacturer for details on the particular timer your motherboard uses.