Description of problem: In the 5.1 kernel, we added the tick divider option so that you can effectively run a machine at a lower HZ than 1000, which is important for certain applications and for virtualization. In order to use this feature, you need to pass "divider=<int>" on the kernel command-line when booting. However, the Release Notes for 5.1: http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/release-notes/RELEASE-NOTES-U1-x86_64-en.html say that you need to pass "tick_divider=<int>" on the kernel command-line, which is not true. This has been causing confusion with CentOS people, and will probably confuse customers as well. I'd like to change the online version of the notes to say "divider=" instead of "tick_divider=" to reduce the confusion.
Further, the whole section is actually wrong. It's not a sysfs parameter at all. Here's the current text: =============================================================================== tick_divider The tick_divider=<value> option is a sysfs parameter that allows you to adjust the system clock rate while maintaining the same visible HZ timing value to user space applications. Using the tick_divider= option allows you to reduce CPU overhead and increase efficiency at the cost of lowering the accuracy of timing operations and profiling. Useful <values> for the standard 1000Hz clock are: * 2 = 500Hz * 4 = 250Hz * 5 = 200Hz * 8 = 125Hz * 10 = 100Hz (value used by previous releases of Red Hat Enterprise Linux) Note that the virtualized kernel does not support multiple timer rates on guests. dom0 uses a fixed timing rate set across all guests; this reduces the load that multiple tick rates could cause. ============================================================================== This would be much more clear and useful as the following: ============================================================================== divider The divider=<value> option is a kernel command-line parameter that allows you to adjust the system clock rate while maintaining the same visible HZ timing value to user space applications. Using the divider= option allows you to reduce CPU overhead and increase efficiency at the cost of lowering the accuracy of timing operations and profiling. This can be useful in virtualized environments as well as for certain applications. Useful <values> for the standard 1000Hz clock are: * 2 = 500Hz * 4 = 250Hz * 10 = 100Hz (value used by previous releases of Red Hat Enterprise Linux) Note that the virtualized (Xen) kernel uses a 250HZ clock by default, and thus does not need the divider= option either in dom0 or in paravirtualized guests. ============================================================================== Chris Lalancette
thanks Chris. i have revised the release note accordingly in source, the online release notes will be updated on the next cycle.