Undervolting Piledriver FX
What if you wanted to take things the other way? After all, if power consumption goes way up when the voltage is increased a bit, then it only makes sense that it would go down with a decrease. With so much overclocking headroom with just 0.1v added to the CPU, we should be able to run as stock speeds with the voltage decreased by at least that amount.
How to Undervolt Piledriver
To find out how low I could take the voltage, the first step was to disable turbo mode and all the power saving functions (C1E, C6, Cool n Quiet) and run the CPU at turbo speed by default. This puts the CPU in its “stock worst case power load”. Once we find the lowest voltage it would use at full load with this speed, we can go ahead an enable the power saving functions again.
It is also important to keep an eye on vdroop. Vdroop occurs when the voltage drops below specified value while under load. This is built into both AMD and Intel processors, and allows them to run with as low a voltage as possible. This enhances durability, and even performance (as Turbo mode will be enabled more often, for instance). However vdroop is the worst enemy of both the overclocker and undervolter. You want a steady, consistent flow of voltage whenever running out of spec. When overclocking, vdroop can cause instability where a steady voltage would not. When undervolting, vdroop can bring the CPU voltage lower than you want, and again causing instability.
The answer to this problem is LLC or “Load Line Calibration”. Different motherboard manufacturers may have different names for it, but the Gigabyte 990FXA-UD3 uses descriptions from “Normal” to “Extreme” depending on the level being used. Different levels can have different effects, and sometimes using too high a level will cause voltage to shoot even higher.
The key to undervolting is finding the voltage that allows you to run the speed you want, and keep it there with the right LLC setting.
After testing various voltages and LLC combinations, I settled on 1.25v as the lowest voltage for this FX-8350 running at 4.2 GHz. One step lower, and Windows wouldn’t boot. This is down from a standard setting of 1.35v without turbo, and 1.45v with turbo.
Once the power saving functions were enabled, voltage would drop to 0.8v at idle (at stock, this would be 0.9v) and it never went above 1.25v at full load. Let’s take a look at the power savings:
We saw a 14% reduction in power consumption, just by lowering the voltage 0.1v. Considering performance remains exactly the same, this is pretty impressive! I tested single threaded performance, and it didn’t drop, nor did idle mode. It looks like Piledriver only really requires a lot of power when all 8 threads are going full out.
This brings our CineBench power efficiency score down to 33.5, from 38.36. Still nowhere near Ivy Bridge numbers of course, but any improvement is encouraging, especially when it’s this easy to do.