What is better from an energy-saving perspective to run task minimum energy consumption

Question

The CPU frequency and CPU usage are the main factors that impact energy consumption (as far as I know). however what is better from an energy-saving perspective to run task minimum energy consumption:

Option 1: Maximum CPU frequency with minimum usage

Option 2: Maximum CPU usage with min frequency.

Answer 1

Work per time scales approximately linearly with CPU frequency. (A bit less than linear because higher CPU frequency means DRAM latency is more clock cycles).

CPU power has two components: switching (dynamic) power which scales with f ³ (because voltage has to increase for higher frequency, and transistors switch are pumping that V^2 capacitor energy more often); and leakage power which doesn't vary as dramatically. At high frequency dynamic power dominates, but as you lower the frequency, eventually it becomes significant. The smaller your transistors, the more significant leakage is.

System-wide, there's also other power for things like DRAM that doesn't change much or at all with CPU frequency.

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Min frequency is more efficient , unless the minimum is far below the best frequency for work per energy. (Some parts of power decrease with frequency, others like leakage current and DRAM refresh don't).

Frequencies lower than max have lower work per energy (better task efficiency) up to a certain point. Like 800 MHz on a Skylake CPU on Intel's 14 nm process. If there's work to be done, there's no gain from dropping below that; just race-to-sleep at that most efficient frequency. (Power would decrease, but work rate would decrease more below that point.)

https://en.wikichip.org/wiki/File:Intel_Architecture,_Code_Name_Skylake_Deep_Dive-_A_New_Architecture_to_Manage_Power_Performance_and_Energy_Efficiency.pdf is slides from IDF2015 about Skylake power management covered a lot of that general-case stuff well. Unfortunately I don't know where to find a copy of the audio from Efraim Rotem's talk; it was up for a year or so after, but the original link is dead now. :/

Also in general about dynamic power (from switching, not leakage) scaling with frequency cubed if you adjust voltage as well as frequency, see Modern Microprocessors A 90-Minute Guide! and

• https://electronics.stackexchange.com/questions/614018/why-does-switching-cause-power-dissipation
• https://electronics.stackexchange.com/questions/258724/why-do-cpus-need-so-much-current
• https://electronics.stackexchange.com/questions/548601/why-does-decreasing-the-cmos-supply-voltage-also-decrease-the-maximum-circuit-fr