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Power Consumption

AMD Athlon II X2 / Phenom II X2 And Low-Power CPU Bonanza

All of our benchmarks were run with power-saving features enabled. AMD actually weighed in just after the entire suite had been completed to recommend against testing this way because, currently, the two cores on its Athlon II X2 are changing P-states independently. An upcoming BIOS code update will have both cores shifting P-states together. The problem with independent switching, according to AMD, is that single-threaded workloads with a tendency to hop from one core to the other will experience a slow-down due to operating system scheduling inefficiencies.

This phenomenon is best-illustrated with an example. If you have two cores and are running a Lame .mp3 encode, then one thread is idle (since the encoder is only single-threaded). Scaling that idle core back to 800 MHz while the utilized core does its work at 3 GHz helps cut back on power, reduce heat, and so on. But if Vista’s scheduler bounced Lame over to the idle core running at 800 MHz, you’d incur a significant performance impact all of the sudden. While it is common to see a single thread of Prime95 bouncing all over the place, I kept an eye on Lame and WinZip and am fairly positive these apps weren’t getting affected by this potential issue.

AMD’s implementation is the “right” way to go about optimizing for efficiency, but it’s hampered by Microsoft. Phenom II “fixed” this behavior by keeping all cores running at the same speed. If I understand AMD correctly, the upcoming BIOS will shift from Phenom- to Phenom II-like operation. With all of that said, testing with Cool’n’Quiet enabled works to AMD’s benefit when it comes time to measure power, since all of these CPUs are able to throttle down to 800 MHz while they idle.

The company’s low-power quad- and triple-core CPUs are its most impressive. They consume just about as much power as the Phenom II X2 at idle, but use less than even the 65 W Athlon II X2 under load.

Intel’s Core 2 Quad Q8400 and Pentium E6300 actually serve up even better idle numbers. However, the Core 2 Quad ends up using about as much power as AMD’s 80 W Phenom II X2 under load (not bad considering the Intel chip is rated at a 95 W TDP).

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