On Wed, Nov 03, 2021 at 11:55:41AM +0000, Stuart Henderson wrote: > On 2021/11/03 05:47, Crystal Kolipe wrote: > > > 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. > > using mwait is a years-old change
It certainly is. I'm not sure what if anything that has to do with the current discussion. When the CPU is manually throttled to a lower speed, which is effectively what the OP was doing, logically it will be placed into the lower C states less often if at all, as there is less idle time. THIS changes the RF noise profile of the equipment, as does many other things. Have you measured the typical RF of any system with the OP's configuration before and after the commit he mentioned? I see a detectable but insignificant to me difference.
