Voltages, Power Consumption And Temperatures
Why We Can't Actually "Undervolt"
Wouldn't it be great if you could simply adjust the GPU voltage? Well, PowerTune wouldn't be PowerTune if it made things that easy. Instead, it plays the arbitrator of the GPU's continued health by closely managing all things electric. This is probably one of the reasons that AMD had such a hard time with its decision to finally make the voltage target adjustable.
Taking a closer look at the voltage, the first thing we see is what appears to be a giant contradiction. Comparing the red curve, which represents the results at the default target, to the blue curve, which represents the results for the adjusted target, we see that the average voltage (VDDC) of 1.10V actually increases slightly to 1.12V! However, the 1.2V peaks drop to 1.1375V—extreme fluctuations in both directions are much smaller and the curve as a whole is smoother.
We also see two drops that show us our voltage reduction is taking the GPU very close to its limits. Going any lower would jeopardize the processor's stability. Even though the voltage increased slightly, the average strength of the electric current decreased significantly, resulting in lower power consumption.
So, how much does the smoother curve impact AMD GPU's power consumption? Let's take a look at the three graphs below. The first graph shows a 60-second window of the stock graphics card's power consumption. Right below is the load, nicely represented by the frame times.
We see that the card is running at maximum capacity all the time, and there are barely any differences between the scenes. Let's take one last look at frame times and then immediately shift our attention to the next graph.
The power consumption shows very clearly what's going on. Apart from the reduced power consumption, the peaks are much lower. If this was the whole story, we'd already be happy with the result. However, it also means that PowerTune is now able to react to changes in load and power demands much more precisely.
In the end, this pattern of results strongly suggests that we didn't actually undervolt, or permanently drop the voltage by a fixed offset at all. Instead, we increased the Power Estimation Engine's value in the telemetry's overall balancing act. The lower voltage number in MSI Afterburner just got rid of the reserve that AMD gave its early Fiji GPUs. At this point in time, the reserve seems to have outlived its usefulness.
Is The GPU Actually Cooler?
All of the above should mean that the GPU stays a lot cooler. There's a lot less waste heat, after all. Unfortunately, there's an enormous problem with this logic. The graphics card's arbitrator also controls the speed of the card's fans. Dependent on the firmware settings, AMD's graphics card recognizes the lower GPU temperatures and, consequently, doesn't spin the fans as fast.
That means the GPU actually ends up getting hotter. This isn't too much of a problem, since the Radeon R9 Fury Nitro's target temperature is set to 85 degrees Celsius. If you're wondering why the temperatures are going up, though, then take a look at how the arbitrator regulates the fan speed. Many different factors go into this. There’s the GPU temperature, of course, but there are also the currents reported by the VR chip. This is taken into consideration to make sure that the VRM and the rest of the board is cooled well, even if additional load is placed on the voltage regulators. So, the power draw is taken as an indicator for the amount of waste heat that’s generated.
Since our "undervolting" efforts are really just a decrease of the current, and with the GPU sensor weighted less heavily, our foray results in a less aggressive fan curve. The end result can be seen in this temperature graph:
If the card draws power like there's no tomorrow during our stress test, then its fans give their all to keep the GPU nice and frosty. As soon as it's "undervolted," the fans take it easy, the GPU runs warmer than it did before and the relevant parts of the board get just as hot. This counterintuitive outcome of the arbitrator's efforts also means that the card operates a lot more quietly, though.
At its hottest spot, the difference between the stock and the "undervolted" graphics card is less than one degree Celsius, which is basically within the measurement's margin of error.
Those who want a cooler graphics card, and not just one that's more quiet, need to manually adjust the fan curves.
Summary And Bottom Line
Lowering the voltage target really doesn't have any drawbacks. It's good for the electric bill, the environment and your ears. Even though it looks like we're undervolting, this setting really offers a way to adjust how PowerTune works. Voltage spikes can be reined in and overregulation dialed back. How far each individual GPU can go down this road before PowerTune can't keep it going any more needs to be ascertained on a case-by-case basis.
Our recommendation is to start at -48mV and run the card using this setting for a while. If everything works perfectly, then -72mV could be the next step down before trying to go for the maximum. Interestingly, this feature is now available for a number of AMD's other graphics cards as well, so their owners can also benefit from better efficiency. We'll test more boards and put our findings in an update or follow-up story.
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Igor Wallossek is a Senior Contributing Editor for Tom's Hardware Germany, covering CPUs and Graphics. Follow him on Twitter, Facebook and Google+.
