Scalable Link Interface

For other uses, see SLI (disambiguation).
Scalable Link Interface
Manufacturer NVIDIA
Type Multi-GPU
Release date 2004
Predecessor Scan-Line Interleave

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 an algorithm of parallel processing for computer graphics, meant to increase the processing power available for graphics.[1]

The acronym 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.

Example of 3-way SLI using a rigid bridging connector
A dual-GPU graphics card for a high-performance gaming laptop, utilizing on-board SLI.
Simple 2-way SLI Bridge
Simple 2-way SLI bridge.
Computer with 2-way SLI Graphics Cards Installed
Computer with 2-way SLI graphics cards installed.

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. An 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.[1]

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.

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. This method does not scale geometry or work as well as AFR, however.

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. A Quad SLI system is capable of up to SLI 32x antialiasing.[2]

Hybrid SLI

Hybrid SLI is the generic name for two technologies, GeForce Boost and HybridPower.[3]

GeForce Boost allowed the rendering power of an IGP and a discrete GPU to be combined in order to increase performance.[3]

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.[4][5] 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.[6][7]

HybridPower was later renamed as Nvidia Optimus.

Caveats

See also

References

  1. 1 2 3 "What is SLI?". www.geforce.com. Retrieved 24 November 2014.
  2. "Introducing SLI Antialiasing: The Ultimate in Visual Quality". www.nvidia.com. Retrieved 24 November 2014.
  3. 1 2 "Hybrid SLI Technology". www.nvidia.com. Retrieved 23 March 2015.
  4. Valich, Theo (2007-06-26). "NVIDIA's Hybrid SLI attacks AMD's PowerXPress". The Inquirer. Retrieved 2007-09-26.
  5. Shilov, Anton (2007-06-25). "NVIDIA Readies Hybrid SLI Technology". X-bit labs. Retrieved 2007-10-17.
  6. Abazovic, Faud (2007-08-08). "Hybrid SLI first for AMD". Archived from the original on 2007-11-14. Retrieved 2007-10-17.
  7. "Growth Opportunities." (PDF). NVIDIA. 2007-06-20. p. 9. Retrieved 2007-10-17.
  8. "Motherboards". NVIDIA. Retrieved 2014-08-20.
  9. 1 2 "SLI FAQs". NVIDIA. Retrieved 2008-12-04.
  10. Kreiss, Tino; Töpelt, Bert; Schuhmann, Daniel (2005-12-02). "Performance Comparison Between Single Configurations And SLI Setups". Tom's Hardware. Retrieved 2007-06-01.
  11. Bit-Tech interview (page 2), retrieved January 23, 2008
  12. Raffael Vötter (2008-02-08). "Video proof: Micro stuttering may destroy the performance gains from current multi GPU technologies". PC Games Hardware Online. Retrieved 2009-11-03.

External links

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