AMD Cool 'n' Quiet, Turbo Core Impact On Linux

When there's no load on the FX-8150 with Cool 'n' Quiet enabled, the frequency drops down to 1400MHz rather than running at the default 3600MHz. (Worth noting is that when Turbo Core is enabled, the higher CPU frequency doesn't seem to be indicated by any of the normal interfaces.)

New to the AMD Bulldozer (Family 15h) CPUs is a new register for being able to read the estimated power consumption of the CPU itself. This is without any special monitoring device, motherboard-specific, or other special requirements. Added to the Linux kernel this summer was the "fam15h_power" driver to expose this estimated Bulldozer power reading to a sysfs node and can be read using LM_Sensors. However, my testing has shown the figures to be wildly inaccurate. For example, when idling, even with Cool 'n' Quiet dropping the CPU down to 1400MHz the reported CPU power consumption was still 125 Watts. The value did not change at all while idling regardless of CnQ or Turbo Core, but stayed at 125.3 Watts, which is the TDP for the FX-8150 eight-core.

The CPU temperature as reported by AMD's hwmon driver also seems to be slightly off. The CPU cooling was the AMD water-cooling kit, but with an idle temperature of around 10°C is much lower than expected. However, when Cool 'n' Quiet was enabled it reflected a drop of nearly 2°C. When Turbo Core was enabled while still idling (so no clocks should be ramped up), the temperature was higher by 1°C.

The Phoronix Test Suite automatically monitored the system's AC power consumption via an USB-based WattsUp power meter. During the idle process, enabling Cool 'n' Quiet knocked down the power consumption by five watts. Having Turbo Core enabled even while idling led to an increase in power consumption by 10 Watts.