Scalable Link Interface

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

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.

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.

Implementation[edit]

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.

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 no SLI bridge is used on two high-end graphics cards, the performance suffers severely as the chipset does not have enough bandwidth.

Configurations currently include:

  • Two-Way, Three-Way, and Four-Way SLI. Uses two, three, or four individual graphics cards respectively.
  • Two GPUs on one graphics card. Examples include the GeForce GTX 590, GeForce GTX 690 and the GeForce GTX Titan Z. This configuration has the advantage of implementing Two-Way SLI, while only occupying one PCI-Express slot and (usually) two expansion I/O slots. This also allows for Four-Way SLI using only two cards.

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. For more information on SLI-optimized games, visit NVIDIA's SLI Zone.

SLI Modes[edit]

SLI allows for the following modes.

Split Frame Rendering (SFR)[edit]

This analyzes the rendered image in order to split the workload 50/50 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)[edit]

Each GPU renders entire frames in sequence – one GPU processes even frames, and the second processes odd frames, one after the other. When the slave card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master card, which then outputs the completed frames. Ideally, this would result in the rendering time being cut in half, and thus performance from the video cards would double. In their advertising, NVIDIA claims up to 1.9x the performance of one card with the dual-card 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 and input response.

SLI Antialiasing[edit]

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 (only available on newer, higher-end models starting with the 8800 series). A Quad SLI system is capable of up to SLI 64X antialiasing.

Hybrid SLI[edit]

This is another mode that is not for performance enhancing. 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.[1][2] 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.[3][4]

NVIDIA has a similar technology called Nvidia Optimus.

Caveats[edit]

  • In an SLI configuration, cards can be of mixed manufacturers, card model names, BIOS revisions or clock speeds. However, they must be of the same GPU series (e.g. 8600, 8800) and GPU model name (e.g. GT, GTS, GTX).[5] There are rare exceptions for "mixed SLI" configurations on some cards that only have a matching core codename (e.g. G70, G73, G80, etc.), but this is otherwise not possible, and only happens when two matched cards differ only very slightly, an example being a differing amount of video memory, stream processors, or clockspeed. In this case, the slower/lesser card becomes dominant, and the other card matches. Another exception is the GTS 250, which can SLI with the 9800 GTX+, as the GTS 250 GPU is a rebadged 9800 GTX+ GPU.
  • In cases where two cards are not identical, the fastest card – or the card with more memory - will run at the speed of the slower card or disable its additional memory. (Note that while the FAQ still claims different memory size support, the support has been removed since revision 100.xx of NVIDIA's Forceware driver suite.[5])
  • SLI doesn't always give a performance benefit – in some extreme cases, it can lower the frame rate due to the particulars of an application's coding.[6] This is also true for ATI's CrossFire, as the problem is inherent in multi-GPU systems. This is often witnessed when running an application at low resolutions.
  • In order to use SLI, a motherboard with an nForce4, nForce 500, nForce 600 or nForce 700 SLI chipset must be used, although with the use of hacks[7] one can make SLI work on motherboards with Intel, ATI and ULi chipsets. NVIDIA has stated that only their own chipsets can allow SLI to function optimally, and that they will not allow SLI to work on any other vendor's chipsets. Some early SLI systems used Intel's E7525 Xeon chipset, which caused problems when NVIDIA started locking out other vendor's chipsets as it limited them to an outdated driver set. In 2007, Intel has licensed NVIDIA's SLI technology for its SkullTrail platform, and select motherboards supporting the Intel X58 (Tylersburg) chipset have unlocked SLI capabilities. Not all X58 motherboards support this technology, as NVIDIA offered it to motherboard manufacturers at the cost of $5 per motherboard sold.[8] NOTE: As of the release of the AMD 900 chipset series, SLI can be run on an AMD mainboard provided that it has the 970, 990X, or 990FX chipset on the board.[9]
  • Vsync + Triple buffering is not supported in some cases in SLI AFR mode.
  • Users having a Hybrid SLI setup must manually change modes between HybridPower and GeForce Boost, while automatically changing mode will not be available until future updates become available. Hybrid SLI currently supports only single link DVI at 1920×1200 screen resolution.[10]
  • When using SLI with AFR, the subjective framerate can often be lower than the framerate reported by benchmarking applications, and may even be poorer than the framerate of its single-GPU equivalent. This phenomenon is known as micro stuttering and also applies to CrossFire since it's inherent to multi-GPU configurations.[11]

See also[edit]

References[edit]

  1. ^ Valich, Theo (2007-06-26). "NVIDIA's Hybrid SLI attacks AMD's PowerXPress". The Inquirer. Retrieved 2007-09-26. 
  2. ^ Shilov, Anton (2007-06-25). "NVIDIA Readies Hybrid SLI Technology". X-bit labs. Retrieved 2007-10-17. 
  3. ^ Abazovic, Faud (2007-08-08). "Hybrid SLI first for AMD". Archived from the original on 2007-11-14. Retrieved 2007-10-17. 
  4. ^ "Growth Opportunities." (PDF). NVIDIA. 2007-06-20. p. 9. Retrieved 2007-10-17. 
  5. ^ a b "SLI FAQs". NVIDIA. Retrieved 2008-12-04. 
  6. ^ Kreiss, Tino; Töpelt, Bert, Schuhmann, Daniel (2005-12-02). "Performance Comparison Between Single Configurations And SLI Setups". Tom's Hardware. Retrieved 2007-06-01. 
  7. ^ "SLI Patch - Multi-GPU on ANY Motherboard". Retrieved 2011-03-01. 
  8. ^ "Mainboard Makers Set to Pay NVIDIA $5 per Mainboard for SLI License". Retrieved 2009-01-01. 
  9. ^ "SLI on AMD". 2011-04-28. Retrieved 2011-06-26. 
  10. ^ Bit-Tech interview (page 2), retrieved January 23, 2008
  11. ^ 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[edit]