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Power consumption and efficiency
We're trying something a little different with power consumption. Our Extech 380803 power meter has the ability to log data, so we can capture power use over a span of time. The meter reads power use at the wall socket, so it incorporates power use from the entire system—the CPU, motherboard, memory, video card, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor and speakers into a separate outlet, though.) We measured how each of our test systems used power during a roughly one-minute period, during which time we executed Cinebench's multithreaded rendering test. All of the systems had their power management features (such as SpeedStep and Cool'n'Quiet) enabled during these tests.

You'll notice that I've not included some members of the Athlon 64 family here. That's because our "simulated" Athlon 64 CPUs are underclocked versions of faster processors, and we've not been able to get Cool'n'Quiet power-saving tech to work when CPU multiplier control is in use. I expect to have in-the-flesh examples of the Athlon 64 X2 4400+ and 5600+ soon, and I'll provide an update once I've tested their power consumption.

I have included our simulated Core 2 Duo E6600 and E6700, because SpeedStep works fine on the D975XBX2 motherboard alongside underclocking. The simulated processors' voltage may not be exactly the same as what you'd find on many retail E6600s and E6700s. However, voltage and power use can vary from one chip to the next, since Intel sets voltage individually on each chip at the factory.

The differences between the CPUs are immediately obvious by looking at these plots of the raw data. We can slice up the data in various ways in order to better understand them, though. We'll start with a look at idle power, taken from the trailing edge of our test period, after all CPUs have completed the render.

Among dual-core setups, the Core 2s have a slight advantage in idle power consumption, but not more than 10 watts or so. There's a much larger gap between the quad-core rigs, since the Quad FX platform is a dual-socket solution with dual core-logic chipsets. Meanwhile, the Intel quad-core systems draw only about 10-15 watts more than their dual-core brethren.

Next, we can look at peak power draw by taking an average from the five-second span from 10 to 15 seconds into our test period, during which the processors were rendering.

The peak power consumption numbers tell us very good things about the Core 2 processors. The system based on Intel's power-hungriest dual-core processor, the Core 2 Extreme X6800, draws 15W less under load than the one based on the lowest power AMD processor here, the 65nm version of Athlon 64 X2 5000+. AMD's 65nm chip does shave off some power draw—roughly 12W—versus its 90nm variant. The Athlon 64 FX-74, meanwhile, is just plain silly.

Another way to gauge power efficiency is to look at total energy use over our time span. This method takes into account power use both during the render and during the idle time. We can express the result in terms of watt-seconds, also known as joules.

The Core 2-based systems led in terms of both peak and idle power draw, so it's unsurprising they do so well over the total duration of our test period.

However, we can quantify efficiency even better by considering the amount of energy used to render the scene. Since the different systems completed the render at different speeds, we've isolated the render period for each system. We've chosen to identify the end of the render as the point where power use begins to drop from its steady peak. There seems to be some disk paging going on after that, but we don't want to include that more variable activity in our render period.

We've computed the amount of energy used by each system to render the scene. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.

These results tell us several things. First, multi-core processors can be very energy efficient when handling parallel tasks like rendering. The quad-core Q6600 and QX6700 run away with the top two positions, and the FX-70 turns out to be the most efficient Athlon 64, even with its considerable built-in platform power handicap. Second, Intel's Core 2 processors are much more efficient overall than the current Athlon 64s. It's really no contest. And finally, the Athlon 64 does gain some efficiency when moving to AMD's 65nm process, but not yet enough to put it back into contention against the Core 2.