
Scalable Link Interface

Scalable Link Interface (SLI) is a brand name for a multi-GPU technology developed by Nvidia for linking two or more video cards together to produce a single output. SLI is a parallel processing algorithm for computer graphics, meant to increase the available processing power.
The initialism SLI was first used by 3dfx for Scan-Line Interleave, which was introduced to the consumer market in 1998 and used in the Voodoo2 line of video cards. After buying out 3dfx, Nvidia acquired the technology but did not use it. Nvidia later reintroduced the SLI name in 2004 and intended for it to be used in modern computer systems based on the PCI Express (PCIe) bus; however, the technology behind the name SLI has changed dramatically.
Implementation.
SLI allows two, three, or four graphics processing units (GPUs) to share the workload when rendering real-time 3D computer graphics. Ideally, identical GPUs are installed on the motherboard that contains enough PCI Express slots, set up in a master–slave configuration. All graphics cards are given an equal workload to render, but the final output of each card is sent to the master card via a connector called the SLI Bridge. For example, in a two graphics card setup, the master works on the top half of the scene, the slave the bottom half. Once the slave is done, it sends its render to the master to combine into one image before sending it to the monitor.
The SLI bridge is used to reduce bandwidth constraints and send data between both graphics cards directly. It is possible to run SLI without using the bridge connector on a pair of "low-end to mid-range" graphics cards (e.g. 7100GS or 6600GT) with Nvidia's Forceware drivers 80.XX or later. Since these graphics cards do not use as much bandwidth, data can be relayed through just the chipsets on the motherboard. However, if there are two "high-end" graphics cards installed and the SLI bridge is omitted, the performance will suffer severely, as the chipset does not have enough bandwidth.
Configurations currently include:
Nvidia has created a set of custom video game profiles in cooperation with video game publishers that will automatically enable SLI in the mode that gives the largest performance boost.
Nvidia has 3 types of SLI bridges:
The Standard Bridge is traditionally included with motherboards that support SLI and is recommended for monitors up to 1920×1080 and 2560×1440@60 Hz. The LED Bridge is sold by Nvidia, EVGA, and others and is recommended for monitors up to 2560×1440@120 Hz+ and 4K. The LED Bridges can only function at the increased Pixel Clock if the GPU supports that clock. The High-Bandwidth Bridge is only sold by Nvidia and is recommended for monitors up to 5K and Surround.
The following table provides an overview on the maximum theoretical bandwidth for data transfers depending on bridge type specifications as found on the open market:
SLI Modes.
Split Frame Rendering (SFR).
This analyzes the rendered image in order to split the workload equally between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry. For example, in a scene where the top half of the frame is mostly empty sky, the dividing line will lower, balancing geometry workload between the two GPUs.
Alternate Frame Rendering (AFR).
Each GPU renders entire frames in sequence. For example, in a Two-Way setup, one GPU renders the odd frames, the other the even frames, one after the other. Finished outputs are sent to the master for display. Ideally, this would result in the rendering time being cut by the number of GPUs available. In their advertising, Nvidia claims up to 1.9x the performance of one card with the Two-Way setup. While AFR may produce higher overall framerates than SFR, it also exhibits the temporal artifact known as Micro stuttering, which may affect frame rate perception. It is noteworthy that while the frequency at which frames arrive may be doubled, the time to produce the frame is not reduced - which means that AFR is not a viable method of reducing input lag.
SLI Antialiasing.
This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern (for example, slightly up and to the right), and the second GPU uses a pattern offset by an equal amount in the opposite direction (down and to the left). Compositing both the results gives higher image quality than is normally possible. This mode is not intended for higher frame rates, and can actually lower performance, but is instead intended for games which are not GPU-bound, offering a clearer image in place of better performance. When enabled, SLI Antialiasing offers advanced antialiasing options: SLI 8X, SLI 16X, and SLI 32x (for Quad SLI systems only).
Hybrid SLI.
Hybrid SLI is the generic name for two technologies, GeForce Boost and HybridPower.
GeForce Boost allowed the rendering power of an IGP and a discrete GPU to be combined in order to increase performance.
HybridPower, on the other hand, is another mode that is not for performance enhancement. The setup consists of an IGP as well as a GPU on MXM module. The IGP would assist the GPU to boost performance when the laptop is plugged to a power socket while the MXM module would be shut down when the laptop was unplugged from power socket to lower overall graphics power consumption. Hybrid SLI is also available on desktop Motherboards and PCs with PCI-E discrete video cards. NVIDIA claims that twice the performance can be achieved with a Hybrid SLI capable IGP motherboard and a GeForce 8400 GS video card.
HybridPower was later renamed as Nvidia Optimus.
SLI HB.
In May 2016 Nvidia announced that the GeForce 10 series would feature a new SLI HB (High Bandwidth) bridge; this bridge uses 2 SLI fingers on the PCB of each card and essentially doubles the available bandwidth between them. Currently, only GeForce 10 series cards support SLI HB and only 2-way SLI is supported over this bridge for single-GPU cards. SLI HB interface runs at 650 MHz, while legacy SLI interface runs at slower 400 MHz.
Electrically there is little difference between the regular SLI bridge and the SLI HB bridge. It is similar to two regular bridges combined in one PCB. The signal quality of the bridge improved, however, as the SLI HB bridge has an adjusted trace-length to make sure all traces on the bridge have exactly the same length.
A PC gaming magazine has done research with x-rays for comparing SLI bridges with their SLI HB successors and found adjusted signal lengths by planned Meandering of certain wires so that clock rates can go up from 400 MHz to 650 MHz and thus the data rates along with that. With the increased bus width a noticeable bandwidth increase should be expected. Tests with a GTX 1080 GPU-board model showed that the improvements in gaming performance is quite marginal. Other than that it was discovered or determined, that LED illuminated bridges (often back-side-illuminating some logo) will mainly result in a noticeable increased market price at a comparable base functionality.