Updated: Tuning C'n'Q: Maximize Power And Performance, Part 2

Performance And Power Management, The Best Of Both Worlds

We have now looked at six different AMD processors from several different perspectives. We’ve measured their actual power consumption. We’ve analyzed them from a performance standpoint, with and without power management. We’ve tweaked their voltages. We then overclocked them to gauge the relationship between voltages and power consumption, and how far they can scale while still consuming power in proportion to performance.

We've done all of this without changing a single BIOS setting. Amazing, right? It's all been a result of simply manipulating AMD's Cool'n'Quiet technology, the power management feature inherent to modern AMD processors.

In the process, we’ve learned that BIOS versions can have a significant impact on power consumption. The choice of motherboard is also critical. While the Gigabyte GA-MA790GP DS4H offers lots of tweaking options, it cannot deliver idle power consumption close to the Biostar TA790GX 128M. We believe this is mostly because the motherboard is designed for high overclockability rather than low power consumption.

It’s Not As Easy As It Sounds

Indeed, simultaneously achieving the best performance and power management is more complicated than it seems. However, the rewards are worth the effort. By strong-arming Cool'n'Quiet to apply core frequency changes globally (on the models not already designed to do so), we’ve regained most of the performance lost when power management features are enabled in Windows Vista. We’ve gained slightly more performance by changing p-state transition times. Exactly how much?

Without changing voltages, we’ve seen power savings between 1-9% when execution cores are running full speed on AMD's Phenom II X4 945 and 1-15% on the Phenom II X4 955. By lowering voltages, savings range between 13-18% on the Phenom II X3 710. We saw larger savings with the Phenom II X4 945 (26%) and Phenom II X4 955 (25-32%), but that’s largely due to the motherboard originally applying higher voltage than it should have. For those who are interested, power savings on the Athlon X2 7750 are similar to the Phenom II X3 710--around 14%. Believe it or not, that’s actually quite an improvement. With default Cool'n'Quiet settings, the savings are around 2%.

Idle numbers are also interesting to discuss. Power consumption while playing hardware accelerated H.264 video roughly equals running typical desktop tasks--around 70 to 80 watts. In percentages, the savings are 12-19% with default Cool'n'Quiet settings for the Phenom IIs. By lowering voltages, that number increases to 25-37%. Savings with the Athlon X2 7750 are about 9%.

Of course, all of these power consumption savings numbers are only meaningful when we compare them to performance. Just how big of a difference is there between running with and without power management, once you optimize the settings? Performance drops between 1-11%, or an average of 3.3% for the Athlon X2 7750. For the Phenom IIs, its between 0-5%--hardly noticeable, and possibly attributable to normal variation between runs.

There is one important note to remember: most of these savings (except for video playback) are observed in tasks that are completed within a certain period. As we demonstrated, there are no difference between average and total power consumption in tasks that do not end, unless you stop the application (video playback, gaming).

Choices, Choices: Which AMD Processor Offers The Best Energy Efficiency?

Those who have been paying attention thus far no doubt already know the answer: the Phenom II X4 945 and 955 may have slightly higher power consumption, but these processors' full complement of cores, cache, and clock rate translates to lower overall power consumption. Another plus is accomplishing tasks faster. The real trick is selecting the right voltages and motherboard. However, if you don’t want to (or can't afford to) make the jump to four cores just yet, the Athlon II X2 250 might be up your alley (Ed.: alternatively, the Athlon II X4 620 might be a better way to go, getting you in the door with four cores).

Some might say that selecting the highest-bin processor to get better power consumption is counterproductive. We believe this article is evidence enough to suggest otherwise. Processors like the Phenom II X4 955 Black Edition represent the best silicon that came out of a wafer from AMD. They do not have disabled cores and are less likely to exhibit leakage (increased heat). This is why we'd be inclined to steer away from processors like the Phenom II X3 710. There is a good reason why AMD disabled the fourth core. Just look at the voltage ramp data we saw.

The highest-bin processors are also likely to be Black Edition processors. These processors, with their unlocked multipliers, let you overclock without altering the HyperTransport interconnect (forcing the necessary voltage adjustments). Even if you’re running your Phenom II X4 955 Black Edition processor at 3.7 GHz, you can still use the default 200 MHz HT clock. Ah, but doesn't overclocking the processor actually result in additional power consumption? Not really--look back at the voltage ramp data in our earlier article. You’ll see that, even at 3.7 GHz, properly optimized processors like the Phenom II 955 Black Edition and Athlon II X2 250 actually consume less power at full load then running at stock voltages.

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    Top Comments
  • nzprogamer
    GO AMD go
    i am telling you my next build AMD/ATI
    """I WILL BE BACK"""
  • Other Comments
  • nzprogamer
    GO AMD go
    i am telling you my next build AMD/ATI
    """I WILL BE BACK"""
  • jedimasterben
    I'd be interested to see the tests performed on Windows 7 to see what the effect of reducing thread "jumping" would be.
  • cnox
    Dammit...how can this part 2 article be posted before the Building the Balanced PC Part 2?

  • melangex3
    Great Stuff. Keep up the good work. This is the type of review that will keep me coming back. How about throwing in the ever popular 720 BE and the new 620 or 630 just for giggles?
  • Ryun
    jedimasterbenI'd be interested to see the tests performed on Windows 7 to see what the effect of reducing thread "jumping" would be.

    I was thinking the same thing as well.

    Also, were the BIOSs all updated? The asynchronous clocks problem you're experiencing with Athlon II X2 was supposed to be fixed with updated CPU microcode.
  • Summer Leigh Castle
    620 and 720? :D
  • redgarl
    I must admit that lately AMD is impressive. I got a PII X3 720 BE unleashed at PII X4 20 fully stable with an Asus M4A78T-E latest BIOS. Let simply add that my 2 radeon 4850 OC in Crossfire are running as fast as 2 stock 4870...

    If you take into account that the 2 cards only cost 82$ each for a total of 165$ for the two... I can hardly believe that so little money can give so much results.
  • JimmiG
    With my Phenom X4 9650, I found Cool n Quiet to be pretty much worthless without tweaks. There were huge performance drops across the board, especially with tasks that didn't use all four cores, or only loaded cores partially. Videos and games would stutter and skip every couple of frames, compressing files would take longer etc. I basically had a 1.1 GHz CPU that would sometimes run at 2.3 GHz, if it felt like it. Too bad there was no tweak guide available then. I just disabled CnQ which solved all problems but made the system use more power and run hotter.

    With my 955BE, I haven't really had a need to tweak CnQ. It might cause a slight performance hit in some rare cases, but generally when I need a 3.2 GHz CPU, that's what it delivers.
  • tacoslave
    Nice, amd owns in the graphics department now with that $1.2 billion im sure amd is heading to pwn BOTH markets.
  • saint19
    Good!!!, I have my 955 to 3.8GHz at 1.5V....
  • marraco
    For this test, we've selected a 616MB folder full of files (the installer for Adobe Photoshop CS4)

    Bad choice. You are "compressing" already compressed files.
    A better choice would be to copy 600 MB from windows "program files" folder, and play with it.
  • eyemaster
    marracoBad choice. You are "compressing" already compressed files.A better choice would be to copy 600 MB from windows "program files" folder, and play with it.

    Bad choice if you're trying to compress file, but not a bad choice if you're just trying to stress a CPU. It will still have to do a whole lot of calculations regardless of the files.
  • b23h
    Thank you very much for this article. It came at the perfect time for me. I’ve just upgraded from a 65 watt AMD Athlon 64 X2 4600 to the AMD Phemon II X3 720. Since I’m running a fanless CPU heatsink (ZEROtherm BTF95) I was concerned that the 95 watts of power of the 720 would be too much for the BTF95. I was planning on underclocking the CPU in order to approximate what I thought the heatsink could handle. However with the help of the article I don’t need to underclock it at all. Using the chart of the 710 I estimated some beginning settings for the 720. While I may be able to further lower the voltages I’ve stressed test my current settings by running a program called the Intel Burn Test plus an immoderate amount of Borderlands.

    The 720 seems to have four p-states. The defaults were 1.25/1.15/1.05/.95 I am currently running the CPU at 1.15/1.125/1.0250/.9 I really haven’t thoroughly stress tested all the possibilities so I expect I may still be able to optimize further P-states one through three.

    The timing of this article was excellent for me and I appreciate all the information I’ve gotten at Tom’s Hardware Guide all these years.
  • volks1470
    Typo! Phenom II X3-X4 720-965 only have 6MB of L3 cache, not 8MB. Not a big deal but for a second there I though the 955 had more L3 cache than my 965.
  • 4ILY45
    melangex3Great Stuff. Keep up the good work. This is the type of review that will keep me coming back. How about throwing in the ever popular 720 BE and the new 620 or 630 just for giggles?

    YES PLEASE!!!! :)
  • shreeharsha
    I still need to sell my intel Pentium 660 system to build a AMD system. I am a AMD fan (converted) stuck with intel processor system.
  • JohnnyLucky
    Thanks for a very informative article that helps put things in perspective.
  • chaitanya_mkin
    Im Telling u AMD wll be the king in HISTORY theres no other name than AMD.
    AMD the BEST
  • chaitanya_mkin
    And this comparision is great for amd users, ofcourse for me tooooo cause im using AMD since 6 years.(my brain is AMD ATHLON X2 6000+ WINDSOR 3.02GHz)
  • arnawa_widagda

    We've updated this article (and the first part) with results from an AM3 motherboard and an Athlon II X4 620. The p-state settings we tested with the Athlon II X4 620 can be found in the first part (along with some power consumption numbers).