On Tue, 2 Nov 2021, Theo de Raadt wrote: > Paul de Weerd <we...@weirdnet.nl> wrote: > > > A recent commit by Theo changed the hw.perfpolicy behavior to always > > run at full speed when AC power is on. This means that my workstation > > (and servers, once I upgrade them) now consumes significantly more > > power, even though they usually idle. > > Did you measure how much more power? > > You must measure, to make such a claim. > > Your OptiPlex 9020 is probably a modern i5/i7, which probably contains > C states similar to this: > > acpicpu0 at acpi0: C2(200@148 mwait.1@0x33), C1(1000@1 mwait.1), PSS > > Which means when the idle loop calls the "mwait" instruction, the cpu > will 'instantly' slow down to a lower clock and make other power use > reductions, until an interrupt happens and requires labour again.
See also https://en.wikichip.org/wiki/race-to-sleep - it's generally more energy efficient to run the CPU at full speed so it can finish its work faster and get back to a low-power state sooner
