SLI

Today I want to talk about SLI. This technology brings a war among Graphics giant. You can get thousands of articles in net. But, here I tried to give a snapshot of SLI. 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.

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.

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.

Nvidia has also revealed a triple SLI setup for the nForce 700 series and the nForce 680i series motherboards, which currently only works on Windows 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). 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 2560×1600 and with 8x antialiasing. Can you imagine how fast and crystal clear will be your monitor (supported one)?

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 and 780i chipsets, whereas CrossFire X can be theoretically used on multiple Radeon HD 2400 cards.

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.

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.

In the mean time, Hybrid SLI will also be available on desktops, 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.

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.