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Timekeeping best practices for Linux guests

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

Notes:

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

32-bit Kernels

Linux Flavor	Version	Kernel Parameters	Notes
RHEL
	RHEL 6.x		No additional kernel parameters required.
	RHEL 5.7		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	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 4.9	clock=pmtmr divider=10 hpet=disable
	RHEL 4.8	clock=pmtmr divider=10 hpet=disable
	RHEL 4.7	clock=pmtmr divider=10 hpet=disable
	RHEL 4.6	clock=pmtmr hpet=disable
	RHEL 4.5	clock=pmtmr hpet=disable
	RHEL 4.4	clock=pmtmr hpet=disable
	RHEL 4.3	clock=pmtmr hpet=disable
	RHEL 4.2	clock=pmtmr hpet=disable
	RHEL 4.1	clock=pmtmr hpet=disable
	RHEL 4.0	clock=pmtmr hpet=disable
	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 (All updates)		No additional kernel parameters required.
	SLES 10 SP4 on ESX 5.0 and later	clock=pmtmr hpet=disable
	SLES 10 SP4 on ESX 4.x		Use a VMI enabled kernel.
	SLES 10 SP3 on ESXi 5.0	clock=pmtmr hpet=disable
	SLES 10 SP3 on ESX 3.5 and 4.x		Use a VMI enabled kernel.
	SLES 10 SP3 on ESX 3.0.x and earlier	clock=pmtmr hpet=disable
	SLES 10 SP2 on ESXi 5.0	clock=pmtmr hpet=disable
	SLES 10 SP2 on ESX 3.5 and 4.x		Use a VMI enabled kernel.
	SLES 10 SP2 on ESX 3.0.x and earlier	clock=pmtmr hpet=disable
	SLES 10 SP1	clock=pmtmr hpet=disable
	SLES 10	clock=pmtmr hpet=disable
	SLES 9 (All updates)	clock=pmtmr hpet=disable
	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 hpet=disable
	SLED 9	clock=pmtmr hpet=disable
Suse Linux
	Suse Linux 10.3	clocksource=acpi_pm
	Suse Linux 10.2	clocksource=acpi_pm
	Suse Linux 10.1	clock=pmtmr hpet=disable
	Suse Linux 10	clock=pmtmr hpet=disable
	Suse Linux 9.3	clock=pmtmr hpet=disable
	Suse Linux 9.2	clock=pmtmr hpet=disable
	Suse Linux 9.1	clock=pmtmr hpet=disable
	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 11.04		No additional kernel parameters required.
	Ubuntu 10.10		No additional kernel parameters required.
	Ubuntu 10.04		No additional kernel parameters required.
	Ubuntu 9.10		No additional kernel parameters required.
	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 ESXi 5.0	clocksource=acpi_pm
	Ubuntu 8.04 on ESX 3.5 and 4.x		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 hpet=disable
	Ubuntu 6.06	clock=pmtmr hpet=disable
	Ubuntu 5.10	clock=pmtmr hpet=disable
	Ubuntu 5.04	clock=pmtmr hpet=disable
Mandriva
	Mandriva Corporate Desktop 4.0	clock=pmtmr hpet=disable
	Mandriva Corporate Server 4	clock=pmtmr hpet=disable
	Mandriva Linux 2008	clocksource=acpi_pm
	Mandriva Linux 2007	clock=pmtmr hpet=disable
	Mandriva Linux 2006	clock=pmtmr hpet=disable
	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 hpet=disable
	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 SP3		No additional kernel parameters required.
	Asianux 3.0 SP2		No additional kernel parameters required.
	Asianux 3.0 SP1		No additional kernel parameters required. Use kernel 2.6.18-53.17AXS3 or later.
	Asianux 3.0	divider=10 clocksource=acpi_pm
CentOS
	CentOS 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.3	divider=10 clocksource=acpi_pm
	CentOS 5.2	divider=10 clocksource=acpi_pm
	CentOS 5.1	divider=10 clocksource=acpi_pm
	CentOS 5.0	clocksource=acpi_pm
	CentOS 4.9	clock=pmtmr divider=10 hpet=disable
	CentOS 4.8	clock=pmtmr divider=10 hpet=disable
	CentOS 4.7	clock=pmtmr divider=10 hpet=disable
	CentOS 4.6	clock=pmtmr hpet=disable
	CentOS 4.5	clock=pmtmr hpet=disable
	CentOS 4.4	clock=pmtmr hpet=disable
	CentOS 4.3	clock=pmtmr hpet=disable
	CentOS 4.2	clock=pmtmr hpet=disable
	CentOS 4.1	clock=pmtmr hpet=disable
	CentOS 4.0	clock=pmtmr hpet=disable
Oracle
	Oracle Linux 5.7		No additional kernel parameters required.
	Oracle Linux 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.3	divider=10 clocksource=acpi_pm
	OEL 5.2	divider=10 clocksource=acpi_pm
	OEL 5.1	divider=10 clocksource=acpi_pm
	OEL 5.0	clocksource=acpi_pm
	OEL 4.9	clock=pmtmr divider=10 hpet=disable
	OEL 4.8	clock=pmtmr divider=10 hpet=disable
	OEL 4.7	clock=pmtmr divider=10 hpet=disable
	OEL 4.6	clock=pmtmr hpet=disable
	OEL 4.5	clock=pmtmr hpet=disable
Debian
	Debian 5.0		No additional kernel parameters required.
	Debian 4.x	divider=10 clocksource=acpi_pm

64-bit Kernels

Linux Flavor	Version	Kernel Parameters	Notes
RHEL
	RHEL 6.x		No additional kernel parameters required.
	RHEL 5.7		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	RHEL 5.3	notsc divider=10 nohpet
	RHEL 5.2	notsc divider=10 nohpet
	RHEL 5.1 with RHSA-2007:0993-13	notsc divider=10 nohpet
	RHEL 5.1 without RHSA-2007:0993-13	notsc nohpet
	RHEL 5.0		No additional kernel parameters required.
	RHEL 4.9	notsc divider=10 nohpet
	RHEL 4.8	notsc divider=10 nohpet
	RHEL 4.7	notsc divider=10 nohpet
	RHEL 4.6	notsc nohpet
	RHEL 4.5	notsc nohpet
	RHEL 4.4	notsc nohpet
	RHEL 4.3	notsc nohpet
	RHEL 4.2	notsc nohpet
	RHEL 4.1	nohpet	Does not support notsc. Avoid using if possible.
	RHEL 4.0	nohpet	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 (All updates)		No additional kernel parameters required.
	SLES 10 SP4 on ESX 4.0 and later		No additional kernel parameters required.
	SLES 10 SP3 on ESX 3.5 and later		No additional kernel parameters required.
	SLES 10 SP3 on ESX 3.0.x and earlier	notsc nohpet
	SLES 10 SP2 on ESX 3.5 and later		No additional kernel parameters required.
	SLES 10 SP2 on ESX 3.0.x and earlier	notsc nohpet
	SLES 10 SP1	notsc nohpet
	SLES 10	notsc nohpet
	SLES 9 with kernel version 2.6.5-7.312 or later	ignore_lost_ticks nohpet
	SLES 9 with kernel version 2.6.5-7.311 or earlier	nohpet	Has no workaround for lost tick overcompensation. Avoid using if possible.
SLED
	SLED 11		No additional kernel parameters required.
	SLED 10	clock=pmtmr nohpet
Suse Linux
	Suse Linux 10.3	clocksource=acpi_pm
	Suse Linux 10.2	notsc nohpet
	Suse Linux 10.1	notsc nohpet
	Suse Linux 10	notsc nohpet
	Suse Linux 9.3	notsc nohpet
	Suse Linux 9.2	nohpet	Does not support notsc. Avoid using if possible.
	Suse Linux 9.1	nohpet	Does not support notsc. Avoid using if possible.
Ubuntu
	Ubuntu 11.04		No additional kernel parameters required.
	Ubuntu 10.10		No additional kernel parameters required.
	Ubuntu 10.04		No additional kernel parameters required.
	Ubuntu 9.10		No additional kernel parameters required.
	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 nohpet
	Ubuntu 6.06	notsc nohpet
	Ubuntu 5.10	notsc nohpet
	Ubuntu 5.04	nohpet	Does not support notsc. Avoid using if possible.
Mandriva
	Mandriva Corporate Desktop 4.0	notsc nohpet
	Mandriva Corporate Server 4	notsc nohpet
	Mandriva Linux 2008	clocksource=acpi_pm
	Mandriva Linux 2007	notsc nohpet
	Mandriva Linux 2006	notsc nohpet
	Mandrake Linux 10.1	nohpet	Does not support notsc. Avoid using if possible.
Turbolinux
	Turbolinux 10 Desktop	nohpet	Does not support notsc. Avoid using if possible.
	Turbolinux 10 Server	nohpet	Does not support notsc. Avoid using if possible.
Asianux
	Asianux 3.0 SP3		No additional kernel parameters required.
	Asianux 3.0 SP2		No additional kernel parameters required.
	Asianux 3.0 SP1		No additional kernel parameters required. Use kernel 2.6.18-53.17AXS3 or later.
	Asianux 3.0	notsc nohpet
CentOS
	CentOS 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	CentOS 5.3	notsc divider=10 nohpet
	CentOS 5.2	notsc divider=10 nohpet
	CentOS 5.1	notsc nohpet
	CentOS 5.0		No additional kernel parameters required.
	CentOS 4.9	notsc divider=10 nohpet
	CentOS 4.8	notsc divider=10 nohpet
	CentOS 4.7	notsc divider=10 nohpet
	CentOS 4.6	notsc nohpet
	CentOS 4.5	notsc nohpet
	CentOS 4.4	notsc nohpet
	CentOS 4.3	notsc nohpet
	CentOS 4.2	notsc nohpet
	CentOS 4.1	nohpet	Does not support notsc. Avoid using if possible.
	CentOS 4.0	nohpet	Does not support notsc. Avoid using if possible.
Oracle
	Oracle Linux 6.x		No additional kernel parameters required.
	Oracle Linux 5.6		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.5		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.4		No additional kernel parameters required. See the note below about the optional use of divider=10.
	OEL 5.3	notsc divider=10 nohpet
	OEL 5.2	notsc divider=10 nohpet
	OEL 5.1	notsc nohpet
	OEL 5.0		No additional kernel parameters required.
	OEL 4.9	notsc divider=10 nohpet
	OEL 4.8	notsc divider=10 nohpet
	OEL 4.7	notsc divider=10 nohpet
	OEL 4.6	notsc nohpet
	OEL 4.5	notsc nohpet
Debian
	Debian 5.0		No additional kernel parameters required.
	Debian 4.x	notsc nohpet

Recommended Configurations

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

RHEL 6 32-bit or 64-bit running on ESX 4.0 or later
RHEL 5.4 or later 32-bit or 64-bit running on ESX 3.5 or later
SLES 10 SP2 or later 64-bit running on ESX 3.5 or later
SLES 10 SP2 or later 32-bit running on ESX 3.5 or ESX 4.x
SLES 11 32-bit or 64-bit running on ESX 4.0 or later
Ubuntu 8.04 32-bit running on ESX 3.5 or later
Ubuntu 8.04 or later 64-bit running on ESX 4.0 or later
Ubuntu 8.04 or later 32-bit running on ESX 3.5 or ESX 4.x

Among different versions of RHEL 5 and RHEL 4, RHEL 5.4 or later has the best timekeeping behavior. For older versions, those supporting divider=10 have better timekeeping behavior than those that do not.

VMI is supported in ESX 3.5 and ESX 4.x. Support for VMI is not present in ESX 5.0. For more information related to VMI enabled kernels, see:

If you are running Java inside a virtual machine, some of the above parameters may affect the performance of your virtual machine. See the following articles:

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: VMware recommends you to use NTP instead of VMware Tools periodic time synchronization. NTP is an industry standard and ensures accurate time keeping in your guest. You may have to open the firewall (UDP 123) to allow NTP traffic.

This 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

This 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, perform one of these options:

Set tools.syncTime = "FALSE" 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. To enable time syncing again, use the same command with "0 1" instead of "1 0".

For more information, see Disabling Time Synchronization (1189).

For ESX 4.1 and later, use these parameters for Linux, Solaris, and FreeBSD:

To display the current status of the service:

vmware-toolbox-cmd timesync status
Disables periodic time syncronization:

vmware-toolbox-cmd timesync disable

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.

To disable one time synchronization events, see Disabling Time Synchronization (1189)

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.

Note for uses on divider=10

For some operating systems, divider=10 is a supported kernel configuration option, but might not be necessary for accurate timekeeping. Using it reduces the frequency of timer interrupts by 10x, which reduces the CPU overhead of processing timer interrupts. This overhead is especially noticeable for idle virtual machines. The only drawback of using divider=10 is that the granularity of wakeups provided by the kernel changes from 1ms to 10ms. The vast majority of applications are not affected by this, but using divider=10 may not be the right tradeoff for some time sensitive applications. For some operating systems, specifically older versions, divider=10 greatly improves timekeeping accuracy and is strongly recommended.

Additional Information

In addition to the above parameters, you can also add the clock=pit parameter to the Linux bootloader file. The clock=pit parameter causes the Linux 2.6 kernel to use a more efficient algorithm to synchronize time between the virtual machine and the ESX/ESXi host. This parameter causes the virtual machine to skip adjustment for ticks resulting in no time gain.

Update History

05/10/2011 - Updated for RHEL 5.6. 04/06/2011 - Added information for RHEL, OEL, and CentOS 4.9. 03/10/2011 - Ubuntu 10.04 added. 01/31/2011 - Added information for RHEL 5.4, 5.5, and 6.0, CentOS 5.4 and 5.5, and OEL 5.4 and 5.5. 06/21/2011 - Added information on why to use NTP and not VMware Tools and information to re-enable tools time sync with the command line 09/19/2011 - Added additional Kernel parameters

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