The divider=<value> option is a kernel command line option 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 is useful when running RHEL-4 under virtualized environments to
help keep more accurate time and reduce load on the hypervisor.
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)
As (partly) discussed in
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=220281, the 1000 Hz timer
interrupt (tick) rate in early 2.6 kernels has some bad effects. Most
obviously, the frequent timer interrupts have a performance cost. Also
significantly, ticks get lost fairly often at this high rate, so the kernel has
code to detect and compensate for lost ticks, but this code is buggy (see
http://bugzilla.kernel.org/show_bug.cgi?id=5127) and can add spurious ticks,
making the clock run faster than real time. If the lost tick compensation is
disabled (which is possible on 32-bit kernels using the clock=pit command-line
flag), lost ticks occur and the clock runs slower than real time.
All these effects occur on real hardware, but are much more severe in a virtual
machine. (Of course one can avoid the problems by paravirtualizing time or
moving to newer kernel versions that don't base timekeeping on counting
interrupts, but this bug report is about stock RHEL4 kernels.)
The divider= patch that was added to RHEL5 in response to
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=220281 helps with these
issues by letting the user select a lower timer interrupt rate without
recompiling the kernel. Can this patch be backported to RHEL4 as well?
committed in stream U7 build 68.5. A test kernel with this patch is available
*** Bug 429010 has been marked as a duplicate of this bug. ***
release note copied over from RHEL5.1 to RHEL4.7 for this bug. thanks!
the RHEL4.7 release notes deadline is on June 17, 2008 (Tuesday). they will
undergo a final proofread before being dropped to translation, at which point no
further additions or revisions will be entertained.
a mockup of the RHEL4.7 release notes can be viewed here:
please use the aforementioned link to verify if your bugzilla is already in the
release notes (if it needs to be). each item in the release notes contains a
link to its original bug; as such, you can search through the release notes by
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.
Added New Release Notes Contents.