On Tue, Nov 02, 2021 at 12:30:56PM -0600, 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.
This high frequcency cycling causes detectable (*) RF noise. In the vast majority of cases this shouldn't be an issue, but it seems worth noting just in case we start seeing bug reports of humming on speakers once this change sees it's way into the next release and more people are exposed to it. The change in noise profile can be tested and measured in a crude way using a simple loop of unsheilded wire connected to some small speakers. (*) Detectable when deliberately looked for. In a correctly shielded system I haven't observed and don't expect any negative effects. In terms of power consumption, I see no significant measurable change on two systems tested here.
