If you're buying a video card for one or two games, focusing in on the relevant benchmarks for those games is a fine approach, but if you're buying a card for the long haul, considering performance averaged over an entire game suite is an even better approach. And as shown below, GTX 1080 Ti SLI definitely comes out looking pretty impressive:
With scaling that averaged around 60%, the GTX 1080 Ti SLI duo roared past every previous GeForce setup we've tested, hitting a monumental 101fps average framerate in our 4K testing. As we've mentioned several times in this article, the benefits of such high framerates will really only make themselves known when we have more capable 4K monitors. With current-gen 4K monitors locked at 60fps, getting over 100fps in games doesn't do a whole lot of good, but it does provide a nice bit of future-proofing!
Note that we excluded Fallout 4 from our average here, as the dynamic day/night cycles allowed the 1080 Ti to produce absurdly-high framerates since it was nighttime in the Fallout 4 world when we tested this setup (luckily, it was still daytime in the real world where we exist!).
The CPU Bottleneck
All right, it's time to address the elephant in the room, which we've noted over and over again in this article: CPU bottlenecking. Oftentimes, we'll see reviewers mention that they run their benchmarks at 4K and max out their CPU overclocks to "eliminate" bottlenecks, but when you've got the kind of firepower offered up by a GTX 1080 Ti SLI duo, there's absolutely nothing you can do to get around this age-old problem. That's because GPUs have advanced far faster than CPUs over the past few years, and even our overclocked Core i7-6900K, running at a whopping 4.3GHz, simply couldn't keep up with the frames that the 1080 Ti SLI setup delivered.
Below we're providing a screenshot of the Windows Task Manager during an in-game run through Rise of the Tomb Raider. A couple of important notes to keep in mind: First this is not the canned benchmark, which makes almost zero use of the CPU, but rather an actual in-game run, and second, the core clock was 4.3GHz, not 3.2GHz as reported by Windows.
If you were to look at CPU usage in the aggregate, which you can see in the upper-left-hand corner, you'd think you couldn't possibly have a CPU bottleneck - the CPU is only running at around 30% capacity overall. But of course, this is entirely misleading, as our core-by-core graphs prove. Core 0 on our eight-core CPU is completely maxed out, and is bottlenecking this "GPU-limited" game. In other words, when running GTX 1080 Ti SLI, Rise of the Tomb Raider is CPU-bottlenecked, not GPU-bottlenecked. That's why we were only able to achieve 50% SLI scaling in this game, despite getting around 80% in prior SLI testing using less powerful cards.
Now, we know some people are going to say we should have tested with the Core i7-7700K, and indeed our 7700K sample, which can easily run at 5GHz, might have topped the 6900K in this test. But in reality, the Z270 platform used by that mid-range chip introduces its own bottlenecking, in the form of half the number of PCIe lanes dedicated to graphics, so we're not convinced that stepping down to that platform is the right choice when running dual ultra-high-end GPUs. In fact, in our previous testing of the Z170 versus X99 platforms, we found that the greater PCIe bandwidth provided by the X99 platform evened the playing field, even though quad-core CPUs on the Z170 platform offered up a bit more raw single-threaded speed. Furthermore, you'd lose out in truly multi-threaded games, like Battlefield 1, which we've found to run much faster on eight-core processors, even when clocked lower than four-core processors.
Cooling, Noise, and Power Use
While this article wasn't meant to be a full-on review, we thought we'd touch upon a few factors that may be of concern to enthusiasts considering a high-powered SLI setup: cooling, noise, and power. As we found when running a custom fan profile to keep our GPUs from hitting the 84 °C thermal throttling threshold, this system could get pretty loud. During our benchmarking of Witcher 3, which taxes GPUs to an extreme degree, we hit 80 °C on our top card and 74 °C on our lower card, with fan speeds hitting 80% and 62% respectively. This generated a noise level reading of 50 dB measured six inches in front of the case. When we overclocked the GPUs, the temperatures increased by 1 °C on both GPUs, and the noise level went up to 50.5 dB due to the slightly faster fan speeds required to keep the temperatures under control. To learn how to tame this savage beast, check out our follow-up review of the EVGA Hybrid cooler for the 1080 Ti.
To test power use, we use 3DMark Combined, which pushes the GPUs to near 100% capacity due to excellent scaling, while also taxing the CPU significantly.
Well, you'd best have a very stout power supply if you're going to be running a setup like this one. Keep in mind that we have a highly-overclocked Core i7-6900K processor in our benchmarking rig, so it contributes significantly to the power use. That being said, it's pretty clear that GTX 1080 Ti SLI draws power like no other SLI setup. You'll need at least a 1000W PSU to support a gaming system like this. You might think you could get away with an 850W unit, but we'd caution you that it will be running at the ragged edge of stability, and well outside of its peak efficiency level.
There's really no doubt about it: a GeForce GTX 1080 Ti 11GB SLI setup is able to conquer any game, even when running at the ultra-high-def resolution of 4K. Gamers looking for the ultimate 4K gaming experience will definitely want to consider making the leap, and anyone even considering investing in the forthcoming 144Hz 4K monitors should add a couple of 1080 Ti GPUs to the budget, as no single GPU will be able to make proper use of such a monitor.
The fly in the ointment for 1080 Ti SLI, however, is CPU bottlenecking. The 1080 Ti is such a powerful GPU that when running two in SLI, it's just too fast for current CPUs to handle. That means a bit of performance is going to be left on the table. Hopefully we'll be seeing more powerful CPUs in the near future, but with many games still taxing one core far more than the rest, we're not all that confident that this bottleneck will be alleviated anytime soon. This article is being published on the eve of Computex 2017, and we already know that both AMD and Intel are going to be trying to win the hearts and minds of gamers everywhere with their massively multi-cored CPUs. Will 12, 14, or 16 cores really help gamers all that much if instructions per clock cycle stay stagnant? We doubt it. And that's what we're likely to see with "Skylake-X", "Kaby Lake-X", and "Threadripper" CPUs. None of these is actually going to exceed the IPC efficiency of the Kaby-Lake-based quad-core i7-7700K, and at stock speeds, most will be slower than our system, which used a Core i7-6900K@4.3GHz. A renewal of competition in the CPU market had bred progress, however, and progress is good. That being said, more progress would be even better....
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