What is SLI?

Scalable Link Interface or SLI, is the name of NVIDIA’s multi-GPU solution devised for NVIDIA video cards. It allows for the parallel processing to use up to four video cards for a computer to process its graphics. By sharing the workload between multiple video cards instead of a single one, the video processing power of a computer can be increased and multiplied for a lower cost than buying a single premium video card.

How SLI Works


SLISLI increases the performance and video processing power of a computer through parallel processing to share the workload of a computer. This is done by effectively splitting the work between multiple GPU, each of which share the workload. However, this workload is shared in a variety of different ways. There are two primary rendering options: Split Frame Rendering and Alternate Frame Rendering. To put it simply, Split Frame Rendering works by splitting each frame into two parts horizontally, and allowing each GPU to work half of the screen (in a two-GPU SLI setup). This allows the GPU in SLI to only carry the workload of half the screen for each individual frame. On the other hand, Alternate Frame Rendering works by having each GPU in SLI render the entire frame, which, ideally, would mean double the performance. However, there is the risk of input latency. This mean that while Split Frame Rendering doesn’t scale as well with the architecture and geometry compared to Alternative Frame Rendering, it is better for action games where it is hard to predict and produce the next frame. This customization can be adjusted to suit the needs of the user when the graphics cards are in SLI setup.

Configuration


For cards to work in an SLI configuration, they must be of the same GPU series and model name. This is because in parallel processing, the workload of the computer is divided evenly between the cards, meaning that the graphics power of the machine can only be double that of the weaker card. However, there are no limits on card manufacturer, models, BIOS revisions or clock speeds. In addition, for users with multiple screen setups, the SLI supports up the three screens of the same resolution.

Performance


NVIDIA advertises SLI configurations operating in Alternative Frame Rendering to have the processing power of 1.9 times the single GPU. While the figure may be closer to 80% in operation due to driver inefficiencies and optimization problems when running two or more cards, that is still a noticeable performance boost over that of a single card. And furthermore, the price of two video cards of the last generation can often be significantly less than the price of a single current-generation video card. This is particularly true for NVIDIA, which has had supply issues with its latest line of video cards.
In addition, NVIDIA has arguably the best driver and software support in the industry which can help with game optimization. The list of current generation SLI optimized games include titles such as Metro: Last Light, Assassin’s Creed III, Call of Duty: Black Ops II, Borderlands 2, and Dishonored, among hundreds of other titles. For example, users running dual GTX 670 cards in SLI setup can expect a score of 5497 in 3DMark, compared to 2851. That is enough to run current-generation such as Battlefield 3 and Skyrim at Ultra with average FPS of over 100.

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