Changing clock source in Linux

[Edge100x]

A customer asked us in our support system how to change the clock source in Linux, and I'm posting about it here, because this seems like something that will come up again.

The clock source is used for kernel timers, and it is important to use a high-quality one because game servers use kernel timers constantly. Using a poor clock source can lead to problems with the game skipping or with the FPS being unstable/dipping.

Normally the kernel picks the best clocksource automatically. To see its current choice, use this command:

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cat /sys/devices/system/clocksource/clocksource0/current_clocksource

This will usually print out "tsc", on a Gentoo Linux VDS, and TSC (the CPU time stamp counter) is usually the right choice, because it is fast and stable on Nehalem CPUs. On CentOS, you should see "xen", which is good as well, since it mirrors Xen's clocksource (Xen usually picks TSC, but can change things up to be better at times).

You can see what other clock sources are available with this command:

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cat /sys/devices/system/clocksource/clocksource0/available_clocksource

Which will print out a number of possible sources that is a subset of these:

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xen acpi_pm hpet tsc

If you are using Gentoo (or a different kernel without PV-on-HVM drivers), you won't see then "xen" option, and if you have HPET turned off in the "Advanced" section of the Server control page, you won't see the "hpet" option.

Usually "tsc" is the best choice, but you can try any of the others out, as well, by changing the setting on the fly. To do this, just echo the one you want into the current_clocksource. For example:

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echo hpet > /sys/devices/system/clocksource/clocksource0/available_clocksource

If it seems to work better for you, just set your startup script (/etc/conf.d/local.start on Gentoo, /etc/rc.local on CentOS) to do this on boot.