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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  • Spoelie - Thursday, January 08, 2009 - link

    Only one little gripe: why was a mid-range motherboard used for the phenom while the intel processors got enthusiast versions?

    there IS a difference apparently: http://www.legitreviews.com/article/795/5/">http://www.legitreviews.com/article/795/5/

    Not that it would change the conclusions.
    Reply
  • melgross - Thursday, January 08, 2009 - link

    "Not that it would change the conclusions. "

    You answered your question yourself.
    Reply
  • duploxxx - Thursday, January 08, 2009 - link

    Nice review, always my firts site to read for a review. A bit basic on oc potential but you hint there is more to come, lets hope we don't have to wait another month like we had to wait for the 790GX board reviews.

    I don't see why AMD launched the unicore @1.8ghz.

    You are stating that it is because of yields, might be but shanghai launched @2.0-2.2. Phenom2 would scale a lot better performance wise against penryn with a 2,2GHZ NB speed. for sure on the BE part that is a real advantage against the q9400-Q9550

    Is this to give the am3+ an additional performance gain when launched? Retail chips hit NB speeds of 2,4-2,6 easy, they also showed up to 3.5-3.6ghz oc on stock vcore, your oc gain was real low, perhaps you show in future oc review what phenom can actually do.

    no overview of total system power consumption idle and load?
    Reply
  • ssj4Gogeta - Thursday, January 08, 2009 - link

    Since most of the people have Intel now, it'll take them only a processor upgrade if they decided to buy a better Intel processor. But if they choose to switch to AMD, they'll have to buy the mobo as well.

    So for *most* people, getting a Q9400 (or Q9550 if the prices drop) will cost around $270, while getting a Phenom II 940 will cost around $470. And since this is the case for the majority, I don't see Phenom II being price competitive at all.
    Reply
  • RadnorHarkonnen - Thursday, January 08, 2009 - link

    There are more people with AM2+ Motherboards than you can think of.
    They may not spew they writings on the forums or comment actively saying "I'm upgrading!!!".

    Units shipped, i would say you r are really short sighted. And the AMD2/AMD3 compatibility is great.
    Reply
  • KikassAssassin - Thursday, January 08, 2009 - link

    Yeah, for people building new systems right now who don't want to spend the money on an expensive i7 mobo and DDR3, the Phenom II looks really nice. Intel probably isn't going to make any more LGA775 CPUs, whereas an AM2+ system might have more room for future upgrades with AM3 being backwards compatible. Reply
  • melgross - Thursday, January 08, 2009 - link

    But if you do go the i7 route now, you won't have to upgrade for a longer time than if you go with Phenom 1. Overall costs over time will still be lower. Reply
  • melgross - Thursday, January 08, 2009 - link

    Oops! meant Phenom 11, or course. Anyway, the higher performance vs the price is worthwhile for many people. Reply
  • plonk420 - Thursday, January 08, 2009 - link

    ask Dark Shikari of x264 fame .. i'm sure he could tell you an approximation of Phenom's L3 cache latency... and possibly Phenom II latency soon. Reply
  • hameed - Thursday, January 08, 2009 - link

    In the first table here http://www.anandtech.com/cpuchipsets/showdoc.aspx?...">http://www.anandtech.com/cpuchipsets/showdoc.aspx?... the percentages are hard to understand since they need to be flipped (i7 is before Quad) and btw in Cinebench the Quad advantage is 12.8% not 4.8% and the CS4 percentages are also not accurate. Reply

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