Power consumption and efficiency
Now that we've had a look at performance in various applications, let's bring power efficiency into the picture. 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, graphics solution, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor into a separate outlet, though.) 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.
Almost all of the systems had their power management features (such as SpeedStep and Cool'n'Quiet) enabled during these tests via Windows Vista's "Balanced" power options profile. The exception here was the Skulltrail system, since its BIOS didn't support SpeedStep.
Anyhow, here are the results:
Let's slice up the 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.
Somewhat unexpectedly, the Phenom X3 8750's idle power consumption is literally no lower than the quad-core Phenoms', despite the fact that one of its execution cores is entirely disabled. Interesting.
Next, we can look at peak power draw by taking an average from the ten-second span from 30 to 40 seconds into our test period, during which the processors were rendering.
The Phenom X3's peak power draw looks to be quite a bit lower than its quad-core counterpart's. Even so, the X3 pulls more juice than the Core 2 Quad Q6600, a quad-core processor produced on Intel's older 65nm process tech. Intel's 45nm dual-cores, the closest performance and price competition, use substantially less power.
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.
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 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.
The Phenom X4 9750 may draw more power under load than the X3 8750, but it finishes first, allowing it to consume less energy while rendering the scene. More strikingly, Intel CPUs are vastly more power efficient overall than AMD's, by every measure. Quite the reversal from several years ago.
