AMD Cool'n'Quiet (PowerNow!) for Windows XP (SP2) and Linux (Fedora 3)

This article contains what I found when I added Cool'n'Quiet support to my machine for Windows XP (SP2) and Linux (Fedora 3). If you are in a rush to get to the setup, just scroll to the BIOS Support section, then continue on to the OS of your choice.

The Intro

A few days ago, my friend Rally emailed me a pointer to a web page about the AMD Cool'n'Quiet feature and how to enable it. We have very similar Athlon-64 setups, so we try to keep each other informed about tips and tricks that relate to the chip. A few days later, He showed me exactly how it worked at a LAN party, and from then on I just had to investigate.

The System

Just as background info, my machine is a 754-pin Athlon-64 3200+ with 1MB of cache and a 2GHz max clock speed. It sits on a MSI mobo with 1GB of PC3500 RAM and a Radeon 9600 Pro. The whole thing is wrapped in an Antec Sonata case. Not the latest tech, but fast enough to play all of the latest games pretty well. The machine was already reasonable quiet, and I never noticed any thermal problems. But the engineering Geek in me couldn't stand the fact that there was a way to make my machine more efficient, and I didn't have it. That's plenty of motive for me.

The Basics

First of all, What is Cool'n'Quiet? Once you get past the obvious marketing title, you find that Cool'n'Quiet is a nice little feature in AMD chips that allows the clock speed and voltage of the chip to be dialed down to save power. The idea is that you probably don't need all of the CPU's 'horsepower' all of the time. Usually, you only need top performance for things like games, compilation, video rendering, and other compute intensive tasks. For mundane tasks like word processing, email, and most importantly, screensavers, you really don't need all of the cycles available to you. All of those unused cycles don't do much more than consume power and generate heat.

Cool'n'Quiet is really just a different perspective on the dynamic clock settings that have been used for overclocking gaming machines for the past few years. Most of these other dynamic clock schemes are used to push the CPU speed up as far as it can possibly go. They usually rely on temperature readings to tell them where the limits are. They know they have reached the limit that they can push the clock when the temperature exceeds a predetermined level. The trick is setting that level to a value that will be reached before the CPU crashes

Cool'n'Quiet is an attempt to use a CPU monitor to determine when the CPU is lightly loaded, and set the CPU voltage and frequency to a lower level. It isn't terribly difficult to monitor how much time the CPU spends in it's idle state and use this data to judge whether it is lightly loaded or not. Unfortunately, I haven't been able to prove that Linux can do this. I'll explain later.

The Cool'n'Quiet hardware seems to require 2 levels of software support. First of all, it requires BIOS support. This is usually supplied by the motherboard manufacturer. The AMD web site maintains a list of recommended motherboards that also tells whether they support PowerNow or not.

Next it needs OS support. In the case of Windows XP, AMD supplies this OS support in the form of an installable driver that can be downloaded from AMD's Web Site for free. In the case of Linux, powernow has been compiled into most recent kernels.

BIOS Support

BIOS support on my system was pretty easy to set up. I'm currently running version 1.9 of my BIOS which contains the AMIBIOS SetupUtility Version 3.31a. I just selected the Frequency/Voltage Control page and changed the Cool'n'Quiet setting to Enabled. Other systems are likely to have a slightly different procedure for enabling the feature, but the end result should be the same. If you are currently in the market for a motherboard and want to make sure it supports Cool'n'Quiet, check the AMD web page. They have a list of recommended boards that indicates which support the feature.

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