Friday, 30 September 2011

Why is my CPU Frequency Limited?

Sometimes the Scaling Maximum Frequency of a CPU is reduced below that of the possible top frequency and finding out why this is can be problematic.  The limitation could have been imposed by:

* Thermal limits
* Hardware limitations (e.g. ACPI _PPC object).
* Program that wrote to a /sys/devices/cpu/cpu*/cpufreq/scaling_max_freq

Fortunately Thomas Renninger introduced /sys/devices/cpu*/cpufreq/bios_limit that exports to user space the BIOS limited maximum frequency for each CPU.  This feature is available in Ubuntu Maverick 10.10 upwards.

So, if you have a machine that you believe should have CPUs running at a higher frequency, inspect the bios_limit files to see if the BIOS is mis-configured.

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