Finally, Cool 'n' Quiet You Can Use

Modern day microprocessors have many operating frequencies they can choose from; these are called p-states. The original Phenom only had two p-states: full frequency and 1/2 frequency. A Phenom 9950 2.6GHz would either run at 2.6GHz or 1.3GHz. The original Phenom was the first quad-core x86 CPU to allow each core to operate at an independent p-state. All of Intel's quad-cores at that point required all four cores to run at the same p-state.

In theory, the AMD design made sense. If you were running a single threaded application, the core that your thread was active on would run at full speed, while the remaining three cores would run at a much lower speed. AMD included this functionality under the Cool 'n' Quiet umbrella. In practice however, Phenom's Cool 'n' Quiet was quite flawed. Vista has a nasty habit of bouncing threads around from one core to the next, which could result in the following phenomenon (no pun intended): when running a single-threaded application, the thread would run on a single core which would tell Vista that it needed to run at full speed. Vista would then move the thread to the next core, which was running at half-speed; now the thread is running on a core that's half the speed as the original core it started out on.

Phenom II fixes this by not allowing individual cores to run at clock speeds independently of one another; if one core must run at 3.0GHz, then all four cores will run at 3.0GHz. In practice this is a much better option as you don't run into the situations where Phenom performance is about half what it should be thanks to your applications running on cores that are operating at half speed. In the past you couldn't leave CnQ enabled on a Phenom system and watch an HD movie, but this is no longer true with Phenom II.

Honestly, AMD's initial Phenom approach is more elegant, but unfortunately the current task scheduling mechanism causes problems. The other issue is that Phenom wasn't switching core speeds quickly enough; ideally it shouldn't matter that a high-priority thread got bounced to a new core, as the new core should simply scale up to full speed in a fraction of a second. Regardless, Phenom II addresses the issues with Phenom CnQ performance not being where it should be.

The Phenom II now supports a maximum of four p-states, with a minimum clock speed of 800MHz. The states for each chip are defined below:

Processor Max P-State P2 P3 Min P-State
AMD Phenom II X4 940 3.0GHz 2.3GHz 1.8GHz 800MHz
AMD Phenom II X4 920 2.8GHz 2.1GHz 1.6GHz 800MHz

Intel still has the technological advantage with Core i7; while it too runs all of its cores at the same frequency, idle cores can be turned off completely thanks to the use of Intel's power gate transistors. While this would be nice to have with Phenom II, at least we finally have a working CnQ.

I ran SYSMark 2007 to demonstrate the performance impact of CnQ on Phenom and Phenom II:

Processor SYSMark 2007 Overall Score
CnQ On
SYSMark 2007 Overall
CnQ Off
% Increase When Disabling CnQ
AMD Phenom II X4 940 182 185 1.6%
AMD Phenom 9950BE 136 157 15.4%

Note that the performance on Phenom goes up by over 15% when I disable CnQ, while Phenom II shows less than a 2% gain. This is actually a best case scenario for the original Phenom, however; in my testing I've seen situations where performance is cut in half. Bottom line? The Cool'n'Quiet problems are now resolved, and Phenom II is starting to look recommendable.

Cache and Memory Controller Comparison 45nm and Low Power Consumption
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  • Kromis - Thursday, January 08, 2009 - link

    *Stands up and applause* Reply
  • wowo - Thursday, April 23, 2009 - link

    how x264?

    x264 benchmark is 819,very old.

    now is 1139.Improved a lot

    please ues new x264,more scores will be Changed.

    Reply
  • cioangel - Wednesday, February 13, 2013 - link

    I have been looking through forum sites for hours. This is the most complete answer I have managed to get so far. Just to make things clear: I am using an AM2+ motherboard and it supports some AM3 processors and says so in the manual. What I am confused on is the memory I will have to use with it. If I use my old AM2+ mb and put a AM3 cpu in there, do I need to run DDR2 or DDR3? I would like to use my old memory for a while to defer the cost of the processor upgrade. Reply

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