SLI: Myth and Reality
Several weeks after the release of the GeForce 6
series, NVIDIA announced the rebirth of SLI. For the
uninitiated, the moniker stands for Scalable Link Interface
(and not Scan Line Interleaving from the ol' 3dfx days).
The concept of SLI is pretty simple - instead of using a
single card, you use two cards to render one screen by
dividing the screen into two parts. If that's not possible,
the two cards can take turns rendering the screen, ie. the
first card renders all odd frames and the second card
renders even frames. An intriguing idea and early test does
show some promise. However, as most reviewers found out
(later), SLI was anything but simple.Now, more than a year after it's introduction, SLI has garnered more and more attention, mostly from gamers who want to play graphics rich games such as F.E.A.R, Quake 4 and Call of Duty at the highest settings. Of course, the engineers and developers at NVIDIA haven't been sitting idle during that time. With every driver version, NVIDIA strive to make SLI easier and more compatible with games. With the promise of faster performance (and better visuals), SLI was fast becoming the choice of many games, it certainly was the choice for many Futuremark's 3DMark ORB users. Even NVIDIA's archrivals, ATI who scoffed SLI at it's launch, have followed NVIDIA's footsteps with Crossfire.
The arrival of Crossfire can be considered a boon for SLI users. Newer Forceware drivers allows you to mix cards from different manufacturers or even different series of cards, ie. a GeForce 7800GTX from ASUS and a GeForce 7800GTX from Gigabyte or any GeForce 7800GTX and a 7800GT. NVIDIA have even certified Intel's latest chipset - the 955 series - as SLI capable chipset. No such luck for AMD users, more likely because AMD does not make chipsets for their processors..
So, after a year, is SLI ready for more 'maintream' use? Granted, installation and use are easier than before, even requiring less hassle than ATI's Crossfire. What interest us more is whether or not SLI is a viable option, both from a performance and quality standpoint for gamers. Let us elaborate.
Understanding SLI
We won't be discussing much of the technical aspects of SLI here. There are already countless SLI articles dealing with that subject available on the Internet. Now, you might dismiss this as just another SLI article, but we beg to differ. In this article, we'll look at what does SLI really bring to the table for gamers. What you say? Well, in a nutshell, SLI brings two things to the table for gamers: performance and quality. By splitting the rendering load between two cards, you should be able to enjoy faster frame rates - half the load per card or twice the performance. At least in theory. This is obviously what we mean with performance. However, you can also force the use of higher sample antialiasing with SLI - 8x and 16x. The use of more samples over the standard 4x or 8xS AA (may) offer slightly improved visuals - very important for those looking for better visuals.Evaluating performance should be easy to do. All we have to do is run an SLI setup in both single and multi card mode. Well, that's exactly what we intend to do (as did other reviewers in their SLI articles). However, before we do that, let's look at this from a different perspective - in what system will you likely use SLI to gain performance? Using a pair of GeForce 6800GT or even the GeForce 7800GT / GTX may provide us with the highest frames rates under the sky but that wouldn't tell us much about SLI at all. Why? For one, these cards are already system limited with the Athlon 64 3500 or even the Athlon FX 53. The only way to make most benchmarks graphics limited is to crank up AA and AF. With these settings, we believe more readers using (or planning to) those kind of rigs would be more interested in quality rather than performance. For those looking for quality, we will run benchmarks with a pair of GeForce 7800GTX, focusing on performance with AA and AF. In addition to that, we will discuss SLI antialiasing - whether or not it's viable option and does it offer any significant improvement over the standard 4x. However, the results of that test will not be shown here, but on the next article, which hopefully will be ready next week. .
To test 'pure' SLI performance, we decided to use the slowest card with SLI support - a pair of GeForce 6600. These cards are basically the same as GeForce 6600GT, but they're equipped with a slower core and memory - the slower DDR2 (though still with 128 bit bus). This might suprise you, but think about it for a minute. This setup is basically the cheapest SLI you can buy and the single GeForce 6600 badly needs the extra performance SLI (may) offer. So in a way, we're testing SLI in a setup that will likely give us the most performance increase.
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