Pinning CPU frequency

[mkatliar]

For consistent performance measurement, we want to run the CPU at a constant frequency. The frequency can be set by e.g.

sudo cpupower frequency-set -g userspace
sudo cpupower frequency-set -f 2.60GHz

The userspace governor is not available if intel_pstate driver is active. To disable it, add intel_pstate=disable to the Linux kernel command line in /etc/default/grub:

GRUB_CMDLINE_LINUX="intel_pstate=disable"

Then do sudo update-grub and reboot the system, and use the cpupower commands above to pin the CPU frequency.

This is what I also found using ChatGPT:

When you disable the intel_pstate driver, the system falls back to the older acpi-cpufreq driver. Even though acpi-cpufreq doesn’t display the turbo frequency as a selectable max frequency, the CPU may still boost beyond 2.6 GHz under the right conditions. Turbo boost is often controlled by the CPU firmware and BIOS settings, independent of the frequency scaling driver.

On many modern kernels, there’s a global switch to control CPU turbo/boost for acpi-cpufreq:

cat /sys/devices/system/cpu/cpufreq/boost

A value of 1 typically indicates that boost/turbo is enabled.
A value of 0 indicates that it is disabled.
To disable boost/turbo:

echo 0 | sudo tee /sys/devices/system/cpu/cpufreq/boost

Check again:

cat /sys/devices/system/cpu/cpufreq/boost

You should see 0, indicating that boost is now disabled.

[mkatliar]

@roversch did you try to disable intel_pstate and pin the frequency as written above?

[mkatliar]

Tips on reducing variance in benchmarks: https://github.com/google/benchmark/blob/main/docs/reducing_variance.md#reducing-variance-in-benchmarks

They also suggest disabling Hyperthreading/SMT.

[roversch]

Good that I documented it

https://github.com/mkatliar/blast#making-benchmarks-give-more-consistent-results

[roversch]

I also remember trying frequency-set, added it here #51