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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  • poohbear - Thursday, January 08, 2009 - link

    this is fantastic news! and just when i was about to upgrade from my ancient s939 system to a C2D system, seems i might be sticking to AMD after all! thanks for review! Reply
  • PrezWeezy - Thursday, January 08, 2009 - link

    For less than $20 more the i7 920 looks like it wins in every single test by a fair margin, doesn't seem like this is really all that competitive, considering the i7 is still in the "high price" phase. I can't believe it wont drop to the $275 mark rather soon which would put the XII 940 back to the same position the original Phenom was, too little too late. Reply
  • Roland00 - Thursday, January 08, 2009 - link

    More expensive Motherboard+More expensive Ram makes i7 about 400 dollars more in cost Reply
  • strikeback03 - Friday, January 09, 2009 - link

    How you figure? By the chart on page 4, it is less than $200. Even if you go for one of the $300 motherboards, you won't see a $400 difference.

    When I built my current system, E6600/P965/2GB DDR2 cost me over $600, and that was considered a decent mid-range system. As my primary use of computing power is Photoshop, I would definitely go for i7 even if cheaper motherboards do not become available.
    Reply
  • Roland00 - Friday, January 09, 2009 - link

    It isn't quite 400 but here

    motherboard p45 vs x58 most x58 are 300 vs 100-120 for p45,
    Ram, 6 gb of ddr3 is about 200, vs 50 or 60 for 6gb of ddr2.
    For a nonstock cpu cooler you are talking 60 to 70 bucks with the i7 for it is a new socket and their is very few products for it. You can get a good cpu cooler for intel quad for 30 to 40 dollars.

    Savings about 200+140+30=370

    If you get things on sale you might be able to find 6gb ram for 150, cpu for 230, and you may be able to get the motherboard cheaper if you get one of the basic versions but you are still talking about 300 more.
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    I am not saying i7 isn't worth the extra money, it is still new tech but it does show beneficial gains (on encoding, minimum frame rate on games, and overclocking) but right now the motherboards and ram is expensive.
    Reply
  • BSMonitor - Friday, January 09, 2009 - link

    You are rounding up on i7 side and rounding down on the core 2 side. It is not $50 for 6GB of DDR2. It is not only $300 for x58. I have seen them for $200. I have seen 6GB DDR3 kits for $140 too.

    It's like we are talking about bare entry into Core 2 and Phenom II, but enthusiest for i7. Why does one need 6GB to entry into i7? 3GB would be reasonable and ~$70-80.

    Phenom II is cute yes, but nothing to jump on.
    Reply
  • Roland00 - Friday, January 09, 2009 - link

    I am rounding up on the I7 side for like most people I buy things with tax (for my state charges tax on internet transactions.) In addition many people buy their equipment in stores such as fry's, microcenter, comp usa, etc.

    370 times 8.25% tax rate (my area's sales tax) is...400 dollars and 52 cents

    ------

    And no I am not overpricing the ram or similar equipment. Go to Fry's, Microcenter, or some other store and you will see the prices I listed or much higher.

    ------

    Regardless you seem to be missing the point, the original poster I was responding to was saying i7 was only 25 dollars higher, and I said that was wrong for you have to figure in the platform costs.
    Reply
  • PrezWeezy - Friday, January 09, 2009 - link

    You were right, I had forgotten about the new socket and DD3. Even so, using the parts Anandtech used, the i7 is about $187 more expensive than the PII (pun totally intended). The C2D with a Q9400 though is only $44 cheaper than the i7. Almost all of that has to do with the motherboards used here, and I'm sure you could find a combo of motherboard/CPU that would bring the price closer but that's besides the point. Reply
  • calyth - Thursday, January 08, 2009 - link

    "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."

    Anand, read that sentence again.

    The problem isn't AMD designing a chip with broken CnQ. The problem is that Microsoft, after so many years, still can't write a scheduler. The problem persists on XP too. The thread that handles the mouse would rev up, causing the chip to switch p-state. Switching p-states takes time, and because of exclusive caching on AMD chips, when the scheduler puts the same thread on different cores, it causes the L1 & L2 to be ineffective.

    I have trouble in WinXP with CnQ on if I move my mouse, but not surprisingly, the same Phenom chip works like a chap in Linux. Because the scheduler isn't an idiot, and 1GHz is more than enough to handle mouse input.

    AMD erred in fixing a software problem in hardware. Independent p-states saved some power if only a single thread needed the speed.
    Reply
  • Zak - Thursday, January 08, 2009 - link

    Well, I hope AMD won't lose the momentum, because right now there isn't that much to celebrate: they've barely caught up with Intel's 2 years old CPU line:(

    Z.
    Reply

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