Scalable Link Interface

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Scalable Link Interface (SLI) is a brand name for a multi-GPU solution developed by Nvidia for linking two or more video cards together to produce a single output. SLI is an application of parallel processing for computer graphics, meant to increase the processing power available for graphics.

The name SLI was first used by 3dfx under the full name 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 intends 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.

Contents 1 Implementation 2 Other implementations 2.1 Two GPUs on one PCI-E slot 2.2 Quad SLI 2.3 3-Way SLI 2.4 Physics calculation 2.5 Hybrid SLI 3 Caveats 4 See also 5 References 6 External links

[edit] Implementation

The basic idea of SLI is to allow two or more graphics processing units (GPUs) to share the work load when rendering a 3D scene. Ideally, two identical graphics cards are installed in a motherboard that contains two PCI-Express x16 slots, set up in a master-slave configuration. Both cards are given the same part of the 3D scene to render, but effectively half of the work load is sent to the slave card through a connector called the SLI Bridge. As an example, the master card works on the top half of the scene while the slave card works on the bottom half. When the slave card is done, it sends its output to the master card, which combines the two images to form one and then outputs the final render to the monitor.

In its early implementations, motherboards capable of SLI required a special card which came with the motherboard. This card would fit into a socket usually located between both of the PCI-Express x16 slots. Depending on which way the card was inserted, the motherboard would either channel all 16 lanes into the primary PCI-Express x16 slot, or split lanes equally to both PCI-Express x16 slots. This was necessary as no motherboard at that time had enough PCI-Express lanes for both to have 16 lanes each. Thanks to the advancement in available PCI-Express lanes, most modern SLI-capable motherboards allow each video card to use all 16 lanes in both PCI-Express x16 slots.

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.

SLI offers two rendering and one anti-aliasing method for splitting the work between the video cards:

• Split Frame Rendering (SFR), the first rendering method. 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), the second rendering method. Here, 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.
• 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 (8800-series only). A Quad SLI system is capable of up to SLI 64X antialiasing.

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. It is also possible to create custom game profiles or modify pre-defined profiles using their Coolbits software.

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

[edit] Other implementations

[edit] Two GPUs on one PCI-E slot

In February 2005, Gigabyte Technology released the GV-3D1^[1], a single video card that uses Nvidia's SLI technology to run two 6600-series GPUs. Due to technical issues with compatibility, at release the card was supported by only one of Gigabyte's own motherboards, with which it was bundled. Later came the GV-3D1-68GT, functionally similar and possessing similarly-limited motherboard compatibility, but with 6800 GPUs in place of the GV-3D1's 6600 units.

Around March 2006, ASUS released the N7800GT Dual. Similar to Gigabyte's design, but mounting two 7800GT GPUs on one video card. Again, this faced several issues, such as high price (it retailed for around US$800, while two separate 7800GTs were cheaper at the time), limited release, and limited compatibility. It would only be supported on the nForce4 chipset and only a few motherboards could actually utilize it. It was also one of the first video cards with the option to use an external power supply if needed.^[2]

In January 2006, Nvidia released the 7900 GX2, their own attempt at a dual-GPU card. Effectively, this product is a pair of slightly lower clocked 7900GTX cards "bridged" together into one discrete unit, with separate frame buffers for both GPUs (512MB of GDDR3 each). The GeForce 7900 GX2 is only available to OEM companies for inclusion in quad-GPU systems, and it cannot be bought in the consumer market. The Dell XPS, announced at the 2006 Consumer Electronics Show, used two 7900 GX2's to build a quad-GPU system. Later, Alienware acquired the technology in March.

The official implementations of dual-GPU graphics cards work in the same fashion. Two GPUs are placed on two separate printed circuit boards (PCBs), with their own power circuitry and memory. Both boards have slim coolers, cooling the GPU and memory. The 'primary' GPU can be considered to be the one on the rear board, or 'top' board (being on top when in a standard ATX system). The primary board has a physical PCIe x16 connector, and the other has a round gap in it to provide cooling for the primary HSF. Both boards are connected to each other by two physical links; one for 16 PCI-Express lanes, and one for the 400 MHz SLI bridge. An onboard PCI-Express bridge chip, with 48 lanes in total, acts as the MCP does in SLI motherboards, connecting to both GPUs and the physical PCI-Express slot, removing the need for the motherboard to support SLI.

A newer version, the GeForce 7950 GX2, addresses many issues in the 7900 GX2, and is available to consumers for separate purchase. More recently, in August 2006, Nvidia has released the drivers to allow end-users to build their own quad-GPU systems using the 7950 GX2.^[3] The GeForce 9800 GX2 appears to be the successor in this setup, combining two G92 GPUs.

[edit] Quad SLI

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In October 2005, Gigabyte Technology released the GA-8N SLI Quad Royal. Essentially it was a motherboard with four PCI-Express x16 slots. At the time of release however, Nvidia stated that this would not be the direction it would take SLI.^[4]

In early 2006, Nvidia revealed its plans for Quad SLI. When the 7900GX2 was originally demonstrated, it was with two such cards in an SLI configuration. This is possible because each GX2 has two extra SLI connectors, separate from the bridges used to link the two GPUs in one unit – one on each PCB, one per GPU, for a total of two links per GPU. When two GX2 graphics cards are installed in an SLI motherboard, these SLI connectors are bridged using two separate SLI bridges. (In such a configuration, if the four PCBs were labeled A, B, C, D from top to bottom, A and C would be linked by an SLI bridge, as would B and D.) This way, four GPUs can contribute to performance. The 7950GX2, sold as an enthusiast-friendly card, omits the external SLI connector on one of its PCBs, meaning that only one SLI bridge is required to run two 7950GX2s in SLI.

Quad SLI did not show any massive improvements in gaming using the common resolutions of 1280x1024 and 1600x1200, but has shown improvements by enabling 32x anti-aliasing in SLI-AA mode, and support for 2560x1600 resolutions at much higher framerates than is possible with single or dual GPU systems with maximum settings in modern games. It was believed that high latencies severely marginalized the benefits of four GPUs, however much of the blame for poor performance scaling is due to Windows XP's API which only allows for a maximum storage of 3 extra frames. Windows Vista is not limited in this fashion and shows promise for future multi-GPU configurations.

In March 2008, Nvidia released the GeForce 9800GX2 GPU. Targeted at the high-end gaming populace with a suggested retail price of $599.99 US, the 9800GX2 is essentially two updated G92 8800GTS cores on a dual-PCB graphics card to compete with ATI's HD3870 X2. The 9800GX2 features a total of 256 stream processors, 1GB video memory buffer and clock speeds only slightly slower than the cheaper, single-core but otherwise comparable G92 8800GTS. Though Nvidia did not release Quad SLI drivers for the 9800GX2 at time of release, the telltale SLI connector on the top of the card leaves little doubt that users in the future will be able to equip themselves with 2 9800GX2s, thus allowing for a total of 4 GPUs in one system via only 2 PCI Express x16 graphics slots, a feat impossible since the 7950GX2. Note that Nvidia no longer supports Quad SLI on Windows XP (Nvidia will automatically prevent you from using two 9800GX2s without Windows Vista.)

[edit] 3-Way SLI

Nvidia has also revealed a triple SLI setup for the nForce 700 series and the nForce 680i series motherboards, which in the Windows series only works on Vista. The setup can be achieved using three high-end video cards with two MIO ports (currently only for the 8800 GTX and 8800 Ultra and 9800 GTX) and a specially wired connector (or three flexible connectors used in a specific arrangement).^[5][6] The technology was officially announced in December 2007, shortly after the revised G92-based 8800GTS made its way out of the factory. In practical terms, it delivers up to a 2.8x performance increase over a single GPU system, giving high-end gamers approximately 60 frames per second at resolutions as high as 2560x1600 and with 8x antialiasing.^[7]

Unlike traditional SLI, or CrossFire X, 3-way SLI is limited to the GeForce 8800 GTX, 8800 Ultra and 9800 GTX graphics cards on the 680i, 780i and 790i chipsets, whereas CrossFire X can be theoretically used on multiple Radeon HD 2400 cards.^[8]

[edit] Physics calculation

In response to ATI offering a discrete physics calculation solution in a tri-GPU system, Nvidia announced a partnership with physics middleware company Havok to incorporate a similar system using a similar approach. Although this would eventually become the Quantum Effects technology, many motherboard companies began producing boards with three PCI-Express x16 slots in anticipation of this implementation being used.

Nvidia have now, in February 2008, acquired physics hardware and software firm Ageia and plans are already under way to increase the market penetration for PhysX beyond its fairly limited use in games currently, notably the Unreal Engine 3.

[edit] Hybrid SLI

Also in response to the PowerXpress technology from AMD, a configuration of similar concept named "Hybrid SLI" was announced on January 7, 2008. 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.^[9][10]

In the mean time, Hybrid SLI will also be available on desktops,^[11] with PCI-E discrete video cards, reports have that twice of the performance can be achieved with a Hybrid SLI capable IGP motherboard and a GeForce 8400 GS video card.^[12][13]

Currently, the hybrid SLI for desktop platform only supports GeForce 8500 GT to be paired with nForce 700 series chipsets and GeForce 8400 GS graphics cards with GeForce Boost feature only. Also, another feature, the HybridPower, is not supported for the nForce 730a.^[14]

[edit] Caveats

• Currently, all SLI configurations work only with a single digital video output. When switching to multi-GPU SLI mode, Dualview gets disabled under Windows,^[15] respectively TwinView and multiple screens get disabled under Linux.^[16] It is possible, however, to use a wide resolution and split it into multiple monitors using an external device such as the Matrox TripleHead2Go.
• To bypass the problem of having SLI and dual monitors functioning at the same time a new method has been devised by the members of slizone forums. By purchasing a low end video card (7100gs,8400gs,8500gt and so on) it's possible to put it inside an empty pci-e or pci slot and use it to power the 2nd monitor. By using latest drivers that have a modified .inf file, it is possible to boot to safe mode and install the same drivers onto main SLI cards and the low end card. Following the install and a boot to windows, you can disable the low end video card, go into Nvidia control panel, enable SLI which at that point will ask you to reboot. Following the reboot you can plug in your 2nd monitor into the low end card which will now power it and allow to use full SLI on your primary monitor. This has been confirmed working on various SLI systems, and even the new 9800gx2 allows this to work. ^[17]
• Cards from two separate retail companies will work together in SLI mode, but they must be the same GPU model (e.g. G70, G73, G80, etc). The cards may have different BIOS revisions, different default clock speeds, or even different memory sizes. However, the fastest card – or the card with more memory - will run at the speed of the slower card or disable its additional memory.^[18]
• SLI doesn't always give a performance benefit – in some extreme cases, it can lower the framerate due to the particulars of an application's coding.^[19] 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 and older drivers, 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.
• Initial versions of drivers for just-released video cards may not show immediate performance increase over a single GPU setup or a dramatic increase in performance.^{[citation needed]}
• Vsync + Triple buffering is not supported in some cases in SLI AFR mode.
• A single high-end GPU is generally preferable to using SLI with low to medium-end GPUs when considering the above caveats and the price-performance ratio of the setup.
• Users having a Hybrid SLI setup must manually change modes between HybridPower and GeForce Boost, while automatically changing mode will not be available until the future updates. The Hybrid SLI currently support only single link DVI at 1920x1200 screen resolution.^[20]

[edit] See also

• Scan-Line Interleave by 3Dfx
• ATI CrossFire – ATI's competing solution

[edit] References

[edit] External links

• Nvidia's Official SLI Technology website
• Official SLI Forums
• Nvidia's Windows Vista Capable GPUs
• Article "Multiple Graphics Card Technology" by Tim Smalley

• Article "Nvidia's SLI: An Introduction" by Ryszard Sommefeldt
• Article "Dell's Quad SLI: A Story in Pictures" by Charlie Demerjian
• Article "Nvidia SLI Support - Getting Better" by Brent Justice
• Nvidia's Quad SLI: Demystifying the rumors by Brandon Bell
• Article "Quad SLI: GeForce 7900 GX2" by Tim Smalley
• Article "Quad SLI part deux: Build It Yourself" by Tim Smalley
• Article "Nvidia and Havok bring SLI physics to life."
• Article "SLI vs. CrossFire" by Gabriel Torres

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