Power consumption and efficiency
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 draw at the wall socket, so it incorporates power use from the entire systemthe CPU, motherboard, memory, graphics solution, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor into a separate outlet, though.) In this case, we tested power use while running Cinenbench's multithreaded rendering benchmark. We chose this benchmark because it scales up pretty well with multicore processors, and it runs in a reasonable amount of time.
Here's the power draw of the various processors before, during, and after the Cinebench test run. I've broken the results into three different graphs and overlaid the 65nm Athlon 64 results on each to make comparison easier.
We've also learned that our early Core 2 Duo E6300 sample is something of a runt, requiring more power than our newer retail Core 2 Duo E6400. I decided to go ahead and include its results here as a reminder of how much power consumption can vary from one chip to the next.
We can break down the power consumption data in various useful ways. We'll start with a look at idle power, taken from the trailing edge of our minute-long test period, after all CPUs had completed the render.
As for the Core 2 processors, their higher idle power use may be in part due to higher overall platform or motherboard power draw, but it may also be related to the aggressiveness of the power-saving mechanisms involved here. AMD's Cool'n'Quiet takes the CPU all the way down to 1GHz, but Intel's SpeedStep only slows the Core 2 processors to a minimum of 1.6GHz.
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 all of the processors were rendering.
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, equivalent to joules.
Finally, we can consider 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 then computed the amount of energy used by each system to render the scene, expressed in watt-seconds. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.
The top spots here are taken by Intel's Core 2 Extreme processors, thanks in part to their modest power draw but more prominently because of their outstanding performance. Because they finish rendering the scene sooner, they're able to consume less energy for the duration of the task. AMD's 65nm processors have improved on one part of the so-called "performance per watt" equation by reducing power draw, but not the other.