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Performance And Power Management, The Best Of Both Worlds

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