Measured Power Consumption And Methodology
Of course, tweaking voltage is just a means to an end: measuring the performance per watt of these processors. We'll start the traditional measurements of system power consumption at idle and with load.
Testing for idle consumption is done on the Windows desktop, from when the system is turned on until 15 minutes after boot. We think 15 minutes is enough time for all Windows background process to settle down, providing the least amount of burden on the processor. For this test, we did not disable SuperFetch, indexing, or the Windows Aero interface.
Testing under load was accomplished by running WinRAR's built in benchmark, which puts heavy load on all cores. The key here is heavy load, but not full load. For those who are interested in peak load numbers, we also measured the power consumption of all processors when running Cinebench R10 multi-threaded benchmark, which does fully utilize all cores.
A Side Note
Please note that the Athlon X2 7850, Athlon II X2 250, Phenom II X4 945, and 955 are running on the Gigabyte GA-MA790GP-DS4H, while the Athlon X2 7750 and Phenom II X3 710 are running on the Biostar TA790GX 128M. There is a good reason why we chose to use two motherboards for these tests; the two boards represent different market segments.
A quick check on Pricegrabber revealed the Biostar TA790GX is selling for around $100, while the Gigabyte GA-MA790GP-DS4H is about $150. The $50 you save going with the Biostar board can go into the processor instead, which means you only have to pay around $25 more to get the Athlon X2 7750 or $for the Phenom II X3 710. However, the Gigabyte GA-MA790GP-DS4H offers more tweaking options, so if you really want to squeeze as much performance as possible, it’s a very good board and a suitable match for the Phenom II X4 945.
There are a few interesting tidbits we observed with Biostar's TA790GX 128M. The motherboard will only run our Phenom II X4 945 and 955 samples at 800 MHz, no matter which BIOS version or settings we used. The company's official response was that the board does not support these processors. The company won’t go into details about it other than “it’s a hardware limitation,” and recommends that enthusiasts who want to use the Phenom II X4 945 or 955 choose the newer TA790GX A3+ with DDR3 memory instead. It also assured us that that board will work with a Phenom II X4 965 BE.
We did find a workaround for this problem. By using AMD’s Overdrive utility we were able to change the multiplier so these two processors operate at their default clock rates. Obviously, you’ll need to adjust the voltage too. One caveat with this trick is that the northbridge and memory controller will only run at 1.6 GHz, rather than 2 GHz.
The TA790GX 128M would also “forget” the p-state changes we applied when the system would wake from sleep/hibernation, resetting the values to default Cool'n'Quiet settings. With the HyperTransport interface overclocked, the system would crash, requiring a reset. We did not see this behavior with either of the Gigabyte boards.
These measurements were taken with different power profiles: No Management (we turned off Cool'n'Quiet from the BIOS), Balanced (Cool'n'Quiet is enabled on the BIOS, Windows power policy set to Balanced), and Optimized (Cool'n'Quiet enabled in the BIOS, Windows power policy set to Balanced, the processor running with lower core and northbridge voltages, shorter p-state transition time and synchronous clock changes or “Ganged” in K10Stat).