Chapter 14. KVM Guest Timing Management
- Interrupts cannot always be delivered simultaneously and instantaneously to all guest virtual machines. This is because interrupts in virtual machines are not true interrupts - they are injected into the guest virtual machine by the host machine.
- The host may be running another guest virtual machine, or a different process. Thereofore, the precise timing typically required by interrupts may not always be possible.
ntpdservice. For more information, see the Red Hat Enterprise 6 Deployment Guide.
14.1. Constant Time Stamp Counter (TSC)
constant_tscflag is present. To determine if your CPU has the
constant_tscflag run the following command:
$ cat /proc/cpuinfo | grep constant_tsc
constant_tscbit. If no output is given follow the instructions below.
14.1.1. Configuring Hosts without a Constant Time Stamp Counter
constant_tscbit, disable all power management features (BZ#513138). Each system has several timers it uses to keep time. The TSC is not stable on the host, which is sometimes caused by
cpufreqchanges, deep C state, or migration to a host with a faster TSC. Deep C sleep states can stop the TSC. To prevent the kernel using deep C states append
processor.max_cstate=1to the kernel boot options in the
grub.conffile on the host:
title Red Hat Enterprise Linux (2.6.32-330.x86_64) root (hd0,0) kernel /vmlinuz-2.6.32-330.x86_64 ro root=/dev/VolGroup00/LogVol00 rhgb quiet \ processor.max_cstate=1
cpufreq(only necessary on hosts without the
constant_tsc) by editing the
/etc/sysconfig/cpuspeedconfiguration file and change the
MAX_SPEEDvariables to the highest frequency available. Valid limits can be found in the