so Athlon XP CPUs run pretty hot at full speed all the time, and my PC makes lots of noise as a result. I have a temperature-sensitive CPU fan, so reducing the CPU temp will reduce noise, too.
A while back, I came across this doc, the Athlon Powersaving HOWTO, which contains a great tip — namely a way to put the processor in ‘STPGNT Mode’ (Stop Grant Mode), which disconnects it from the FSB and turns off parts of the CPU when not in use.
It works perfectly, in most respects, although the Ensoniq 5880 onboard sound chip goes crazy when it’s active, as it can’t deal with the changed timings from the CPU. But when I’m playing music, I can’t hear the fans anyway ;)
The details — to keep it brief, just take a look at the commands for my chipset as described here. I’m using ACPI in the kernel anyway, since I’m using software suspend-to-disk as well.