Nvidia's high-refresh-rate "power bug" rears its head again

— 10:39 AM on March 8, 2016

A couple months back, some reviewers noticed that pairing their Nvidia graphics cards with high-refresh-rate displays—those with maximum refresh rates greater than 60Hz—caused the idle clock speeds of those cards to surge at the Windows desktop. That clock speed surge resulted in excessive power consumption. Nvidia later issued a fix for that issue in version 361.43 of its drivers, and all was well—or so we thought. A TR reader emailed us recently and reported that he was experiencing the same issue with his triple-Asus-PG278Q setup. He noted that setting any one of those displays above a 60-Hz refresh rate at the desktop would cause this "power bug" to recur.

Since we have an Asus PG279Q and several other monitors in our labs, we figured we'd try and see what was up. Our testbed system is running the most current drivers from Nvidia, version 364.47. First, we confirmed that even with the single PG279Q running at 165Hz, the GeForce GTX 980 Ti in our system was running at 135MHz at the Windows desktop. That's the expected behavior from Nvidia's fixed driver.

Next, we added an LG 27MU67-B to our system using that screen's DisplayPort connection. That 4K screen runs at a maximum of 60Hz. In that configuration, the GeForce GTX 980 Ti's clocks rose to 925MHz. Weird. Since that behavior tracked with our reader's experiences, we then set the PG279Q's refresh rate to 60Hz. Like magic, the GTX 980 Ti's clocks fell back to 135MHz.

It gets weirder, though. While my all-DisplayPort setup demonstrated the issue, TR coder Bruno Ferreira's multi-monitor system didn't. His system uses one DisplayPort connection for an Acer XF270HU running at 144Hz, plus a DVI connection for a smaller 60Hz screen. I confirmed that the problem doesn't occur with the PG279Q and a DVI monitor connected.

While this "power bug" may sound like a minor issue, its effects are more insidious than one might expect. Using our trusty Watts Up power meter, I measured the idle power consumption of our testbed system at the wall. When the GTX 980 Ti is running at 135MHz as expected, that system draws between 80 and 90 watts at the wall. Kick up the refresh rate in a multi-monitor setup, though, and that figure jumps to between 130 and 150 watts. An affected PC running 24/7 is nearly doubling its idle power consumption, and that'll make for a hefty increase in one's power bill. Even weirder still, this problem doesn't seem to go away even if you turn off all of the monitors attached to the PC.

We tested some other parameters and found that display resolution and G-Sync settings don't appear to have any effect on the issue. We've reported our findings to Nvidia, and we'll update this post if we hear back. Hopefully, this bug can be solved with another simple driver update.

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