Power consumption and efficiency
We have pages and pages of performance data to show you, but let's start with our power consumption tests, since that's a very important characteristic of a CPU these days and is especially relevant to our key match-ups.
For these tests, we used an Extech 380803 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 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.) 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.
We'll start with the show-your-work stuff, plots of the raw power consumption readings. We've broken things down by socket type in order to keep them manageable. Please note that, because our Asus H57 motherboard tends to draw more power than we'd like, we've tested power consumption for the Core i5-530 and the Core i5-661 on our P55 mobo, instead.
You'll notice that the Pentium 4 670 couldn't finish rendering the scene before our test period ended. That presented us with a real problem. In fact, we had to extend the P4's test period to nearly twice the usual length in order to capture the full scene render.
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.
System power draw at idle is still very much influenced by how well the CPU is able to shut down parts of itself when dropping into lower power states, as these numbers attest. The latest Intel processors, both the Clarkdales and Lynnfields, come out looking very good. The newer LGA775 CPUs have nice, low power draw at idle, as well. The results for the Q6600 and the Pentium 4 670 tell the story of considerable progress for Intel over time, even in the same socket type.
Interestingly, our quad-core Phenom II-based systems just match our Q6600-based one, suggesting AMD is a couple of generations behind in terms of power reductions at idle. Fortunately, the Propus- and Regor-based Athlon IIs move the needle a bit. Our Athlon II X2 255-based test rig only pulls 7W more than its Pentium E6500-based rival.
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.
Intel has a more pronounced lead when it comes to peak power draw. Consider the Pentium E6500 versus the Athlon II X2 255: the Athlon II-based system draws 28W more under load. Our Core i3-530 system pulls 50W less than our Athlon II X4 635-based one. The gap between the Core i5-750 and the Phenom II X4 965 systems is 40W, also in Intel's favor. Only the Bloomfield Core i7 processors, with their extra memory channel, draw more power than the X4 965. AMD is capable of making CPUs that require fewer watts, as the results for the Phenom II X4 910e attest. Too bad those low-power Phenom IIs aren't the norm.
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.)
Notice that the Pentium 4 670 isn't included above, since its total test period was much longer.
We can pinpoint efficiency more effectively by considering the amount of energy used for the task. 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.
Let's first pause to collect our breath and wonder at the amount of progress we've seen since on this front since the Pentium 4 days. The P4 670 system requires over seven and a half times the energy that our most efficient contender, the Core i7-870 system, does to render this scene. On the exact same motherboard, in the same socket, the P4 670 uses 6.2 times the power Core 2 Quad Q9400 does to complete the same work. The move from one core to four has no doubt played a big part in this progress, but obviously, advances of many types have produced these gains.
These efficiency numbers tell us Intel's processors generally have solid performance to go with their relatively low power consumption. Notably, the Bloomfield Core i7-975 and i7-920 have moved into the upper ranks, because they're able to finish the job more than quickly enough to compensate for their relatively high peak power draw. That same dynamic propels the Phenom IIs past the Athlon IIs among AMD's offerings.