At the time of writing, we've gotten our hands on three NVIDIA GeForce GTX 1080 graphics since its launch. We typically get a 2-way SLI review out fairly soon after the launch review, but two things pinched us - availability of our second card and the new SLI HB bridge. With the GeForce "Pascal" architecture, instead of driving its GPU lineup toward DirectX 12 native multi-GPU, which has some fascinating features on paper, such as the ability to mix and match any two GPUs that support the same API (D3D 12_0 and above), NVIDIA developed its proprietary SLI technology further and made it even more restrictive than before.
The GeForce GTX 1080 does not support 3-way and 4-way SLI. Not officially or, now, even unofficially. There's very limited support for a few non-gaming apps, including competitive benchmarks, but beyond that, 3-way and 4-way SLI is of no use to gamers. We feel this is unfair, especially when monitor resolutions, color depth, and such bandwidth-intensive features as hardware HDR are finally on the rise. It would have been great if current GTX 1080 users could scale their setup beyond two GPUs.
NVIDIA does have a method to its madness; it's a gamble, but we expect it to pay off in the best interests of gamers. Unlike current-generation AMD GPUs with XDMA CrossFire, which relies on the PCI-Express system bus for inter-GPU communication (which is pretty much also how DirectX 12 native multi-GPU tech works), NVIDIA continues to rely on a direct physical connection between GPUs for some inter-GPU communication. It's not that the PCI-Express bus doesn't have the bandwidth to cope with this sort of thing, but it adds latency. For a primary GPU to receive data from other GPUs in the machine, data must flow between those GPUs and the PCIe root complex, the CPU, and back down to the primary GPU. This creates some latency. SLI, on the other hand, does this by its direct connection. NVIDIA GPUs still use PCI-Express for "some" inter-GPU communication, but NVIDIA won't tell anyone what moves between PCIe and what moves over these bridges.
According to this NVIDIA graphic, the classic SLI bridge was proving to be insufficient for smooth display output at some of the higher resolutions (4K @ 60 Hz, 4K @ 120 Hz, 5K, and HDR-enabled resolutions). You can still very much use a classic 2-way bridge between two GTX 1080 cards, but NVIDIA claims that the performance won't be up to the mark. This is where the new SLI HB bridge steps in.
The SLI HB bridge taps into two SLI connectors and runs at a higher clock speed, providing sufficient bandwidth to link two GPUs for smooth output at the high resolutions we mentioned earlier. Since it eats up both SLI fingers on your card, you're limited to 2-way SLI. This is probably also why NVIDIA isn't supporting 3-way and 4-way SLI, knowing that people will use the 3-way and 4-way bridges that came with their motherboards that link each GPU to the next with just one path, which results in less than smooth output. The company even claims that this could have a negative impact on performance (vs. 2-way SLI).
So now, we have with us two GeForce GTX 1080 cards and an SLI HB bridge to present our GeForce GTX 1080 SLI review. We also had a classic 2-way bridge lying around, so we thought "why not throw in those numbers, too."