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SLI: Myth and Reality - Part 2

Last week, we took a quick look at what NVIDIA's SLI has to offer. In that article, we concluded that while SLI does work, you should not expect tremendous gains in frame rates. In average, SLI can only offer around 30 % increase, occurring mostly when AA and AF are enabled. So, getting a single faster card is always better than putting two slow cards in an SLI setup. There are several reasons why is this so. First, the increase in performance with a faster single card is at least the same or even better and you'll see that increase both with and without AA and AF. Second, a single card will work with every game unlike SLI which can only work its muscles in games that are pixel shader or fillrate bound. Third, you'll save some space inside your PC for additional expansions, not to mention some power savings as well.

However, performance is not the only feature of SLI. In addition to multi rendering, SLI can be used in SLI antialiasing mode. In this mode, the two graphics cards will perform antialiasing with slightly different sample patterns and then combine the results. Once you've enabled SLI antialiasing in the driver panel, you have a choice of either SLI 8x or 16x AA. Just like the name implies, 8x means 8 samples are used to compute an antialiased pixel - this works much like this: each graphics card will perform 4x AA, with the second card using a slightly different sample pattern and 'blends' them together. So, 8x SLI translates to 4x per card while 16x translate to 8xS. You might ask why is this a big deal. Well, since technically each card is only rendering 4x AA, with SLI 8x you can get 8x AA quality at close to 4x AA performance levels. Furthermore, since we're not computing two pixel samples (remember the 'S' in 8xS), SLI 8x will probably be faster than 8xS even in SLI multi rendering mode. We'll be looking at this in the later parts of the article.

We also chose a different (well, two actually) card for this article - the GeForce 7800GTX. With our setup, this card is mostly system limited when running without AA and AF. That's perfect for this article because we want to focus on AA and AF. Remember, from the first article we saw that SLI multi rendering mode works best with AA and AF. Most people who bought the 7800GTX expect to run it with AA and AF, however even a single 7800GTX can still be too slow in some games. So, it will be interesting to see whether or not a pair of 7800GTX will be able to offer higher frame rates in SLI multi rendering mode. This will be the focus of the first part our performance evaluation.

Image Quality Comparison

We're going to look at what SLI 8x has to offer, not just in performance but quality as well. So, we will be looking at several image taken from different AA modes, But before we do that, let's take a look at their sample patterns. Below, you can see the different sample patterns from each mode we will be testing in this article - left to right they are 4x, 8xS, SLI 8x and SLI 16x.



If you look at SLI 8x sample pattern, you'll see it doesn't look like 8xS at all. That's normal since we know it's based on 4x sample pattern. You can definitely see the resemblance. Overall, SLI 8x sample pattern has more 'slant' than 4x, so it should help more when antialiasing nearly horizontal lines. At least that's the theory. Let's see how it translates in real life. Below you'll see screenshots taken (in order) with 4x, 8xS and SLI 8x in Call of Duty.



Did you notice any difference? The most obvious is the fence on the left - 8xS by far has the best quality. Not surprising since this mode also perform supersampling which help antialiased transparent textures. That's about it actually, we can see no significant differences between 4x, 8xS and SLI 8x. There are some very subtle differences if you look very close, but they are not noticeable in gameplay. Let's see the next game.



Hard to see any differences unless you're looking at them real close. 8xS is again superior here, providing us with aliased transparent textures. Even with opaque textures, we can see 8xS is slightly better - look at the track in front of the red Ferrari. SLI 8x is better than 4x, but still not close to 8xS. What about Nascar 2003?



Since these are static screenshots, they really don't do justice to SLI 8x. Look at the far edge of the track on the left - it's very close to being completely horizontal. With extra sample patterns, both 8xS and SLI 8x does a much better job than 4x can. In fact, SLI 8x works better, most probably due to the combined sample pattern. Next we have Homeworld 2.



All the images pretty much look the same. There are differences, but they're not really noticeable. 8xS has a slightly sharper image, again probably due to supersampling. Now, let's look at aliasing in motion.

Aliasing in motion
2xQ
8xS
SLI 8x

We can see that SLI 8x has slightly less aliasing in motion than 8xS. However, it's still not able to eliminate all of them. It's 2xQ that's very effective in this test. Unfortunately, there are no SLI 4x or SLI 4xQ - that would just be perfect for this game. Next, we have Star Wars: Knights of the Old Republic.



Now we're seeing the best example of what SLI 8x can do - quality wise. By far, SLI 8x offers the less aliased image than both 8xS or 4x. Look at the far room, you can see aliasing in 4x and 8xS that's not present or as noticeable in SLI 8x. Let's see how it looks in motion. These zip files are quite big (around 15 to 20 MB each), so they may take a while to download.

Aliasing in motion
4x
8xS
SLI 8x

Whoa! 8xS is pretty much the worst in motion, even compared to 4x. Furthermore, 8xS will not work with frame buffer effects on. Now look at SLI 8x - it's just so good you can hardly believe it. There's very little aliasing and it works with frame buffer effects. This does prove that SLI 8x can offer even better visuals than 4x or 8xS. What's even more good news, at least for those with GeForce 7 series, is that you'll be able to use SLI 8x with transparency antialiasing with supersampling to deal with transparent textures aliasing. We'll come back to this subject later, but for now, let's take a look at what two GeForce 7800GTX in an SLI setup has to offer in terms of performance.

Performance - GeForce 7800GTX SLI

Unlike the GeForce 6600s we used last week, the GeForce 7800GTX is system limited with our setup. So, we limit our testing to game that scales well graphically, which includes older games such as Call of Duty, Richard Burns Rally and newer games such as F.E.A.R, Quake 4, Splinter Cell: Chaos Theory and Serious Sam II. To push these cards extra hard, we ran test with and without 4x AA and 16x AF. In older games, these settings are forced through the driver panel, while in newer games these settings are enabled in game. However, with some games, we had to force 8xS from the driver panel since it was not possible to enable 8xS in game.

In addition to AA and AF, we also perform additional testing with HDR and transparency antialiasing. All other settings are set to the max - full detail except for soft shadows in F.E.A.R. Quake 4 was set to High Quality, with AA and AF set through the command 'r_multisamples 4' and 'image_anisotropy 16'.

We'd like to thank both Tagan and Kingston for supplying with the additional power supply and 1 GB memory modules for this article.

Our test setup
AMD Athlon 64 3500+ socket 939
2 x 1024 MB Kingston KVR 3-3-3 PC3200 DDR-SDRAM
MSI K8N NForce 4 SLI motherboard
2 x GeForce 7800GTX DDR3 256 MB graphics card
(running at reference clocks - core 430 MHz / memory 600 MHz (1200 MHz effective))
Maxtor DiamondMaxPlus9 80 GB Serial ATA 8 MB buffer
ASUS E-616 DVD-ROM
Tagan TG530-U15 530 watts ATX/BTX power supply

Windows XP Professional with Service Pack 2 installed
NVIDIA Forceware 81.98 reference driver
NVIDIA NForce 4 6.70 reference driver
Creative SoundBlaster Live! 24 bit 5.12.1.512 driver.
DirectX 9.0c

Performance

The results:

Unlike what we're seeing last week, there's little difference running this game with and without SLI. Even with AA and AF, there's practically no difference between a single card and SLI in 1600 x 1200. It will take a significantly faster processor than the Athlon 64 3500+ to see any difference between single and multi rendering in this game.


There's no doubt SLI is paying dividends to gamers playing this game who are fortunate enough to have two GeForce 7800GTX. We can see that a single GeForce 7800GTX begin to loose steam in 1280 x 1024 while the SLI setup barely breaking a sweat. In fact, the SLI setup still has enough pixel pushing power to push more than 100 fps with 4x AA and 16x AF at 1600 x 1200. What if we threw transparency supersampling antialiasing into the mix?


It's safe to say, only a pair of GeForce 7800GTX has the processing power for transparency antialiasing in this game. Even then, the SLI setup barely manage passing the minimum 30 fps mark at 1600 x 1200. Of course, the single GeForce 7800GTX fares worse, pushing about the same frame rates at the slightly lower resolution of 1280 x 1024.

These older games rely mostly on textures rather than shaders. So, let's see how SLI fares in newer, shader laden games.


In F.E.A.R, we can see a pair of GeForce 7800GTX in multirendering mode behave in a similar manner to a pair of GeForce 6600. SLI can only push slightly higher frame rates at 1024 x 768, no more. In higher resolutions, there's little difference between SLI and a single card. It looks like, SLI can only work its muscles with textures and not shaders - let's look at other games to be sure.


Quake 4 is not shader heavy, but as you can see, we won't get higher frame rates with an SLI setup. In fact, the added overhead of SLI is wreaking havoc here. We're actually running slower with two GeForce 7800GTX. So it behaves pretty much like F.E.A.R - the SLI single and multirendering results of this game (or at least in this particular level) is similar to last week GeForce 6600 SLI. ..


We had high hopes SLI will be able to push higher frame rates in this game. Particularly, since we saw nearly 100% increase with a pair of GeForce 6600 in SLI multirendering mode. But it was not to be - Splinter Cell : Chaos Theory is already running as fast as it can on this setup. What if we opt for HDR instead of antialiasing?


Same thing. So, out of four newer games, both the 6600 SLI and 7800GTX SLI perform in a similar manner - SLI will not give you faster frame rates most of the time. Now, let's look at the last game in our benchmark suite, Serious Sam II.


Need we say more? Well, yes actually. Serious Sam II is system limited here, so that might explain the lack of difference between single and multirendering results. Thankfully, there's HDR and transparency SSAA. First, let's look at HDR test results.


Hmm, pretty much the same thing. And here are the transparency SSAA results.


Well, SLI is faster but it won't make any difference. Now, we can rule out these results as system limited, but that excuse can't be applied to AA and AF results. Of course, we could also have screwed up the results, but that's unlikely. These SLI test results are obtained during the same session as Richard Burns Rally and F.E.A.R, so we know SLI is working properly. So, it looks like we have to repeat last week's conclusion - SLI should always be considered a complimentary solution. It can and does work, but it won't give you higher frame rates all the time. Based on what we see here, faster frame rates or higher performance should not be the main reason for choosing SLI.

Quality

With that out of the way, let's move on to what we feel is a stronger reason to get an SLI setup - quality. We already can see, there's very tangible image quality benefits from SLI 8x antialiasing mode. So, we took the same games and ran them with 4x, 8xS and SLI 8x antialiasing with both single, multirendering and SLI antialiasing mode. We expect SLI 8x antialiasing performance to be faster than 8xS in both single and multirendering mode, so how did the GeForce 7800GTX fare?


Well, although SLI 8x performance is still pretty far from 4x both in single and multi rendering mode, we can all see it's faster than 8xS in both modes. With higher resolutions, the SLI 8x can still maintain a higher minimum fps than 8xS. The performance difference is around 15 - 25 % across the board.


Here, we're seeing a similar situation. Don't let the graph fool you. While SLI 8x performance levels is definitely nowhere close to 4x, it's still significantly faster than 8xS with any mode. Clearly, computing 8 samples antialiasing under any mode is really taxing on the GeForce 7800GTX SLI.