Scalable Link Interface

 

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. With SLI, it is possible to theoretically double the power of your graphics solution just by adding a second video card with an identical GPU.

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 graphics accelerators. 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.

Implementation

The basic idea of SLI is to allow two (or more) graphics processing units (or 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 game (scene) to render, but effectively half of the work load is sent to the slave card through a connector dubbed the SLI Bridge. For example, in some cases the slave card will work on the bottom half of the screen. The slave then sends its rendered output to the master card, where it is incorporated into the master card's own image (in the frame buffer) and sent to the screen.

In its early implementations, motherboards capable of SLI required a special card (the size of a credit card) which came with the motherboard, which 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 channel the full x16 lanes into the top slot, for a single GPU configuration. If in SLI mode, the motherboard would split the 16 PCI-Express lanes in both slots to only allow a maximum of 8 lanes per card. This was necessary as no motherboard at that time had enough PCI-E lanes for two full x16 slots. Motherboards today do not require this card due to advances in their chipsets and drivers. Also due to the advancement in available PCI-E lanes, most modern SLI-capable motherboards allow each video card to use all 16 lanes in the PCI-Express x16 slot, rendering any method of sharing the lanes obsolete.

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 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, via the PCI-E connectors. This is also possible on ATI CrossFire. Both CrossFire and SLI suffer severe performance penalties when this method is used on two high-end cards as the bus will be loaded down with data both from the CPU as well as the master sending data to the slave and the slave sending data to the master.

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 (so that both cards have an equal share), 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 secondary card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master GPU, 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 1.9 x the performance of one card with the dual-card setup. (There is another mode, AFR2, which can be manually chosen, but NVIDIA has not documented the difference between it and normal AFR.)
• 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..

Other Implementations

Two GPUs on one PCI-E slot

In February 2005, Gigabyte Technology released the GV-3D1-68GT, 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 only supported by one of Gigabyte's own motherboards, with which it was bundled.

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 $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.^[1]

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 400MHz 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.^[2]

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 it would not be the direction it would take SLI.^[3]

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 newer 7950GX2 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 has yet to 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.

NVIDIA has recently released official Quad SLI drivers, marking the first time one can use Quad SLI with official support.^[4] For more information, visit NVIDIA's Quad SLI website.

Triple SLI

NVIDIA has also revealed a triple SLI setup for upcoming nForce 700 series motherboards (currently limited to high-end motherboards), the setup can be achieved using three high-end video cards with two MIO ports (currently the 8800 GTX and 8800 Ultra) with a special connector ^[5] using special wiring techniques ^[6], and a high-end motherboard (nForce 680i, and upcoming 780i/780a) with a specialized PCI-E bridge onboard (see coverage on nForce 700 article). The technology is expected to be announced to accompany with the launch of the nForce 700 chip at later date, shortly after the launch of the GeForce 8800 GT video card.

It is however, compared with the third iteration of ATI CrossFire implementation, all video cards from high-end to lowest-end (the Radeon HD 2400 series, if enabled on PCB level ^[7][8]) video cards support dual CrossFire connectors, it is seemed that the cost for triple SLI (high-end video cards and motherboard) will be higher compared to an unannounced triple-card CrossFire setup.

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.

Hybrid SLI

Also in response to the PowerXpress technology from AMD, a configuration of similar concept named "Hybrid SLI" was announced on June 25, 2007. The setup consists of an IGP as well as an 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].

Caveat

• Currently, SLI works only with a single monitor. When switching to multi-GPU SLI mode, Dualview gets disabled under Windows^[14], respectively TwinView and multiple screens get disabled under Linux^[15].
• 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^[16].
• 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.^[17] This is also true for ATI's CrossFire, as the problem is inherent in multi-GPU systems.
• In order to use SLI, a motherboard with an nForce4, nForce 500, or nForce 600 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 have 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]}

References

See also

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

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."

NVIDIA Gaming Graphics Processors Early Chips: NV1 • NV2 Direct3D 5/6: RIVA 128 • RIVA TNT • RIVA TNT2 Direct3D 7.0: GeForce 256 • GeForce 2 Direct3D 8.0: GeForce 3 • GeForce 4 Direct3D 9.0: GeForce FX • GeForce 6 • GeForce 7 Direct3D 10: GeForce 8 • GeForce 9

Other NVIDIA Technologies nForce: 220/415/420 • 2 • SoundStorm • 3 • 4 • 500 • 600 • 700 Workstations and HPC: Quadro • Quadro Plex • Tesla Graphics Card Related: TurboCache • SLI Software: Gelato • Cg • PureVideo Consumer Electronics: GoForce Game Consoles: Xbox (NV2A) • PlayStation 3 (RSX)

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