I'm sure many of you can remember the days of 3dfx, the first Voodoo Graphics back in 1996 and about a year later the introduction of the Voodoo2. Voodoo2 actually made sure that 3dfx reigned supreme for quite some time as two cards could be combined in something called an SLI, Scan Line Interleave, configuration. Each card rendered half of the image scan lines which resulted in double the performance of a single board and the ability to play OpenGL games such as Quake 2 in a 1024x768 resolution. To date no manufacturer has come up with a similar concept simply because modern graphics accelerators are all AGP based, there's no dual AGP motherboards and PCI simply doesn't have the bandwidth to handle modern graphics accelerators. With the arrival of PCI-E things have changed though, a number of workstations motherboards featuring the Tumwater chipset will have dual PCI-E-x16 slots making dual graphics accelerators a possibility again. Nvidia steps up to the plate today with the re-introduction of the SLI concept on the GeForce 6800 series, again using the SLI moniker but now with a different approach to the same principles that made Voodoo2 SLI a huge success.
Two PCI-E GeForce 6800 Ultra graphics cards running in a SLI configuration.
Whereas Voodoo2 SLI used a ribbon cable to be connected between two Voodoo2 cards internally and a pass through VGA cable externally to distribute the analog signal Nvidia's implementation is all done in the digital domain. Both 6800 series PCI-E cards are connected by means of a SLI, Scalable Link Interface, dubbed the MIO port, a high-speed digital interconnect which connects to a connector on top of both cards. This connector is actually available on all PCI-E GeForce 6800 series graphics cards. Through this MIO port both cards communicate to each other and distribute the workload which is accelerated by dynamic load-balancing algorithms. In essence the screen is divided vertically in two parts; one graphics card renders the upper section and the second graphics card renders the lower section. The load balancing algorithms however allow it to distribute the load across the graphics processors. Initially they'll both start out at 50% but this ratio can change depending on the load. Although Nvidia has remained tight-lipped about what makes their SLI implementation tick exactly it is clear that both hard- and software contribute to making SLI work. Most of the dynamic load balancing between the two graphics processors is handled in software and thus SLI needs driver support, drivers which are as of yet unreleased, to work.
The MIO port connector that is used to connect two PCI GeForce 6800s together in SLI.
Exact performance figures are not yet available, but Nvidia's SLI concept has already been shown behind closed doors by one of the companies working with Nvidia on the SLI implementation. On early driver revisions which only offered non-optimized dynamic load-balancing algorithms their SLI configuration performed 77% faster than a single graphics card. However Nvidia has told us that prospective performance numbers should show a performance increase closer to 90% over that of a single graphics card. There are a few things that need to be taken into account however when you're considering buying an SLI configuration. First off you'll need a workstation motherboard featuring two PCI-E-x16 slots which will also use the more expensive Intel Xeon processors. Secondly you'll need two identical, same brand and type, PCI-E GeForce 6800 graphics cards. For workstation users it is also a nice extra that with a SLI configuration a total of four monitors can be driven off of the respective DVI outputs on the graphics cards, a feature we'll undoubtedly see pitched as a major feature for the Quadro version of the GeForce 6800 series SLI configuration.
The high-speed digital MIO port bridge connecting the two PCI-E cards together.
The dual PCI-E-x16 motherboard however will mean a significant investment, two PCI-E GeForce 6800GT cards could however make more sense than a single PCI-E GeForce 6800 Ultra or Ultra Extreme, as the performance increase will be much larger. Also, workstation motherboards run at a hefty price premium over consumer products, fortunately they do not require dual Xeons, a single Xeon will work just as well. All in all Nvidia's SLI implementation brings back fond memories of the 3dfx days and has all the right ingredients to once again revolutionize 3D graphics provided you're willing and able to pay the hefty price tag associated with it. Unlike Voodoo2 there's no simple upgrade to double your 3D performance; apart from a second PCI-E GeForce 6800 you'll need a new motherboard, memory and CPU(s). That doesn't do much to dampen our spirits though, the best 3D performance available comes at a price much like driving a Porsche or Ferrari and it doesn't come cheap. Kudos to Nvidia for once again raising the bar and making the harts of many gamers rejoice; SLI is back, and with a vengeance.