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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
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 5.4 |
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 4.8 | clock=pmtmr divider=10 | ||
| 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 or clock=hpet* | * AMD Rev F Opteron processors | |
| 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 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 5.3 | 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 4.8 | clock=pmtmr divider=10 | ||
| CentOS 4.7 | clock=pmtmr divider=10 | ||
| CentOS 4.6 | clock=pmtmr | ||
| CentOS 4.5 | clock=pmtmr | ||
| CentOS 4.4 | clock=pmtmr | ||
| CentOS 4.3 | clock=pmtmr | ||
| CentOS 4.2 | clock=pmtmr | ||
| CentOS 4.1 | clock=pmtmr | ||
| CentOS 4.0 | clock=pmtmr | ||
| Oracle | |||
|
|
OEL 5.2 | divider=10 clocksource=acpi_pm | |
| OEL 5.1 | divider=10 clocksource=acpi_pm | ||
| OEL 5.0 | clocksource=acpi_pm | ||
| Debian | |||
| Debian 4.x | divider=10 clocksource=acpi_pm |
64bit Kernels
|
Linux Flavor |
Version |
Kernel Parameters |
Notes |
|
RHEL |
|
|
|
| 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 | ||
|
|
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 4.8 | notsc divider=10 | ||
|
|
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 |
|
No additional kernel parameters required. |
|
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 SP1 |
|
No additional kernel parameters required.
Use kernel 2.6.18-53.17AXS3 or later. | |
| Asianux 3.0 |
notsc |
||
| CentOS | |||
| CentOS 5.3 | notsc divider=10 | ||
| CentOS 5.2 |
notsc divider=10 |
||
| CentOS 5.1 |
notsc |
||
| CentOS 5.0 |
|
No additional kernel parameters required. | |
| CentOS 4.8 | notsc divider=10 | ||
| CentOS 4.7 | notsc divider=10 | ||
| CentOS 4.6 | notsc | ||
| CentOS 4.5 | notsc | ||
| CentOS 4.4 | notsc | ||
| CentOS 4.3 | notsc | ||
| CentOS 4.2 | notsc | ||
| CentOS 4.1 |
Does not support notsc. Avoid using if possible. | ||
| CentOS 4.0 |
Does not support notsc. Avoid using if possible. | ||
| Oracle | |||
|
|
OEL 5.2 |
notsc divider=10 |
|
| OEL 5.1 |
notsc |
||
| OEL 5.0 |
|
No additional kernel parameters required. | |
| Debian | |||
| Debian 4.x | notsc |
Recommended Configurations
The following configurations are expected to have the best timekeeping behavior:
- RHEL 5.4 32bit or 64bit running on ESX 3.5 or later
- SLES 10 SP2 32bit or 64bit running on ESX 3.5 or later
- SLES 11 32bit or 64bit running on ESX 4.0 or later
- Ubuntu 8.04 32bit running on ESX 3.5 or later
- Ubuntu 8.04, 8.10, or 9.04, 32bit or 64bit running on ESX 4.0 or later
Among different version 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.
For more information related to VMI enabled kernels, see:
- How to enable a virtual machine interface in a Linux kernel and in ESX 3.5 (1003644)
- Enabling VMI with SLES 10 SP2 32bit virtual machines on an ESX host (1005701)
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 (UDP 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 = "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.
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.
Note on RHEL 5.4 and divider=10
For RHEL 5.4 and later, divider=10 is a supported kernel configuration option, but is no longer 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 RHEL 5.3 and earlier, divider=10 greatly improves timekeeping accuracy and is strongly recommended.
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- KB Article: 1006427
- Updated: Nov 4, 2009
- Products:
VMware ACE
VMware ESX
VMware ESXi
VMware Fusion
VMware GSX Server
VMware Player
VMware Server
VMware Workstation