ConclusionThe debate whether SLI is on the decline has pretty much been settled a while ago, probably when the promises of DirectX 12 mixed multi-GPU never got fulfilled. Over the past three generations, NVIDIA has narrowed down SLI capability to fewer and fewer market segments. The GTX 960 had at least 2-way SLI, while the GTX 1060 lacked SLI support altogether. The GTX 1070 had SLI support, while the $500 RTX 2070 lacks it. The price of entry for SLI has hence escalated to $1,600 (for two $800 RTX 2080 cards), whereas three years ago you could pick up a pair of GTX 960 cards for $400 to reach performance levels of the GTX 980.
Interestingly, even as NVIDIA was cannibalizing its own market for SLI, it was investing in making the tech itself better. "Pascal" saw the introduction of SLI-HB, which purportedly improves performance and smoothness at higher resolutions such as 4K-60 Hz. With "Turing," NVIDIA leveraged its NVLink interconnect, which was invented for the professional compute space, to enable reliable multi-GPU scaling for even more demanding display formats, such as 4K-120 Hz, 5K, or even 8K. The onus is now entirely on NVIDIA to keep big game studios interested in SLI support because they have to be convinced any investment in developer time (=money) can reach a sufficiently large audience (that can afford to spend $1,600 on graphics cards alone).
The promise of NVLink is fantastic. In the past, the SLI interface was used just to transport the frame output of the SLI slave cards to the master, which would stitch the frame together and send it out over the monitor cable. With NVLink things have changed dramatically. All SLI member cards can now share their memory with the VRAM of each card sitting at different addresses in a flat address space. Each GPU is able to access the other's memory in a completely transparent way—the memory controller will fetch non-local data over the NVLink interface, which is very similar in concept to PCI-Express, with tons of low-latency bandwidth. NVIDIA has managed to make NVLink cache coherent, which means that each GPU will always see the right value for a memory address, no matter where the actual information is located and even if the data was updated just a split second ago. For compute applications, this newfound ability enables solving of large problem sets that would otherwise have overflowed GPU memory.
For existing games, this means nothing though. Utilizing these new address space capabilities requires developers to rebuild their SLI support from the ground up and patch already shipped games—not going to happen. I also have serious doubts whether game developers are willing to implement such a technology for near-future games as they'd not only be developing for the tiny subset of SLI users, they'd only be able to target Turing NVLink users with it, an even smaller piece of the market. Still, not all is lost. NVLink enables much higher throughput rates for the NVLink interface with lower latencies, which might have a positive impact on SLI performance in existing games.
Once you've spent $1,600~2,400 on graphics cards and written the book on negotiating with the significant-other, you might be surprised that only about half the titles in our test suite actually scale with SLI. Out of 23 titles, 11 titles showed negative or no performance scaling when switching to SLI. To be honest, we expected multi-GPU support to be even worse, so we were actually positively surprised. This is because NVIDIA developer relations are solid, and they convince larger studios to add SLI support to most major titles out there. Our results show that you really do not need SLI for 1440p or 1080p regardless of the refresh rate. The single RTX 2080 Ti or RTX 2080 handle these resolutions well above 60 fps, and we even run into platform limitations in some games where the frame rates are so high that the CPU is unable to keep up. "GTA V" is a nice example of a game that shows no scaling at 1080p or 1440p, but a good multi-GPU performance uplift at 4K UHD.
4K Ultra HD is the only resolution at which RTX 2080 Ti SLI and RTX 2080 SLI begin to show their mettle. Even there, performance scaling isn't that impressive, certainly not good enough to justify spending twice the money. At 4K, the RTX 2080 SLI shows +26% performance when averaged over our whole test suite, and the RTX 2080 Ti gains 24%. We include a second data point in the summaries that excludes all the games that don't scale, giving you the averages of SLI-supported games only. Here, things look better: +56% for the RTX 2080 SLI setup and +55% for the RTX 2080 Ti SLI, with the gains varying wildly between games.
"Battlefield 1" is a nice example of a game that scales well, posting 68% scaling with RTX 2080 Ti SLI and a stunning 81% upscaling with RTX 2080 SLI, at 4K UHD. "F1 2018" is another well-scaling game, posting +69% with RTX 2080 Ti SLI and +73% with RTX 2080 SLI. Not all games are CPU-limited at 1440p. "Rise of the Tomb Raider," for example, scales 73% at 1440p with RTX 2080 Ti SLI, and 83% for RTX 2080 SLI.
We have to give a special kudos to the developers of "Strange Brigade." A love letter to PC gaming, this title supports both DirectX 12 and Vulkan APIs, SLI and CrossFire. In our tests (involving DirectX and SLI), it puts out stellar performance increases in all three resolutions, including 1080p. At 4K UHD, both the RTX 2080 Ti SLI and RTX 2080 SLI post an astounding 96% performance uplift. Even GTX 1080 Ti SLI from the previous generation manages an 81% upscale. Our compliments to Rebellion.
"Hellblade: Senua's Sacrifice" is a good example of a game that's notoriously bad at SLI, posting negative 19% scaling at 4K with RTX 2080 Ti SLI and a staggering negative 30% scaling with RTX 2080 SLI. The purported poster-boy of DirectX 12 and other cool new technologies, "Hitman" also posts negative scaling. "Wolfenstein: The New Colossus" suffers from negative scaling, too.
We also included SLI results for the GTX 1080 Ti, a card that delivers very similar performance to the RTX 2080. This lets us compare the scaling of traditional SLI vs. NVLink. If NVLink and SLI were completely identical for gaming, then we'd expect to see both RTX 2080 SLI and GTX 1080 Ti SLI end up with the same performance levels. The opposite is true: RTX 2080 SLI scales much better than GTX 1080 Ti, which is proof that the added bandwidth and lower latency of NVLink is helping SLI—even for gamers. For example, in Deus Ex at 4K, GTX 1080 Ti SLI has an 11% performance uplift, whereas GTX 2080 SLI can deliver +38%. The difference is even bigger in F1 2018 4K, with GTX 2080 SLI outpacing GTX 1080 Ti by a huge margin (+74% vs. +16%) using the same driver. When looking at all SLI-supportive games combined, RTX 2080 SLI gives you an impressive 30%-46% more FPS than a similarly priced GTX 1080 Ti SLI setup. We didn't expect that!
SLI does make 4K a lot more playable, letting you not just max out settings as it also gives you a cushion in case performance drops in pitched battle scenes that bog down your hardware. Unfortunately, fewer new games support it, and only NVIDIA is to blame. It first cut out SLI support from mid-range graphics cards, and with the RTX 2070, even the performance segment is set to lose it. Rival AMD still supports CrossFire across its entire product stack; however, that doesn't translate into every game supporting CrossFire, either. Why? Because AMD's market-share is lower. Game developers will only invest in a technology if they're convinced enough people out there are going to use it.
The new NVLink bridge is an assault on your wallet and an affront to decency. While the NVLink technology itself sounds impressive on paper, we wish that after having spent either $1,600 or $2,400 on graphics cards alone, consumers weren't made to spend another $80 on the bridge, something with a $5 bill of materials and not a cent more. The bridge contains no logic besides what is required for the LED-lit NVIDIA logo. $49 gets you a fully fledged motherboard complete with chipset, CPU socket, slots, controllers, VRMs, heatsinks, UEFI BIOS license, and a much bigger multi-layer PCB, etc. I really wish the NVLink bridge were bundled for free with Founders Edition graphics cards, just like AMD Radeon graphics cards used to bundle CrossFire X bridge cables in the past. It really doesn't cost much and makes people like you more.
For this generation, two variants of the Turing GPU exist for each SKU. One with an -A suffix intended for use on factory-overclocked cards (including Founders Edition) and a non-A variant that's cheaper, for boards that are targeted at the MSRP price, but are forbidden to be overclocked out of the box. Each variant comes with its own device ID and would thus be unable to pair with the other, if going by previous requirements. We reached out to NVIDIA to check whether SLI is possible between -A and non-A GPUs, and the company promptly confirmed that such setups are possible indeed. Good! Just to clarify, that doesn't mean that you can SLI the RTX 2080 with the RTX 2080 Ti (which nobody would expect anyway).
So the burning question: should you spend $1,680 on RTX 2080 SLI? Absolutely not. Averaging all our tests, RTX 2080 SLI is within single-digit percentage performance with RTX 2080 Ti SLI. Get a single RTX 2080 Ti, overclock it further yourself, and you've definitively set yourself up for 4K 60 Hz bliss. Should you spend $2,480 on RTX 2080 Ti SLI? This is a different beast. For the sheer fact that there is no alternative to RTX 2080 Ti SLI, it becomes a halo option. You spend top dollar for the highest performance you can have. Even so, the RTX 2080 Ti SLI is unlikely to make 4K 120 Hz gaming a breeze without watering down the eye candy, and it's still not fast enough for higher resolutions, such as 5K or 8K.