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POV-Ray 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 asked POV-Ray to render our "chess2.pov" scene at 1024x768 resolution with antialiasing set to 0.3.

Before testing, we enabled the CPU power management features for Opterons and Xeons—PowerNow! and Demand Based Switching, respectively—via Windows Server's "Server Balanced Processor Power and Performance" power scheme.

Incidentally, the Xeon's I've used here are brand-new G-step models that promise lower power use at idle than older ones. I used a beta BIOS for our SuperMicro X7DB8+ motherboard that supports the enhanced idle power management capabilities of G-step chips. Unfortunately, I'm unsure whether we're seeing the full impact of those enhancements. Intel informs me that only newer revisions of its 5000-series chipset support G-step processors fully in this regard. Although this is a relatively new motherboard, I'm not certain it has the correct chipset revision.

Anyhow, here are the results:

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.

The new Opterons draw a little more power at idle than the old ones, as might be expected with so many more transistors on the chips. Still, the Barcelonas draw much less power at idle than the Xeons. Part of the Xeons' problem is a platform issue: FB-DIMMs draw quite a bit more power per module than DDR2 DIMMs.

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.

True to its billing, the Opteron 2350 draws no more power under load than the Opteron 2220, its dual-core predecessor. Of course, AMD had to compromise on clock frequency in order to do it, but this still is an impressive result—especially since the 2350 draws less power under load than the low-power Xeon L5335, whose TDP rating is 50W. Then again, this is total system power draw, and we've already established that the Xeons have a handicap there—one they're tied to, nonetheless.

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.

By using more cores to finish the rendering work sooner, the Opteron 2350 is able to use less power through the course of the test period than the Opteron 2220, despite having similar peak power consumption and higher idle power consumption. Sometimes, power efficiency is partially about finishing first. However, the Xeons' strong performance alone can't redeem them here.

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. We've sometimes seen 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.

This may be our best measure of power-efficient performance under load, and in this measure, the Barcelona Opterons again excel. The Xeons are close here due to their short render times, but the new Opterons place first and second.