SLI-explained – What it is and how it works

SLI is a relatively new technology that makes it easy to bridge GPU’s together to enhance your GPU’s performance. This new technology is much preferred over the old method of simply buying a more expensive card to upgrade your computer, now you can simply double your existing graphics card to achieve those higher framerates you want.

Both Nvidia and AMD have their own methods for multi-card configurations, Nvidia’s is called SLI meaning scalable link interface. AMD‘s equivalent to SLI is known as AMD CrossFireX.

What is SLI (Scalable Link Interface)

SLI is a multi-card scaling system, which in layman’s terms means an engine used to simultaneously use more than one graphics card to increase the performance of the computer in a video game by up to double (or 100%) for each added graphics card. SLI functions by splitting the workload between the two graphics cards to increase gaming performance. We can look at SLI a bit closer in the following paragraph.

The SLI engine allows multiple graphics cards to work on each frame independently, this parallelism can in theory double or even triple the frame rate depending on how many cards you are using. Once the CPU is done working on the pre-render objects such as the mathematics and the vertex tracing, the graphics driver will feed the dating to the GPU’s, the GPU’s can render the Frame via two methods:

  • SLI AA (Antialiasing) this is to drastically increase the image quality
  • Split Frame Rendering (SFR) drastically increases framerate and performance.
  • AFR (Alternate Frame Rendering) drastically increases framerate and performance.

The three above methods are used by SLI to either increase performance or to drastically increase the quality of each frame’s image, all methods increase performance to some degree and make your computer a better machine overall.

SLI Modes

SLI AA (SLI Anti-Aliasing)

SLI Antialiasing focuses mainly on producing good image quality and increasing performance, it achieves this by using a rendering engine that divides the workload to render antialiasing on 2 or more GPU’s to reduce performance lag and increase image quality. In a dual-SLI configuration, both GPU 1 and GPU 2 will actually render the same frame but it will increase the depth at which the GPU scans for any aliasing and the resultant merged frame looks much better than a frame produced by only one GPU performing all the Anti-Aliasing work.
This mode is usually great when you want to increase image quality but you don’t have any real problems with performance. To give you an example, Dual card- SLI will provide with SLI Anti-aliasing much better Image quality than single card anti-aliasing but it wouldn’t provide much of a performance boost.
You can find this option in the Nvidia control panel, just select the anti-aliasing mode in application profiles.

Split Frame Rendering

Split Frame Rendering method signals the engine to analyze the frame and split it up into “n” parts for each GPU to render and deliver back to the monitor to display. Split frame rendering dynamically allocates screen space and frames based on each frame, it won’t just deliver each frame and randomly chop it up to be rendered for example.

A good example is to imagine an outdoor scene with you, a gun, some characters, and a sky. The sky will be pretty easy to render as it is not really resourced intensive, the moving gun, characters and your arm will be pretty resource intensive. The engine will split up the frame in a way so that for example, GPU 1 would only have to render the arm, gun, and only half of the resource-intensive parts of the frame, while the other GPU will have to render the other half of the resource-intensive parts of the frame as well as the sky. This is because the engine doesn’t want to overwork one card by assigning it 50/50 and not telling the difference between a static sky and moving arms, characters and objects.

Alternate Frame Rendering

Alternate Frame Rendering is a rendering mode for SLI that uses alternating frame subsets to render a set of frames, for example, GPU 1 in a dual GPU SLI configuration will render all frames that have odd numbers while GPU 2 will render all frames that have even numbers. If you are using more than 2 GPU’s the system will scale up and will assign multiples of numbers to each GPU to render. This is meant to increase the performance by milking each GPU’s individual performance as much as possible, in order to achieve this your driver must be updated (You need this for overclocking as well and GeForce Experience will automatically do this for you)because the principle behind SLI Alternate Frame Rendering requires a well-defined SLI rendering profile. While these are well established for many games already,  you can submit a request for an updated SLI profile to Nvidia via their website here: Nvidia SLI Profile Request forum page.

The main problem with this method is that it can cause things such as micro-stuttering when a card fails to render a frame in time, the delayed “zombie-frame” is made up of two frames that aren’t supposed to be combined together, causing a stuttering action to occur on screen.

How can I use SLI?

Know the compatibility

Not all GPU’s are compatible with SLI. You can’t just slap two GPU’s together and watch it work, you first have to have two Nvidia GPU’s as SLI is only compatible between Nvidia devices.

The other requirements are mostly straightforward, the GPU’s have to be equivalent meaning that a GTX 970 and a GTX 980 won’t work together, unfortunately. The reasoning behind this is that SLI relies on both GPU’s being able to handle the same performance, without it the system will fall apart as the SLI engine is incapable of handling two cards that render at different rates.

The two graphics cards must also have the same memory capacity, meaning that you cannot have a GTX 1080 ti with 4 GB of RAM and another GTX 1080 ti with 8 GB of RAM work in SLI configuration, it goes back to the earlier principle that we’ve talked about in the previous paragraph.

Also, not all Nvidia GPU’s are SLI compatible, if they don’t have an SLI port it doesn’t have SLI compatibility. Also GTX 10 Series is not compatible with higher than 2-card SLI.

Your power supply must also be able to support two or more GPU’s running at the same time and the power connectors, a lot of lower-end power supply’s are unable to do this. You also need a motherboard with SLI support and enough card slots to connect each GPU.

SLI, unfortunately, does not double or triple your RAM supply, due to the fact that the SLI engine is not able to dynamically split the data stored on VRAM between different cards. You will only really get the amount of VRAM that one single graphics card has.

The last requirement for running an SLI system is an SLI bridge, using PCIe slots and the Motherboards default bus to communicate between the two graphics cards is way too slow compared to using an individual SLI bridge that physically connects two different GPU’s together. There are two options for an SLI bridge, either a standard bridge or a high bandwidth bridge. You really only need a standard bridge if your card isn’t very powerful, high bandwidth bridges are meant for more powerful cards to use.

More powerful cards can still use standard bandwidth bridges, but you won’t be maximizing the amount of performance you can be getting from them. Use a high bandwidth bridge for any powerful set of graphics cards to maximize their performance boost.

What are the potential performance benefits from using SLI?

While in theory, the usage of SLI can double or even triple your performance in practice it rarely ever does. Mainly because the technology behind SLI is quite immature and doesn’t have the optimization that is required to take benefit from more than 1 graphics card. Games, for the most part, don’t have graphics engines that are optimized for dual card usage, hence they often don’t know how to take advantage of the extra card that SLI gives you. Nvidia and the developer of a game need to cooperate to produce an optimized SLI profile for a video game, this takes time and effort that many games simply don’t have the budget or time to justify, many of these games are targeted towards audiences that rarely ever use SLI.

Unless you are playing an extremely mainstream game, your performance boost may be limited (in some cases the performance actually decreases while using SLI, rarely but it does happen on occasion). You will need to keep this in mind while attempting to use SLI, as many games simply don’t support it and you should be aware of these limitations. You can look up good benchmarks for before and after comparisons with and without SLI.

I encourage people who have the money and are enthusiastic enough about high-resolution gaming to look into SLI to boost performance, but for many lower-end gamers, a budget graphics card will likely do fine. You should keep these factors in mind when considering the upgrade to SLI or just buying a higher end graphics card. (You might be even able to nab a high end used graphics card)

Conclusion

SLI is a great technology that can drastically increase your native performance in video games across the board, by using more than one graphics card to render a frame you can cut down on lag, performance issues and increase image quality while preserving framerates. SLI is quite expensive, and for many, it makes more sense just to upgrade to a newer graphics card (especially if your graphics card is just too old to perform well).

Enthusiast gamers who are looking for the highest framerates possible at the highest resolutions possible should look into SLI or AMD CrossFire as an option to increase performance if they are already at the highest tier of GPU’s already. SLI and AMD CrossFire at 4k resolution can be a blessing for your framerates and overall performance.

As we always have in other articles, please make sure to leave a comment or leave any suggestions. If we have any errors in the writing, please email us or contact us through the comments because we really hate to have any sort of errors in our writing!

For more info on graphics and gaming, check out our guide on history of overclocking CPU’s or on how to check and change your refresh rate

Leave a Reply

Your email address will not be published. Required fields are marked *