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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 5300-series Xeons I've used here are newer 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.

Of course, our Stoakley platform should support the further reductions in idle power offered by the Xeon E5472s.

Anyhow, here are the results:

Without any extra help, you can easily see that the new Xeons bring big reductions in power use over the X5365s. 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 Stoakley platform draws about the same at idle as Bensley does when coupled with low-power Xeons. The E5472s on Stoakley draw 20W less at idle than their 3GHz counterparts on the Bensley platform, but that's still quite a bit more power draw at idle than any of the Opterons.

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 Stoakley/Harpertown pairing brings a drastic drop in power draw versus the Xeon X5365s on Bensley. In fact, the Stoakley/Harpertown combo at 3GHz draws less power than Bensley/Clovertown pairing at 2.33GHz. Notably, the Xeon E5472 system also consumes less power than the Opteron 2360 SE-based one.

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.

When you slice things this way, the Opterons tend to excel, led by the low-power Opteron 2347 HE. However, the Stoakley/Harpertown system isn't far behind, and it edges out the low-power Xeon L5335.

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.

In what may be our best measure of energy-efficient performance, the Xeon E5472/Stoakley system distances itself from the pack. Even AMD's impressive new quad-core Opterons, our previous champs, are well behind it.