MyComputingArt

AMD Cool'n'Quiet

I'll try to answer these questions:

What is AMD Cool'n'Quiet?
Requirements
Does it really work?

What is AMD Cool'n'Quiet (or CnQ)?

AMD defines Cool'n'Quiet as a technology "that can effectively lower the power consumption and enable a quieter-running system while delivering performance-on-demand".
In practice, if a 2 GHz processor at idle normally gets a 1.3V voltage, with CnQ enabled, the same processor at idle gets a 1.1V voltage and its clock speed is set to 1 GHz; but whenever a process runs, and the processor is no more at idle, voltage is increased to 1.3V and clock speed rises to 2 GHz.
The lower voltage allows to reduce power consumption when the processor is at idle. Lower voltage also means less heat. Hence, fans can run slower and quieter (less rpm = less noise). The name "cool and quiet" just means that.
Think about variable displacement technology in car engines, like Chrysler Hemi, where tipically half of the cylinders can be shut off to reduce fuel consumption when there is no need of maximum power, and shut on when the driver asks for high performance. The concept of Cool'n'Quiet is exactly as Hemi's.

Requirements

AMD Athlon 64 processor
Motherboard with a CnQ compatible BIOS. A list of compatible motherboards is available on the AMD site.
Windows or Linux Power Management
Heatsink and/or fans with speed control (to get the "Quiet" part of CnQ)

To Check voltage, clock speed and processor usage, AMD Athlon 64/FX Processor Utilities can be downloaded from AMD.

Enabling and disabling Cool'n'Quiet

On Windows, go to Control Panel > Power Options and select Minimal Power Management to enable CnQ, or Always On to disable it.

Cool'n'Quiet enabled Cool'n'Quiet disabled

Does it really work?

Short answer: Yes, it does, but if your system is already adequately cooled it could be useless.
Long answer: Read on.

On a AMD X2 3800+ (2000 MHz) these are the results of CnQ enabled and disabled:

System idle, with CnQ enabled: Voltage: 1.1v CPU Clock Speed: 50% (1000 MHz)
AMD Clock:	AMD PowerNow! Dashboard Demo:
System idle, with CnQ disabled: Voltage: 1.3V CPU Clock Speed: 100% (2000 MHz)
AMD Clock:	AMD PowerNow! Dashboard Demo:
System running United Devices Agent at full speed [1], with CnQ enabled: Voltage: 1.25v CPU Clock Speed: oscillates between 90% and 100% (1800 MHz - 2000 MHz)
AMD Power Monitor:	AMD PowerNow! Dashboard Demo:
System running United Devices Agent at full speed [1], with CnQ disabled: Voltage: 1.3v CPU Clock Speed: 100% (2000 MHz)
AMD Power Monitor:	AMD PowerNow! Dashboard Demo:

And here are the differences in temperature, as shown by SpeedFan:

System idle, with CnQ enabled	System idle, with CnQ disabled
System at full speed [1], with CnQ enabled	System at full speed [1], with CnQ disabled

I left the system idle with CnQ enabled for 15', waiting for temperature to drop to the lowest levels (32�C for the core temperature).
Then I disabled CnQ and immediately the temperature began raising; just after 3 minutes the core fan accelerated its speed from 1200 rpm to full speed at 3300 rpm; after 5 minutes the core temperature stabilized at 2-3 degrees above the level with CnQ (34-35�C); forcing the core fan speed at 40% the core temperature raised to 4-5 degrees above the level with CnQ (36-37�C): at this level the core fan automatically went again to full speed.
Pushing the system at full speed with UD Agent (see note [1]) the difference between temperatures with CnQ enabled and CnQ disabled tightened to 1-2 degrees because the difference in voltage was reduced to 0.05V. The motherboard temperature instead raised 3 degrees because the airflow was not optimal: before doing the tests I stopped the case fan that should pull the fresh air inside the case from the front intakes.
There's one thing I definitely dislike: running UD Agent the voltage is continuously adjusted between 1.25V and 1.30V, and when I say "continuously" I really mean continuously. This is because UD Agent is not a multithread application, so while with a single core processor it uses 100% of the speed with AMD X2 dual core processor it uses only 50% of the speed, divided between the two cores. Running a multithread application or multiple single thread applications that push the two cores at 100% this problem does not occur and the voltage always stays at 1.30V.

Test conditions:
Ambient temperature: 20.5�C.
System as tested: AMD X2 3800+ with AMD stock heatsink, mobo MSI K9N Platinum, RAM DDR2 Corsair 2 GB PC6400, HD SATA WD2500JS and EIDE WD1200JB, case Antec SLK3000B with one rear exhaust Tricool fan and closed side exhaust, PSU Seasonic S12-500W, ATi Radeon X1300 with passive cooling.
SpeedFan: the core fan speed is set to automatically adjust between 40% and 100%: under 40% the noise is unaudible and as low as 30% the core fan stops revving.
[1] Full speed test: with United Devices Agent running, only 50% of processor speed is used and the work is made by the two cores, using about half of their power.

With the front case fan on

I repeated the test with the front case fan on, a Papst 4412 F/2GLL (one of the most efficient and silent fans agreeing with madshrimps.be): its speed is constant and my motherboard does not allow to adjust it, thus it always revs at full speed.
Here are the temperatures:

System idle, with CnQ enabled	System idle, with CnQ disabled
System at full speed [1], with CnQ enabled	System at full speed [1], with CnQ disabled

At idle Cool'n'Quiet seems to be totally ineffective, despite the reduced voltage, but looking at how low the temperatures are, it could be impossible to push them lower. The pictures show the effectiveness of a better airflow inside the case, especially for hard disks.
Even at full speed [1] the difference is quiet inexistent, except for one hard disk, but I don't think it's related to Cool'n'Quiet.

Test conditions:
Ambient temperature: 20�C.
System as tested: AMD X2 3800+ with AMD stock heatsink, mobo MSI K9N Platinum, RAM DDR2 Corsair 2 GB PC6400, HD SATA WD2500JS and EIDE WD1200JB, case Antec SLK3000B with one rear exhaust Tricool fan and closed side exhaust and one Papst 4412 F/2GLL placed at bottom just behind the front air intakes, PSU Seasonic S12-500W, ATi Radeon X1300 with passive cooling.