Scalable Link Interface or SLI is the brand name for Nvidia’s multi-GPU technology established for linking up two or more graphics cards into a single output using a parallel processing algorithm.
It sounds like a mouthful, but it’s a really cool piece of technology that has also been used by their competitors, AMD, with their CrossFire brand, and it’s worth talking about.
Let’s see how all of this works!
Fun fact: Nvidia cannot claim credit for this technology as they acquired it from 3dfx Interactive, who introduced it in 1998. At that point, it was a little ahead of its time, but in 2004, when SLI made its reappearance, the market for top-performing gaming equipment was ripe.
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What Is Needed To Run SLI?
A friend of mine actually fried his motherboard thinking that just because he had enough slots, he could insert graphics cards and just boot up the computer. Okay, that was me. But still, there are a few more requirements than just a couple of open PCI-Express x16 slots.
First, you must check if the motherboard is SLI compatible. This is a really vital step, so be careful, especially because some motherboards support SLI, others CrossFire, or both, or neither. Of course, if you’re only going for a two-card setup, then cards can be configured to work in SLI mode.
Secondly, you need identical graphics cards. Hooking up a GTX 1080 and GTX 1070 won’t work, despite their similarities. They need to be the same model and series, although it’s possible to get it from a different manufacturer.
For example, if one is made by MSI and the other by Asus, and the third one by Gigabyte, you’ll still be able to configure them together on the same machine. However, on rare occasions, it’s possible to run “mixed SLI” configurations on some cards that only have a matching core codename, like G70, G73, G80, and so on.
Graphics cards are set up in a master-slave configuration, which means that one card will play the role of the “master” even though the workload is distributed equally to all cards. In a two-card setup, the master will work on the upper part of the screen, while the slave will manage the bottom part. When the slave finishes rendering, it sends it to the master, which then combines the two renders and output to the monitor.
In those rare cases where the cards are mismatched, the slower or lesser card will become dominant, with the better one adjusting by either running at the same speed as the other card or disabling its extra memory.
SLI Bridge connects the graphics cards. It’s also known as the SLI Connector as its purpose is to establish a direct connection between the cards. It’s worth noting that it’s possible to run two low-end or mid-range cards without the bridge by using the chipset on the motherboard.
This is also possible for high-end cards, although not recommended, as the results will be very poor considering how the chipset just doesn’t have enough memory. This is where SLI Bridge comes in because it reduces the bandwidth constraints and can send data directly between the cards.
There are three types of SLI Bridges:
- Standard Bridge (400 Mhz Pixel Clock, 1GB/s bandwidth) – This is a traditional bridge included with motherboards that support SLI up to 1920×1080 and 2560×1440@60 Hz.
- LED Bridge (540 MHz Pixel Clock) – Recommended for monitors up to 2560×1440@120 Hz+ and 4K. Sold by Nvidia, EVGA, MSI, Asus, among others. It can only operate on an increased Pixel Clock if the GPU supports that clock.
- High-Bandwidth Bridge or SLI HB Bridge (650 MHz Pixel Clock and 2GB/s Bandwidth) – This is the fastest bridge and is sold exclusively by Nvidia. It’s recommended for monitors up to 5K and surround. SLI HB Bridges are only available in 2-way configurations.
While the two-GPU setup might be the most popular one, SLI can also be configured in a three or four GPU dynamic by either a single bridge that connects every card or by combining two two-way bridges. Combining three or four dual cards set up with a 2-way bridge is impossible as the technology for hexa- or octa SLI configuration doesn’t exist.
The future is uncertain for these layouts as Nvidia is phasing out the support for more than two-card combinations due to high driver complexity. As a matter of fact, cards like GTX 1070, GTX 1080 and higher only support the two-way SLI.
SLI is supported by all Windows versions following Windows Vista, both 32-bit and 64-bit. It can be configured to work on Linux, but as there aren’t many high-end graphics video games made for that platform, it seems useless to even try.
SLI requires at least 2 GB of RAM for a 32-bit system and 4 GB for 64-bit. Back in 2003, Nvidia released NVIDIA ForceWare Unified Driver Architecture (UDA), which has built-in support for SLI technology. As such, there’s no need for additional SLI-specific drivers, but you still need them for each graphics card.
A very powerful and reliable Power Supply Unit is necessary as the GPUs are big power consumers and you’ll be running two or even more at the same time. High-end graphics cards can use up to 200-250 watts of power so beware of this as well.
Important: not all games and applications support SLI. For those that do, Nvidia includes SLI profiles in their driver package so you don’t have to set anything up manually to get a far greater gaming experience.
Split Frame Rendering
SFR analyzes the rendered image to divide the load between the GPUs. To achieve this, the frame is split on a horizontal line, depending on geometry. For example, if the upper part of the screen is rendering the sky, then this line will be lower in order to balance out the geometry workload on the GPUs.
Alternate Frame Rendering
AFR means that each frame is rendered by a different GPU. In practice, this is most commonly executed with one card working on odd frames, and the other on even frames.
Although AFR might have a better frame rate than SFR, it may cause micro stuttering, which changes the way frame rate is perceived. While the frequency at which frames arrive may be doubled, the production time of the frames is not, so this cannot lower the input lag.
This is a separate rendering mode that can double the antialiasing performance by splitting the workload between the GPUs. One card will perform an antialiasing pattern that is offset to the regular pattern, for example, a little to the right and down, while the other one will do the same thing with an equal offset in the opposite direction (left and up in this case).
SLI Antialiasing has some advanced options like SLI 8X, SLI 16X, and SLI 32X, although the last one is only for Quad SLI systems.
This is a generic name for two technologies, GeForce Boost and HybridPower. GeForce Boost used integrated graphics and a discrete GPU combination to increase performance.
In turn, HybridPower is not a performance-enhancing mode. It also uses IGP and a GPU on an MXM module, which is used to easily integrate the GPU on laptops. As it’s a big power consumer, the MXM module will shut down when the laptop is unplugged from the power supply. This isn’t exclusive to laptops as it can also be found in desktop computers.
It’s crucial to note that a high-end SLI setup will require a powerful CPU to prevent bottlenecking.
NVLink – Is This The Death Of SLI?
It would be more accurate to say that the future of multi-GPU setups is in question as the gaming industry still hasn’t fully embraced its advantages. While AMD’s CrossFire appears to be at the brink of extinction, Nvidia still has a few tricks up its sleeve.
This is where NVLink, a wire-based communications protocol serial multi-lane near-range communication link, enters. And you thought SLI’s definition was a mouthful. Much like SLI, NVLink is a way to connect multiple graphics cards for a single purpose, but a little more spiced up.
So far, it only works in a two-card setup, but that should be enough given its use of NVLink Bridge. Like the SLI Bridges, it connects two cards, but it can provide 10 to 12 times better link speed.
The extra speed is additionally accentuated by the fact that NVLink uses mesh networking to communicate instead of a central hub. This suggests that there is no master-slave dynamic, and each card can work to its full capacity all the time.
Following the release of Nvidia’s Ampere and RTX 3000 series cards, it’s safe to say that SLI is officially over. Only the enthusiast-level RTX 3090 has an NVLink interface, while others will support SLI only explicitly, meaning it’ll be handled outside APIs like Vulkan, DirectX 12, or Open CL.
If you’re thinking about using multiple GPUs in the SLI setup, here’s the list of all NVIDIA desktop GPUs that support SLI.
|GeForce RTX 2080 Ti|
|GeForce RTX 2080 SUPER|
|GeForce RTX 2080|
|GeForce RTX 2070 SUPER|
|Nvidia Titan Xp|
|GeForce GTX 1080 Ti|
|Nvidia Titan X|
|GeForce GTX 1080|
|GeForce GTX 1070|
|GeForce GTX TITAN X|
|GeForce GTX 980 Ti|
|GeForce GTX 980|
|GeForce GTX 970|
|GeForce GTX 960|
|GeForce GTX 950|
|GeForce GTX TITAN|
|GeForce GTX 780 Ti|
|GeForce GTX 780|
|GeForce GTX 770|
|GeForce GTX 760 Ti|
|GeForce GTX 760|
|GeForce GTX 690|
|GeForce GTX 680|
|GeForce GTX 670|
|GeForce GTX 660 Ti|
|GeForce GTX 660|
|GeForce GTX 650 Ti BOOST|
|GeForce GTX 480|
|GeForce GTX 470|
|GeForce GTX 465|
|GeForce GTX 460|
|GeForce GTX 460 SE|
|GeForce GTS 450|
|GeForce GTX 555 (OEM)|
|GeForce GTX 560 Ti (OEM)|
|GeForce GTX 560|
|GeForce GTX 550 Ti|
|GeForce GTX 590|
|GeForce GTX 660|
|GeForce GTX 560 Ti|
|GeForce GTX 545 GDDR5|
|GeForce GTX DDR3|
|GeForce GTX 570|
|GeForce 9800 GT|
|GeForce GTX 580|
|GeForce 9600 GT|
|GeForce 8500 GT|
|GeForce 8600 GTS|
|GeForce 8600 GT|
|GeForce 8400 GS|
|GeForce GTX 275X|
|GeForce GTS 150|
|GeForce GT 130|
|GeForce GT 120|
|GeForce GTS 250|
|GeForce GTX 285|
|GeForce GTX 295|
|GeForce GTX 295|
|GeForce 8800 ULTRA|
|GeForce GTX 280|
|GeForce 8800 GTX|
|GeForce 9800 GX2|
|GeForce GTX 260|
|GeForce 9400 GT|
|GeForce 9500 GT|
|GeForce 9800 GTX|
|GeForce 9800 GTX+|