Last Updated: Sep 29, 2019
If you're even only slightly interested in gaming graphics technology, you've no doubt heard about NVidia SLI at some point but perhaps wondered what SLI is, how it works, how much faster it is compared to a single GPU, and what you need for SLI compatibility in terms of hardware. Well, seems it's your lucky day my friend 'cause in this introduction to SLI we'll explain what to know as a beginner to quickly get you up to speed with the basics of planning and building a SLI PC for gaming.
Is SLI for everyone? You already know the answer. But is SLI worth considering for that tiny niche segment of the upper-end of the gaming market who desire even more power than any one single GPU can provide? Yep. Despite what many might parrot on forums, for the right (patient) person with the right setup with the right games, SLI can work out well. Whether you're genuinely weighing up whether to take the plunge into the murky waters of a multi-GPU machine, or are just here to come along for a tire-kicking ride down the rabbit hole of this cool technology, let's dive in and enjoy.
Related: How to Choose a GPU (Specs 101)
SLI stands for Scalable Link Interface, and is NVidia's own multi-GPU technology for linking two or more NVidia-based graphics cards together to share the render processing load of games/applications for improved performance. SLI was actually first introduced way back in 1998 by a company called 3DFX who initially named it Scan-Line Interleave technology, and it was used in their Voodoo2 range of GPUs.
NVIDIA eventually bought 3DFX and improved upon the technology to come up with their own version (and name) which was re-released in 2004 as Scalable Link Interface. Ever since, NVidia has periodically improved upon the technology, with recent notable improvements including the release of SLI HB (High-Bandwidth) bridges along with their Pascal-based GeForce GTX 10 Series GPUs which introduced faster communication between two GPUs, and most recently with the emergence of a new and faster version of SLI called NVLink which is the technology you need to use when linking multiple RTX cards.
There are 3 different types of SLI configurations, but only one that still lives on for modern gaming:
Also worth mentioning for reference is the rarer SLI setup of having two GPUs on the one graphics card, which old cards like the GTX 590, GTX 690 and GTX Titan Z all used which allowed for 4-Way SLI using only 2 cards.
There are also different SLI rendering modes that multiple graphics card setups can use, however modern games pretty much only exclusively utilize the one mode (AFR). These are the 3 most common SLI modes:
An SLI gaming build rocking multiple video cards sounds amazing in theory: install your first graphics card, rinse and repeat with a second graphics card, enable SLI, and unleash heavenly performance with double frame rates! But in reality, it doesn't work that way, and SLI doesn't scale linearly for gaming.
In fact, in games that do support SLI (more on that in a bit) the scaling isn't anywhere near linear. In other words, you're not even going to get close to doubling your performance in the majority of games that are supported by SLI. Plus, when using SLI you also do not get double the VRAM; so if you have a
Exactly how much faster SLI is compared to a single graphics card varies a lot from game to game and setup to setup. On one hand, the games that scale the best using SLI may net you an extra 50-80% extra performance vs a single GPU.
But on the other less-appealing side of the spectrum seeing a disappointing 10-50% boost in frame rate is a very common scenario in many SLI-supported games. In the worst case scenario, which does happen believe it or not, certain SLI-compatible games may even reduce your performance in certain configurations.
SLI these Star Wars themed Titans like Linus did and you have my respect
This generally poor scaling is one reason why building a SLI PC is definitely not for everyone, and absolutely not for anyone seeking value for money and/or looking to be cost-effective, with another big reason being the fact that not all games support SLI (more on that soon). Furthermore, even if a game scales okay and does end up providing faster performance, in some games it's very possible to encounter visual artefacts/glitches which is a phenomenon referred to as micro-stuttering, which can hinder the smoothness of your gaming experience and make SLI not worth it at all for that particular game (if you can't find a way to fix it that is, as there are many workarounds you can find online).
For SLI to work you will need the following things:
An SLI bridge is the actual physical connector that clips on top of your graphics cards, and is used to reduce bandwidth limitations and allow your GPUs to communicate directly to one another for the fastest SLI performance. Without using a SLI bridge, it may be technically possible to run multiple graphics cards in certain situations as the GPUs can chat and keep each other updated on the latest memes via the motherboard instead, but if you want to do SLI properly for the best improvements in performance then you'll need to get a bridge.
Graphics cards do not come with SLI bridges, though some SLI-certified motherboards will come with a SLI bridge, especially if you choose a high-end modern motherboard with 4 full-length PCIe slots. However, there's no telling if the SLI bridge that comes shipped with a motherboard will be the one you need for your particular SLI setup. As an example, if you look closely at the MSI MEG Z390 Godlike specs (Haha, Meg) under "Box Content" you'll see it comes shipped with a SLI HB bridge:
SLI HB bridges are for GTX 10 series GPUs like the GTX 1080 and GTX 1080 Ti, so it could be the one you need to SLI those cards. For RTX cards you'll need a NVLink SLI bridge instead, but we'll get to that in a second. When choosing the right SLI bridge for your system you must also take into account different slot spacing, as you might need a smaller or larger SLI HB bridge depending on which slots you install your GPUs in. In other words, the exact SLI bridge you need to use will be determined by how far apart your SLI cards will be on the motherboard.
Which PCIe slots you choose for your GPUs will depend on your particular motherboard, how thick your cards are, airflow restrictions, etc. If you install a 2-Way SLI setup in the first and third motherboard slots (pretty standard setup), you'll need what's called a 3-slot SLI bridge. If instead you install 2 cards in the first and fourth slots for whatever reason, you would need a 4-slot SLI bridge instead. Check your motherboard manual or research online as to which exact slots you should use for SLI.
If you're building a SLI rig using a RTX 2080, 2080 Super, or 2080 Ti, you will need to buy a NVLink SLI bridge which are sold separately by companies like MSI, EVGA, Asus. RTX GPUs use the new and improved NVLink technology, and won't be compatible with the older SLI HB bridges.
Besides choosing the right SLI bridge, you'll also need to make sure your graphics cards are compatible with SLI. You will need GPUs that are of the same base NVidia model, but you can mix different brands and manufacturers. For example, you can use an EVGA RTX 2080 Ti with an MSI RTX 2080 Ti, but you can't SLI with a 2080 Ti and a 2080. Besides combining the same GPUs, you also need to use GPUs that actually support SLI in the first place.
Here are lists of all recent consumer gaming GPUs that are certified for SLI as of 2019:
Modern GTX and RTX GPUs That Support SLI:
No, the RTX 2060 and RTX 2070 do not support SLI (the non-Super RTX 2070, that is). Neither do mid-range GPUs of the previous GTX 10 series such as the super-popular GTX 1060. Remember the above list of gaming GPUs that support SLI are just the most recent RTX and GTX cards, and the previous Titan graphics cards (Titan X and Titan XP) both support SLI as well as many older cards from the GTX 900 series and earlier generations.
For a motherboard to support SLI, it must officially list support for SLI because not all motherboards that have multiple PCI-Express x16 slots support SLI. As well as having official compatibility, physically your motherboard must also have the room for the amount of GPUs you plan on installing, keeping in mind most modern graphics cards take up more than one PCIe slot (and likely 2 or 2.5).
There's no such thing as a SLI-ready or compatible PSU, so forget about that. But what you will need in your power supply are 3 things if you want to SLI on the safe side:
The actual physical installation of SLI is very simple, and just a matter of installing GPUs in the right slots and attaching the SLI bridge between them. The potentially difficult and time-consuming part is the software side of things, and getting certain games to run smoothly without issues such as visual glitches/micro-stuttering (even in games that do support SLI).
For more exact detail on installing a SLI setup, see the video below, but here are the quick overall steps on how to set up SLI in a nutshell:
How Many Games Support SLI?
To actually take advantage of SLI mode, the game you're playing must have SLI support - either in the initial release or in a later patch/upgrade. As for how many games officially come with SLI support, there's some bad news, but some good news too that can possibly render that bad news as irrelevant. While there are plenty of big AAA games of the past that have official SLI support, few really recent 2019 titles come with official support these days. Some games also have their SLI support dropped after a while due to too many issues, such as Apex Legends.
Can You Manually Force SLI to Work?
However, and this is where it gets interesting. There are indeed ways to get SLI working for a game that doesn't have official support for if you have the time and patience to do some research and tweak some settings using a NVidia tool called Inspector. Results will vary by attempting this - you might get lucky and find a quick fix for a certain game that works well, or you might encounter too many issues such as micro stuttering or other glitches that it ends up not being worth it for that particular game (and your second GPU will just sit there idly).
But it's possible to get SLI working for pretty much any game out there. Yep, that's right, despite official SLI support waning in recent years (tons of older games do have official support), SLI is far from dead thanks to collective efforts of the community around the web. For example, check out how to fix SLI for Apex Legends here. Google and YouTube are your best friends to find fixes for any other game. Good luck!
How to Know if a Game Officially Supports SLI?
To find out if a game officially supports SLI (without having to do manual tweaks yourself like for Apex Legends), you have to do your own research as there's no official list anywhere, although this list of officially supported SLI games seems legit.
Ready to get super impractical, cost-intensive, and downright ridiculous to aim for 100FPS-144FPS+ in 4K (or 60FPS in 8K) to generate envy in the heart's of every other gamer on the planet? Before proceeding further into SLI land, you must pass the following test because with great power comes great responsibility.
If you answer no to any of the following questions; take the blue pill, and be on your way back to your normal life as if you had never encountered this ludicrous article.
If you (somehow) answer yes to ALL questions below; take the red pill, and see how far the SLI RTX rabbit hole goes.
Then SLI may be for you. For now, specific build guides for SLI are beyond what we cover here at BGC, however don't rule it out for the future (subscribe to our newsletter down below to be notified if we do...it's on my near-future To-Do list ;p). But for ideas on which parts to choose for a SLI gaming PC build, here are some starting points, though if you're building such a system you're more than capable of doing your own research. I'd first check out any completed SLI builds over at PCPartPicker for inspriation and to learn a few things.
Also, in our current best gaming PC builds for the money chart we detail a $5000 RTX 2080 Ti SLI build using an i9-9900K which may also help. Also, see the links below for some interesting, trustworthy videos to learn more, including the usual high-tech suspects Jay from JayzTwoCents who setup dual SLI RTX 2080 Ti's and Jay's arch-overclocking-nemesis Steve from Gamer's Nexus who went a step further and linked dual Titan RTX's (the financial equivalent of a small moon).
That wraps our introduction to SLI for beginners, and hope it helped a little. If you have any feedback then leave a comment below as this guide is a continual work in progress and I'd be interested to hear any thoughts you might have after reading this. Thanks guys, and if you are indeed going ahead with building a SLI gaming PC, may the force be with you and your parts - I think you'll be needing the good fortune ;)