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Power consumption and efficiency
We used a Yokogawa WT210 digital power meter to 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, graphics solution, hard drives, and anything else plugged into the power supply unit. (The monitor was plugged into a separate outlet.) We measured how each of our test systems used power across a set time period, during which time we ran Cinebench's multithreaded rendering test.

Please note that, although we tested a range of AMD processors, only the FX-8150 and the Phenom II X6 (the results marked "990FX") were tested on the same motherboard. The others were tested on an 890GX-based board from Gigabyte whose power consumption characteristics differ. Oh, and we tested the FX-8150 with four DIMMs here, since that's the config all of the other dual-channel systems shared, and it only seemed fair to match the DIMM count for power testing. Fortunately, the move to lower memory clocks didn't impact rendering completion times.

We'll start with the show-your-work stuff, plots of the raw power consumption readings.

We can slice up these raw data in various ways in order to better understand them. 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. Next, we can look at peak power draw by taking an average from the ten-second span from 15 to 25 seconds into our test period, when the processors were rendering.

Pretty impressive results. Our Core i7-3960X system draws no more power at idle than a P67-based Sandy Bridge quad-core rig. At peak, the 3960X system uses less power than any of the test systems based on its predecessors in the Core i7-900 series.

We can highlight power efficiency by looking 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. (In this case, to keep things manageable, we're using kilojoules.) Note that since we had to expand the duration of the test periods for the Pentium EE 840 and Core 2 Duo E6400, we're including data from a longer period of time for those two.

We can pinpoint efficiency more effectively by considering the amount of energy used for the task alone. Since the different systems completed the render at different speeds, we've isolated the render period for each system. We've then 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.

Our Sandy Bridge-E test system uses less energy to render the Cinebench test scene than anything else we've tested. The keys to its efficiency are relatively modest power draw at peak and, most crucially, the ability to complete the task in the shortest amount of time.