Value and ConclusionAt close to $1,400 (as low as $1,200 if you consider the $599 baseline price and $1,300 if you consider the $650 mean price of custom-design cards), GeForce GTX 1080 SLI is the fastest dual-GPU solution, although any claims of being the fastest graphics solution money can buy could be disputed by the fact that the GeForce GTX Titan X and the Radeon R9 Fury X support 4-way multi-GPU.
The GTX 1080 duo makes short work of any current game at 4K Ultra HD resolution, with framerates exceeding 60 FPS in most games that scale. 4K@60Hz is also the highest monitor resolution we had at hand. NVIDIA's decision to facilitate higher resolutions such as 5K and 4K@120Hz using modern DisplayPort 1.4 connectors, and the new SLI HB multi-GPU interconnect with over three times the bandwidth of a classic 2-way SLI bridge, show that the company believes that GTX 1080 SLI can provide playable frame-rates at those resolutions.
A GeForce GTX 1080 SLI setup should prove useful if you have a 120 Hz WQHD (2560 x 1440) monitor. For a 60 Hz monitor, even a single card is sufficient as its framerates stay close to or exceed 60 FPS in most of titles. GTX 1080 SLI is way overkill for lower resolutions such as 1080p. Even a single card is overkill for 1080p. Get a GTX 970 or wait for the likes of the AMD Radeon RX 480.
2-way multi-GPU users (typically gamers) make up a significantly larger demographic than 3-way and 4-way multi-GPU users (typically enthusiasts). This is also supported by the fact that you need the costlier HEDT platform for enough PCIe slots for 3-way or 4-way. With the GTX 1080, you're getting smooth frame-rates (60 FPS and above) with just two cards, which puts 4K@60Hz performance within the realm of gamers. You needed 3-4 GTX 980 Ti cards to achieve something like this before, and the demographic was smaller.
When you look online for guidance on SLI HB, you will find forums full of people claiming that you must absolutely use a high-bandwidth bridge with Pascal or performance will suffer. We took a special look at that, comparing performance of the classic SLI bridge with the HB bridge at up to 4K60Hz, and there is no significant difference in framerates or visual smoothness, not even in games running over 200 FPS at 4K resolution.
The underlying reason is that SLI works on a display controller level, which is also why G-Sync uses SLI logic - which can do variable frame rates. Not every single frame that is rendered in AFR gets sent over the SLI bus. Rather, each GPU renders into a single buffer (overwriting it), and SLI will pick up a frame from the second card when it's time to send a frame to the monitor, which happens at half the monitor's refresh rate (the primary GPU provides the other half of the frames).
A single frame at 4K resolution is 32 MB (3840x2160x4 bytes per pixel). At 30 frames per second, that's 960 MB, which is just shy of the 1 GB/s bandwidth classic SLI provides. This means that SLI HB provides no benefit until you exceed that limit, which happens at higher resolutions, like 5K or 4K@120. However, even when bandwidth is exceeded, you will not run at reduced framerates; rather, you will see stuttering on the monitor: the second GPU sends its frame over the SLI bus, but it will arrive too late to be sent to the monitor because of bandwidth limitations - it has to wait its turn, which is two refreshes, which causes the frames to be displayed in the wrong order, something you see as stuttering.
Right now, SLI HB bridges cost $39.99 and are in limited supply, especially the popular three-slot bridge most motherboards use is scarce. So as long as you are gaming at 4K@60Hz or lower, you will be fine with a classic bridge, which also has the advantage of being flexible, letting you pair cards that have their SLI connectors at different positions vertically.
GTX 1080 SLI cannot escape the biggest limitation of multi-GPU setups since the dawn of time - software support. There continue to be big AAA game titles (5 out of 16 in our testing) that lack SLI support due to engine limitations, which NVIDIA cannot fix through a driver update. More and more modern game engines come with such caveats, which is why classic AFR frame rendering doesn't have much of a future looking forward.
With the advent of DirectX12, we are promised new multi-GPU rendering modes thanks to Microsoft giving developers more control over per-GPU resource allocation, but we doubt that we'll see widespread use of those techniques. Nowadays, games are developed for consoles first (which are single-GPU), and publishers have little interest in spending a lot of developer time (= money) on adding support for exotic multi-GPU configurations that are used by only a small percentage of their customers.
The enthusiasts among you will be pleased to know that 3-way and 4-way SLI, using classic bridges, may still work on a very few synthetic benchmarks, such as 3DMark FireStrike, Catzilla, Unigine Heaven, etc. Such machines will work like a machine with 2-way classic-bridge SLI with every other app, such as games.
We therefore recommend that you wait things out until there are bigger Pascal-based GPUs that can tame those resolutions on a single chip unless you really need the muscle power of two GTX 1080s in your display setup.