Nvidia brings back SLI

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Nvidia today announced its new SLI Multi-GPU technology, which greatly increases graphics performance by providing a way for users to link together multiple GeForce 6-series video cards in a single PC system. Nvidia claims its SLI implementation increases performance by up to 87 percent, according to a composite score composed of benchmark results from 3DMark03 Game Tests 2, 3, and 4 and the unreleased Epic Games Unreal Engine 3.

Many may remember 3dfx's original SLI technology, scan line interleave, which connected two Voodoo2 video cards together and divided the graphics workload by having each card draw every other line on the screen. Nvidia did obtain the original SLI technology and brand assets when it acquired 3dfx's intellectual property several years ago, but Nvidia representatives assured us that the SLI Multi-GPU technique is completely new technology--Nvidia is only reviving the SLI name for marketing purposes.

Nvidia's SLI stands for "scalable link interface" and offers major improvements over 3dfx's dual-card implementation. Nvidia's technology links together two GeForce 6800 Ultra or GT cards via a high-speed digital interface. The actual GeForce GPU silicon contains dedicated logic to enable inter-GPU communications, and enthusiast-level PCI Express GeForce 6800 cards will have specialized connectors for SLI implementation.

Instead of having cards divide the graphics workload by drawing every other line, Nvidia uses a "symmetric multi-rendering with dynamic load balancing" technique where a special algorithm divides each frame into two halves based on workload complexity, and one card renders the top portion while the other renders the bottom. While the original 3dfx Voodoo2 SLI setup used analog pass-through cables, the Nvidia multi-GPU system has one card reassemble complete frames in digital form before sending the information to the monitor.

High-end system manufacturer Alienware previously announced a similar multicard "video array" technology that uses an additional add-in card, what the company calls a "video merger hub," to reassemble frames from both cards. Alienware's system will be compatible with both ATI- and Nvidia-based PCI Express video cards since the video array system does not rely on the specialized SLI connectors present only on the Nvidia cards.

Nvidia's SLI technology will only work with matching PCI Express GeForce 6800 GT and Ultra cards of the same vendor type and card version. Users, of course, will also need a motherboard with two PCI Express x16 graphics connectors.

GeForce 6-series SLI systems won't be available for sale until this fall, but Nvidia will have demo systems on display at several major events this summer, including the Electronic Sports World Cup, Fragapalooza, the CPL Extreme Championships, Siggraph, and QuakeCon.