Installation guide

Warning
The divider kernel parameter was previously recommended for Red Hat Enterprise Linux 4
and 5 guests that did not have high responsiveness requirements, or exist on systems with
high guest density. It is no longer recommended for use with guests running Red Hat
Enterprise Linux 4, or Red Hat Enterprise Linux 5 versions prior to version 5.8.
divider can improve throughput on Red Hat Enterprise Linux 5 versions equal to or later
than 5.8 by lowering the frequency of timer interrupts. For example, if HZ=1000, and divider
is set to 10 (that is, divider=10), the number of timer interrupts per period changes from the
default value (1000) to 100 (the default value, 1000, divided by the divider value, 10).
BZ #698842 details a bug in the way that the divider parameter interacts with interrupt and
tick recording. This bug is fixed as of Red Hat Enterprise Linux 5.8. However, the divider
parameter can still cause kernel panic in guests using Red Hat Enterprise Linux 4, or Red Hat
Enterprise Linux 5 versions prior to version 5.8.
This parameter was not implemented in Red Hat Enterprise Linux 3, so this bug does not affect
Red Hat Enterprise Linux 3 guests.
Red Hat Enterprise Linux 6 does not have a fixed-frequency clock interrupt; it operates in
tickless mode and uses the timer dynamically as required. The divider parameter is therefore
not useful for Red Hat Enterprise Linux 6, and Red Hat Enterprise Linux 6 guests are not
affected by this bug.
Usin g t h e Real- T ime C lock wit h Win d o ws Server 2003 and Win d o ws XP g u est s
Windows uses the both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For Windows
guests the Real-Time Clock can be used instead of the TSC for all time sources which resolves guest
timing issues.
To enable the Real-Time Clock for the PMTIMER clocksource (the PMTIMER usually uses the TSC)
add the following line to the Windows boot settings. Windows boot settings are stored in the boot.ini
file. Add the following line to the boot.ini file:
/use pmtimer
For more information on Windows boot settings and the pmtimer option, see Available switch options
for the Windows XP and the Windows Server 2003 Boot.ini files.
Usin g t h e Real- T ime C lock wit h Win d o ws Vist a, Win d o ws Server 2008 and Win d o ws 7
g u est s
Windows supports both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For some
Windows guests, the RTC can be selected for use instead of the TSC for all of the guest's time
sources. This can resolve some guest timing issues.
⁠Chapt er 1 7 . KVM guest t iming management
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