posted by westfale on december 27th, 2004
well, nvidia is making big noise about SLI. what exactly is SLI though? how does it work? and is it worth buying an SLI setup?
SLI is a little technology that allows people to use two graphics cards in a single system to increase the rendering power of their machine. it is potentially useful in professional CAD programs and in games, where nvidia claims that performance nearly doubles.
it works by linking the two identical cards (they must be the same model, and should come from the same manufacturer) together using a small "SLI bridge". the graphics driver then distributes the workload evenly across the two cards. it does not simply divide the screen in half (one half to work on for each card), but actually calculates how much needs to be done on the full frame and then makes sure each card does half of that work.
in theory, this sounds fantastic, richer people can get two high-end cards (6800GT and ultra) to work together, and us regular joes can buy one card now and the other one later (6600GT or 6800GT). there are however several issues with SLI that are worth adressing:
1.- games need to be supported in the graphics driver in order to be recognised by an SLI setup. play a game that isn't recognised, and you gain no performance at all. right now, only about 50 games are supported by the drivers, and although nvidia has made sure that popular benchmarking programs (such as the 3dmark series) and games that are used for benchmarking are indeed recognised, a lot of other less popular titles are not and nvidia isn't saying when more games will be added to that list or even which games will be added. there is also the possibilty that nvidia sponsored games could be given preferential treatment ("the way it's meant to be played" program), and that ATI sponsored ("get in the game" program) games are left without SLI support, or receive driver support much later. once again though, there's no official word from nvidia on this. the same goes for professional apps of course: one program may be on NV's list, and another one may not be.
2.- you need two graphics cards, and an SLI-capable motherboard. that gets quite expensive, and when you consider that both ATI and nvidia are expected to release new architectures in early to mid 2005 (and those new chips might well be faster and cheaper than a current SLI setup), then SLI starts to look much less attractive.
3.- a single 6800ULTRA needs a lot of power, in an SLI setup you need twice as much. of course, if you can afford SLI, you can afford a decent PSU (power supply unit). still, buying a new PSU increases the cost of the whole thing even further.
4.- performance. in NV's benchmarks, the SLI setup is much ffaster than a single card. that is simply because NV decided to run their test games at high resolution and at high anisotropic filter/anti-aliasing levels. it's a very high workload to render a current game at 1600x1200 with high AF/AA. obviously, a two cards will clearly uotperform one. at lower resolutions and AF/AA settings though, the speed of a game is more determined by the speed of the processor rathre than the graphics card, as a single 6800 ultra doesn't really struggle in any game at 1024x768. so at that resolution, the difference between SLI and non-SLI is much smaller, and sometimes the single card system is even slightly faster (as th e driver doesn't have to calculate how to distribute the workload between two cards since there is only one).
i'm not really sold on SLI. still, if you think this might be something for you, here are a few links worth hitting:
list of games that make use of SLI
NV's SLI FAQ
anandtech's SLI tests
techreport's test, i recommend reading this one
