Efficiency & Conclusion
Does undervolting improve efficiency, then? Yes and no. If you increase Radeon RX Vega 64's power limit, then yes. If you don't, then no.
It's only by raising the card's power limit that the telemetry can identify voltage as a limiting factor and optimally adjust the resulting clock rate to the actual graphics load. The result barely exceeds the default Balanced mode, though. Conversely, manually decreasing the voltage to 1.0V without increasing the power limit lowers efficiency.
All of our measurements were taken while using a game to generate the load. The game in question was selected because it produces a constant and reliable load across the entire duration of 30 minutes. We did run Doom as well to check our results, and it demonstrated higher frequencies and lower power consumption. Broken out into percentages, though, the two games’ results are almost identical. Our bottom line stays the same.
The gaming performance shows very clearly what we’ve found time and again so far: undervolting without increasing the power limit is pointless and counterproductive.
Summary & Conclusion
It’s certainly possible to run a Radeon RX Vega 64 more efficiently than AMD made possible with its default modes. However, the unique nature of AMD’s AVFS dictates that manually lowering the voltage only makes sense if the power limit is raised at the same time to allow the telemetry to regain its footing. In that case, the reduction in voltage becomes just one more limiting factor among many others, such as temperatures. Each card is going to have its own boundaries that you'll have to experiment with, since every GPU's quality is different.
Even though there we don't have anything sensational to report when it comes to undervolting, an additional 20W (or seven percent), does produce five percent more performance. This means that the card scales really well, and certainly a lot better than when it's run at at stock settings without lowering voltage. In that case, 41 percent more power consumption gets you just 11 percent-higher frame rates. Consequently, if you want to overclock efficiently, then you need to undervolt. This is a really interesting finding that’s very much unique to AMD’s Radeon RX Vega 64.
It would be great if AMD could handle temperature readings in a more precise manner. Documentation would also be appreciated. Our advice is to use a suitable tool, such as GPU-Z, to read the hot-spot temperature. The readings provided by WattMan just can’t be correct, at least for low-temperature measurements.
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