TECH TALK Nvidia’s RTX Multi-GPU Shifts to NVLink
THE DUST FROM THE TURING launch has settled, and we’re left with the most expensive consumer GPUs in Nvidia’s history. The RTX 2080 Ti takes over where the Titan Xp left off, with a decent increase in performance. Most of us would be happy with that, but
SLI has a long history, going back to 3dfx’s original SLI (Scan-Line Interleave), where two Voodoo2 cards could render alternate lines. Nvidia acquired 3dfx’s assets in 2004, and began offering a new SLI (Scalable Link Interface) with the GeForce 6800 family. Instead of alternating rendered lines, Nvidia’s GPUs used alternate frame rendering (AFR). It was a significant change in functionality, and has led to a host of issues over the years.
SLI systems—and AMD CrossFire PCs—render multiple frames sort of simultaneously. The drivers need to balance things so that frame pacing is relatively consistent, or you get microstutter and other problems. Ideally, with two GPUs, GPU1 starts the first frame, and GPU2 starts work on the next when GPU1 is halfway finished with its frame.
Nvidia later introduced three-way and four-way SLI, but the difficulty of maintaining consistent frame pacing, the additional latencies, and limited scaling in games made them niche solutions that were more trouble than they were worth. Nvidia killed off three-way and four-way SLI with Pascal. With Turing and the RTX 2080, two-way SLI is the only supported mode, but there’s a second change.
Pascal GPUs introduced the HB SLI bridge, which increased SLI link bandwidth from about 1GB/s to 4GB/s. That was necessary to better support 4K60 resolutions, but it still wasn’t enough—not with 4K144 HDR and 8K displays now available. So, Nvidia moved to an NVLink connector for the RTX 2080 and 2080 Ti. The 2080 gets a single NVLink channel that’s good for 50GB/s, while the 2080 Ti has two and 100GB/s. Thanks to the higher bandwidth, there’s no difference between running dual GPUs off x16 PCIe connections versus dual x8 connections, which is good, as the Core i7-8700K and i9-9900K end up being faster in games than any X299 enthusiast platform CPU.
But how does RTX 2080 Ti SLI perform? Results vary. 3DMark shows nearly perfect scaling in Time Spy Extreme and Fire Strike Ultra. GTA5 ,Rise of the Tomb Raider, Strange Brigade, and The Witcher3 also show excellent scaling of 80 percent or more at 4K. Other games, such as Battlefield 1,F12 018, Far Cry5, Rainbow Six Siege,Shadow of the Tomb Raider, and Shadow of War, aren’t so impressive, but manage 30–50 percent scaling. Then there are games that show negative scaling:
Assassin’ s Creed, Call of Duty, Divinity Original Sin, Hell blade, Hit man, Monster Hunter World,
Prey, Wolfenstein, and more. As a $2,500 proposition ($1,200 per 2080 Ti and $79 for the NVLink adapter), this isn’t for the faint of wallet. It’s also sobering to think that even 4K isn’t enough to max out scaling from SLI. Beyond that, we’re years away from 8K gaming being a viable choice. That’s all for the best as far as I’m concerned. When SLI or CrossFire work well, they’re impressive, but far too often, you run into incompatibilities or glitches. I’d rather spend my time playing games instead of futzing around with SLI profiles. Jarred Walton has been a PC and gaming enthusiast for over 30 years.