Timekeeping best practices for Linux

Purpose

This article presents best practices for Linux timekeeping. These recommendations include specifics on the particular kernel command line options to use for the Linux operating system of interest. There is also a description of the recommended settings and usage for NTP time sync, configuration of VMware Tools time synchronization, and Virtual Hardware Clock configuration, to achieve best timekeeping results.

Resolution

Linux Timekeeping Best Practice

Note: When both SMP and UP kernels are available, they must be applied to the appropriate SMP and UP virtual machines, otherwise a mismatch may cause time to drift.

Note: The recommended kernel parameters must be added to the kernel parameters already configured by the distribution.

32bit Kernels

Linux Flavor	Version	Kernel Parameters	Notes
RHEL
	RHEL 5.3	divider=10 clocksource=acpi_pm
	RHEL 5.2	divider=10 clocksource=acpi_pm
	RHEL 5.1	divider=10 clocksource=acpi_pm
	RHEL 5.0	clocksource=acpi_pm
	RHEL 4.7	clock=pmtmr divider=10
	RHEL 4.6	clock=pmtmr
	RHEL 4.5	clock=pmtmr
	RHEL 4.4	clock=pmtmr
	RHEL 4.3	clock=pmtmr
	RHEL 4.2	clock=pmtmr
	RHEL 4.1	clock=pmtmr
	RHEL 4.0	clock=pmtmr
	RHEL 3 (All updates)		No additional kernel parameters required.
	RHEL 2.1		No additional kernel parameters required.
Redhat
	Redhat 9.0		No additional kernel parameters required.
	Redhat 8.0		No additional kernel parameters required.
	Redhat 7.3		No additional kernel parameters required.
	Redhat 7.2		No additional kernel parameters required.
	Redhat 7.1		No additional kernel parameters required.
	Redhat 7.0		No additional kernel parameters required.
	Redhat 6.2		No additional kernel parameters required.
SLES
	SLES 11		No additional kernel parameters required.
	SLES 10 SP2 on ESX 3.0.x and earlier	clock=pmtmr
	SLES 10 SP2 on ESX 3.5 and later		Use a VMI enabled kernel.
	SLES 10 SP1	clock=pmtmr
	SLES 10	clock=pmtmr
	SLES 9 (All updates)	clock=pmtmr
	SLES 8		No additional kernel parameters required.
	SLES 7		No additional kernel parameters required.
SLED
	SLED 11		No additional kernel parameters required.
	SLED 10	clock=pmtmr
	SLED 9	clock=pmtmr
Suse Linux
	Suse Linux 10.3	clocksource=acpi_pm
	Suse Linux 10.2	clocksource=acpi_pm
	Suse Linux 10.1	clock=pmtmr
	Suse Linux 10	clock=pmtmr
	Suse Linux 9.3	clock=pmtmr
	Suse Linux 9.2	clock=pmtmr
	Suse Linux 9.1	clock=pmtmr
	Suse Linux 9.0		Remove desktop kernel command line parameter.
	Suse Linux 8.2		No additional kernel parameters required.
	Suse Linux 8.1		No additional kernel parameters required.
	Suse Linux 8.0		No additional kernel parameters required.
	Suse Linux 7.3		No additional kernel parameters required.
Ubuntu
	Ubuntu 9.04 with kernel 2.6.28-7.18 or later on ESX 4.0 or later		No additional kernel parameters required.
	Ubuntu 9.04 with kernel prior to 2.6.28-7.18 on ESX 4.0 or later		Avoid using if possible. May cause guest operating system to stop responding.
	Ubuntu 8.10 with kernel 2.6.27-12.28 or later on ESX 4.0 or later		No additional kernel parameters required.
	Ubuntu 8.10 with kernel prior to 2.6.27-12.28 on ESX 4.0 or later		Avoid using if possible. May cause guest operating system to stop responding.
	Ubuntu 8.10	clocksource=acpi_pm
	Ubuntu 8.04 on ESX 3.5 and later		Use a VMI enabled kernel.
	Ubuntu 8.04 on ESX 3.0.x and earlier	clocksource=acpi_pm
	Ubuntu 7.10	clocksource=acpi_pm
	Ubuntu 7.04	clocksource=acpi_pm
	Ubuntu 6.10	clock=pmtmr
	Ubuntu 6.06	clock=pmtmr
	Ubuntu 5.10	clock=pmtmr
	Ubuntu 5.04	clock=pmtmr
Mandriva
	Mandriva Corporate Desktop 4.0	clock=pmtmr
	Mandriva Corporate Server 4	clock=pmtmr
	Mandriva Linux 2008	clocksource=acpi_pm
	Mandriva Linux 2007	clock=pmtmr
	Mandriva Linux 2006	clock=pmtmr
	Mandrake 10.1		Does not support pmtmr. Avoid using if possible.
	Mandrake 10		Does not support pmtmr. Avoid using if possible.
	Mandrake 9.2		No additional kernel parameters required.
	Mandrake 9.1		No additional kernel parameters required.
	Mandrake 9.0		No additional kernel parameters required.
	Mandrake 8.2		No additional kernel parameters required.
	Mandrake 8.1		No additional kernel parameters required.
	Mandrake 8.0		No additional kernel parameters required.
Turbolinux
	Turbolinux 10 Desktop		Does not support pmtmr. Avoid using if possible.
	Turbolinux 10 Server	clock=pmtmr
	Turbolinux Enterprise 8		No additional kernel parameters required.
	Turbolinux Workstation 8		No additional kernel parameters required.
	Turbolinux 7.0		No additional kernel parameters required.
Asianux
	Asianux 3.0	divider=10 clocksource=acpi_pm
CentOS
	CentOS 5.2	divider=10 clocksource=acpi_pm
	CentOS 5.1	divider=10 clocksource=acpi_pm
	CentOS 5.0	clocksource=acpi_pm
Oracle
	OEL 5.2	divider=10 clocksource=acpi_pm
	OEL 5.1	divider=10 clocksource=acpi_pm
	OEL 5.0	clocksource=acpi_pm

64bit Kernels

Linux Flavor	Version	Kernel Parameters	Notes
RHEL
	RHEL 5.3	notsc divider=10
	RHEL 5.2	notsc divider=10
	RHEL 5.1 with RHSA-2007:0993-13	notsc divider=10
	RHEL 5.1 without RHSA-2007:0993-13	notsc
	RHEL 5.0		No additional kernel parameters required.
	RHEL 4.7	notsc divider=10
	RHEL 4.6	notsc
	RHEL 4.5	notsc
	RHEL 4.4	notsc
	RHEL 4.3	notsc
	RHEL 4.2	notsc
	RHEL 4.1		Does not support notsc. Avoid using if possible.
	RHEL 4.0		Does not support notsc. Avoid using if possible.
	RHEL 3 Update 9 with RHSA-2008-0973	disable_lost_ticks
	RHEL 3 through Update 8		Has no workaround for lost tick overcompensation. Avoid using if possible.
SLES
	SLES 11		No additional kernel parameters required.
	SLES 10 SP2 on ESX 3.5 and later
	SLES 10 SP2 ESX 3.0.x and earlier	notsc
	SLES 10 SP1	notsc
	SLES 10	notsc
	SLES 9 with kernel version 2.6.5-7.312 or later	ignore_lost_ticks
	SLES 9 with kernel version 2.6.5-7.311 or earlier		Has no workaround for lost tick overcompensation. Avoid using if possible.
SLED
	SLED 11		No additional kernel parameters required.
	SLED 10	clock=pmtmr
Suse Linux
	Suse Linux 10.3	clocksource=acpi_pm
	Suse Linux 10.2	notsc
	Suse Linux 10.1	notsc
	Suse Linux 10	notsc
	Suse Linux 9.3	notsc
	Suse Linux 9.2		Does not support notsc. Avoid using if possible.
	Suse Linux 9.1		Does not support notsc. Avoid using if possible.
Ubuntu
	Ubuntu 9.04 with kernel 2.6.28-7.18 or later on ESX 4.0 or later		No additional kernel parameters required.
	Ubuntu 9.04 with kernel prior to 2.6.28-7.18 on ESX 4.0 or later		Avoid using if possible. May cause guest operating system to stop responding.
	Ubuntu 8.10 with kernel 2.6.27-12.28 or later on ESX 4.0 or later		No additional kernel parameters required.
	Ubuntu 8.10 with kernel prior to 2.6.27-12.28 on ESX 4.0 or later		Avoid using if possible. May cause guest operating system to stop responding.
	Ubuntu 8.10	clocksource=acpi_pm
	Ubuntu 8.04 with kernel 2.6.24-24.52 or later on ESX 4.0 or later		No additional kernel parameters required.
	Ubuntu 8.04 with kernel prior to 2.6.24-24.52 on ESX 4.0 or later		Avoid using if possible. May cause guest operating system to stop responding.
	Ubuntu 8.04	clocksource=acpi_pm
	Ubuntu 7.10	clocksource=acpi_pm
	Ubuntu 7.04		No additional kernel parameters required.
	Ubuntu 6.10	notsc
	Ubuntu 6.06	notsc
	Ubuntu 5.10	notsc
	Ubuntu 5.04		Does not support notsc. Avoid using if possible.
Mandriva
	Mandriva Corporate Desktop 4.0	notsc
	Mandriva Corporate Server 4	notsc
	Mandriva Linux 2008	clocksource=acpi_pm
	Mandriva Linux 2007	notsc
	Mandriva Linux 2006	notsc
	Mandrake Linux 10.1		Does not support notsc. Avoid using if possible.
Turbolinux
	Turbolinux 10 Desktop		Does not support notsc. Avoid using if possible.
	Turbolinux 10 Server		Does not support notsc. Avoid using if possible.
Asianux
	Asianux 3.0	notsc
CentOS
	CentOS 5.2	notsc divider=10
	CentOS 5.1	notsc
	CentOS 5.0		No additional kernel parameters required.
Oracle
	OEL 5.2	notsc divider=10
	OEL 5.1	notsc
	OEL 5.0		No additional kernel parameters required.

Recommended Configurations

The following configurations are expected to have the best timekeeping behavior:

SLES 10 SP2 32bit or 64bit running on ESX 3.5 or later
Ubuntu 8.04 32bit running on ESX 3.5 or later

Among RHEL 4 and 5, versions with the divider=10 option have better timekeeping behavior than those without.

For more information related to VMI enabled kernels, see:

Editing Kernel Configuration

Kernel command line parameters are specified in the /etc/lilo.conf or /boot/grub/grub.conf file, depending on your choice of boot loader.

For LILO, put the kernel command line parameters at the end of the "append" line. For example, if the append line looks like:

append="resume=/dev/hda6 splash=silent"

and you want to add "clock=pmtmr divider=10" the updated text is:

append="resume=/dev/hda6 splash=silent clock=pmtmr divider=10"

Remember to run /sbin/lilo after editing lilo.conf , so that your edits take effect.

For GRUB, put the kernel command line parameters at the end of the "kernel" line. For example if the kernel line looks like:

kernel /vmlinuz-2.6.18 ro root=/dev/hda2

and you want to add "clock=pmtmr divider=10" the updated text is:

kernel /vmlinuz-2.6.18 ro root=/dev/hda2 clock=pmtmr divider=10

For additional information about working with boot loaders, see your Linux distribution's documentation.

NTP Recommendations

Note: In all cases use NTP instead of VMware Tools periodic time synchronization. Also, you may need to open the firewall (UPD 123) to allow NTP traffic.

The following is a sample /etc/ntp.conf :

tinker panic 0

restrict 127.0.0.1

restrict default kod nomodify notrap

server 0.vmware.pool.ntp.org

server 1.vmware.pool.ntp.org

server 2.vmware.pool.ntp.org

driftfile /var/lib/ntp/drift

The following is a sample /etc/ntp/step-tickers :

0.vmware.pool.ntp.org

1.vmware.pool.ntp.org

The configuration directive tinker panic 0 instructs NTP not to give up if it sees a large jump in time. This is important for coping with large time drifts and also resuming virtual machines from their suspended state.

Note: The directive tinker panic 0 must be at the top of the ntp.conf file.

It is also important not to use the local clock as a time source, often referred to as the Undisciplined Local Clock. NTP has a tendency to fall back to this in preference to the remote servers when there is a large amount of time drift.

An example of such a configuration is:

server 127.127.1.0

fudge 127.127.1.0 stratum 10

Comment out both lines.

After making changes to NTP configuration, the NTP daemon must be restarted. Refer to your operating system vendor’s documentation.

VMware Tools time synchronization configuration

When using NTP in the guest, disable VMware Tools periodic time synchronization.

To disable VMware Tools periodic time sync you use one of the following options:

Set tools.syncTime = "0" in the configuration file (.vmx file) of the virtual machine.

OR
Deselect Time synchronization between the virtual machine and the host operating system in the VMware Tools toolbox GUI of the guest operating system.

OR
Run the vmware-guestd --cmd "vmx.set_option synctime 1 0" command in the guest operating system.

These options do not disable one-time synchronizations done by VMware Tools for events such as tools startup, taking a snapshot, resuming from a snapshot, resuming from suspend, or VMotion. These events synchronize time in the guest operating system with time in the host operating system, so it is important to make sure that the host operating system's time is correct.

To do this for VMware ACE, VMware Fusion, VMware GSX Server, VMware Player, VMware Server, and VMware Workstation run time synchronization software such as NTP or w32time in the host. For VMware ESX run NTP in the service console. For VMware ESXi, run NTP in the VMkernel.

Virtual Hardware clock configuration

When configuring the Linux guest operating system, if you are given a choice between keeping the “hardware” clock (that is, the virtual CMOS time of day clock) in UTC or local time, choose UTC. This avoids any confusion when your local time changes between standard and daylight saving time (in England, "summer time").

For additional information, see Timekeeping in VMware Virtual Machines .

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KB Article: 1006427
Updated: May 28, 2009

Products:
VMware ACE
VMware ESX
VMware ESXi
VMware Fusion
VMware GSX Server
VMware Player
VMware Server
VMware Workstation

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