SLI (Scalable Link Interface) is a marvelous technological innovation that allows two or more graphics cards to be simultaneously utilized to gain an overall boost in performance. It can handle 2 to 4 GPUs at once, and it is NVIDIA’s equivalent to AMD’s Crossfire technology. Technically, most of the information in this article will be applicable to both Crossfire and SLI, but the specific topic of this post will be SLI.
SLI gained popularity in recent years largely due to the bitcoin mining craze, but also because of a certain class of gamers who are determined to squeeze every drop of possible performance out of their rigs. But how useful is SLI—and, more importantly, is it worth going all-out and building a rig with four GPUs running in sync?
The short answer is no. It is not worth building a rig with 4-way SLI. And the answer for 3-way SLI is much the same. But depending on your needs and approach, it may sometimes be worth going for 2-way SLI. For more details and specific reasoning, read on:
Dropped Support for SLI
The largest issue with 3-way SLI and 4-way SLI is that NVIDIA dropped most support for it several years ago, except for a select few benchmarking applications. This means that the latest and greatest cards simply aren’t supported. While 2-way support remains, 3- and 4-way would require you to use older and less current cards (i.e. cards from before the GTX 1070 and 1080 release).
Planning to purchase 3 or 4 cards from two generations ago in the hopes of achieving some kind of sneakily amazing rig is a fundamentally bad idea. This is because of heavily diminishing returns, as covered below, but also because of a straightforward historical trend: as GPUs have become increasingly more powerful and complex, so have the programs they’re meant to run and the games they’re meant to play.
Developers are constantly pushing the limits on cards, and within 5 years, a card that could play a brand new AAA game at 60 FPS or run top-of-the-line GPU rendering software might get knocked down to far lower frame rates and somewhat slower renders on new releases. Pure frame rates and speeds aside, newer programs and games throw additional challenges at hardware, such as how lighting operates and how much texture detail there is.
Older graphics cards run slower, have slower memory, have fewer processing cores, feature more limited instruction sets, have reduced compatibility with available software libraries, have older graphics drivers, and lack newly developed features when compared to newer cards. So throwing more graphics cards at the problem might solve the processing core disparity, but none of those other issues.
Diminishing Returns of Multiple GPUs
The other big problem with SLI is that the performance boost is not simple addition: throwing two GPUs together won’t give you double the performance in most applications. Similarly, throwing three or four GPUs together will usually not triple or quadruple it. There are some workstation rendering farm set-ups where using multiple GPUs is nearly additive, but most ordinary PC users (including video editors, graphic designers, streamers, and gamers) will see only tiny benefits from adding a second GPU.
Moreover, while the performance bump from 2-way SLI is often marginal, adding more than two GPUs in SLI can actually decrease performance for some systems. (And that’s without even mentioning how multi-GPU set-ups also encounter monetary diminishing returns, where you’re getting much less performance per dollar for each GPU after your first in a given system.)
There are a couple reasons for the underwhelming boost or even potential performance drop:
For one, each time you add another GPU, you add another sizable element for your CPU to manage under load. Not every CPU even has the capacity to run 4 GPUs at full power simultaneously (especially in systems also using NVMe storage drives).
For another, multi-GPU set-ups also alter how the computer has to divide up the work—and not all tasks are easily divisible. Similar to how games (and many other common programs) do not usually benefit from a CPU having more than 4 cores, they often can not benefit from a system having more than 2 GPUs. It turns out that a lot of program tasks just have to be done sequentially (one after the other), and can not be done in parallel (simultaneously).
Finally, along the same lines as the previous points, the work done to complete a task has to be communicated between GPUs to create a single output. This means the computer spends more time interpreting and coordinating the results. More GPUs is making your computer work harder, but not necessarily smarter.
What About 2-way SLI?
2-way SLI is a bit of a gamble, as the result changes from game to game and program to program. A game like Tomb Raider might respond exceptionally well to 2-way SLI and gain a decent FPS boost at higher resolutions. A game like The Witcher 3 might suffer a drop in minimum framerate and gains some stuttering issues.
Fortunately, SLI can be toggled on and off, which at least reduces the gamble. But the fact of the matter is that SLI is an unreliable and tricky beast, and no two games or programs respond precisely the same to being chopped up and distributed between two GPUs. There is no doubt that 2-way SLI has the capacity to increase overall performance, but it should not be seen as a one-size fits-all guarantee to increased performance like a normal upgrade path.
You will notice that, in our primary build recommendation chart, the only situations in which we recommend having 2 GPUs is when you’ve literally run out of options for getting more performance with a single GPU: in the very highest tiers of the chart. To potentially get better performance than an RTX 2080 Ti, for the time being, a very-high-tier builder would have to consider getting something like 2 GTX 1080 Tis, 2 RTX 2080s, or 2 RTX 2080 Tis.
If at all possible, buying a more powerful single GPU will guarantee an uptick in performance if there are no bottlenecks elsewhere in the system. 2-way SLI doesn’t guarantee increased performance, however, and could in theory throttle your machine. But when you find a game or program where stacking GPUs together does increase performance, then you’ll find you’ve got a route toward getting an edge over the flagship cards of the past couple generations.
To sum things simply, unless you’re a very serious rendering professional, there are really no situations in which seeking to acquire and install 3 or more GPUs in a single system makes any sense; and there only a few situations in which pursuing 2-way SLI is a worthwhile endeavor:
If you happen to have two identical GPUs that are a generation or two behind, as often happens to thrifty PC enthusiasts who cannibalize rigs and keep old parts after helping others upgrade their rigs, there’s no harm in trying 2-way: you’ve got the parts on hand, and you can toggle SLI on and off as needed.
Some lucky folks accidentally wind up with two GPUs instead of one when they order online—don’t ask me how that happens—and get to keep the spare. If that’s you and selling the GPU to fund upgrades in a different area isn’t worth the time or effort to you, then 2-way SLI is the next logical step.
If you’re someone with very deep pockets and you’ve got the upgrade itch, and you have no other way to upgrade your PC (such as a better CPU, improved cooling solutions, bigger and faster RAM), then 2-way SLI is the way to go.
But for all other comers, and those considering 3-way and 4-way SLI, save your money and buy a better GPU if possible.