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.
At idle, the 990FX-based sytem with a Phenom II X6 processor draws 94W at the wall socket. Simply as a result of dropping in an FX-8150 instead, idle power consumption plummets to 76W. That's the impact of Bulldozer's ability to gate off power to its idle cores and north bridge. Also, notice that the Phenom II X6 draws even less power at idle on the 890GX board: 82W. Our 990FX may be a bit of a power hog at idle. We'll have to swap the FX-8150 into a different Socket AM3+ board soon and see if we can't get it down into the mid-60-watt range, like the Sandy Bridge systems. Seems possible.
At peak, the FX-8150 system draws about 22W more than the same system equipped with a Phenom II X6 1100T. That's not an entirely shocking result, even though the chips have the same TDP rating. Bulldozer's improved Turbo Core is more effective at wringing every last ounce out of a given thermal envelope, and that translates into higher measured power draw at peak. With a more modest 95W thermal envelope, the Sandy Bridge-based competition delivers its superior performance at much lower total system power levels.
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.
Even though it ostensibly benefits from finer process technology, the FX-8150 actually uses slightly more power than the Phenom II X6 1100T does in rendering this scene. That's not the sort of progress we had hoped to see.
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