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 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, 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.
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.
I've only shown verbose results for some of the most relevant competitors here. You can see the results for the other processors in prior reviews.
We can slice up this 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.
TDP ratings deal with maximum power draw, but they don't account for what happens when a CPU is unoccupied. Interestingly enough, the systems based on Phenom II quad-cores (including the X4 965) draw quite a bit less power at idle than our Q9550-based test system.
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, during which the processors were rendering.
The X4 965-based system draws roughly 20W more power at the wall socket than a similarly configured system with a Phenom II X4 955 processor. AMD must be at the edge of what it can get from its 45nm quad-core chips with the X4 965 at 3.4GHz. That said, the X4 965-based system draws only 15W more than the Q9550-based one. The gap between the Q9550- and X4 965-based systems is thus smaller than the processors' TDP ratings alone suggest.
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 specifically 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.
By virtue of its lower system power draw at idle and its ability to finish the rendering task sooner, the Phenom II X4 965 fares better than the Q9550 in our two most important measures of energy efficiency. The bottom line here is that the X4 965 may require slightly better cooling to dissipate the heat it produces under load, but it can still be more power efficient overall than the Q9550.
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