Original URL: http://www.theregister.co.uk/2005/09/28/review_ati_crossfire/

ATI CrossFire multi-GPU platform

Worth the wait?

By Trusted Reviews

Posted in Reviews, 28th September 2005 15:43 GMT

Review If you have even a passing interest in 3D graphics, you've probably been waiting for the appearance of ATI's dual-GPU solution. It seems like aeons ago that ATI announced that it would produce a platform to rival Nvidia's tremendously successful SLi, but now, finally I have a CrossFire system in front of me and it's time to see if it was worth the wait, writes Riyad Emeran.

What I'm talking about is the ability to install two graphics cards in a single PC so the two cards share the load when it comes to 3D rendering - resulting in a significant performance boost when you fire up your favourite game.

CrossFire

My first foray into the dual-GPU scene was when I got my hands on two 3dfx Voodoo2 cards. I remember having to keep pretty quiet about it in the office - having one Voodoo2 at the time was considered lucky, but installing two in the same machine was positively greedy! This was the birth of SLi, although as I remember, back then it stood for 'Scan Line Interleave', rather than the 'Scalable Link' Interface that Nvidia now attributes to the acronym.

It was a long time before I gave up my dual Voodoo2 cards, despite advances in 3D technology from other companies. But eventually 3dfx went the way of the Dodo, and it was time to move on. I had to wait quite a while before I saw a PC with two graphics cards again.

The barren spell ended about a year ago when Nvidia started to ship SLi kit into the market. Although there were cards available quite early, it took a while for the mainstream motherboards to appear, but when they did the flood gates opened. Now, dual-graphics card systems are commonplace, even at the low end of the market.

ATI didn't want Nvidia to have everything its own way, and it wasn't long before rumours of a platform to compete with SLi started to appear. But it was at the beginning of June that those rumours started to take shape, and ATI announced the CrossFire branding. Hardware was expected soon, allowing ATI to hit the streets with CrossFire before the launch of Nvidia's next generation 3D solution, the GeForce 7800GTX. Unfortunately, CrossFire didn't appear.

But all good things come to those who wait, so they say, and now that wait is over.

One of the most obvious physical differences between SLi and CrossFire, is the method of connection between the two cards. With SLi a small bridge is employed between the two cards that links them internally, while CrossFire uses a daisy-chained DVI cable that links the cards externally. There's no doubt that the Nvidia solution is more elegant from a PC building point of view, but looks aren't everything.

Another big difference between the two systems is that ATI has promised that CrossFire will work with every single game out there, whereas for SLi to weave its spell there needs to be a driver profile for each game you want to play. Of course, this doesn't mean that CrossFire will enhance the performance of every game, but just that every game should play in CrossFire mode.

CrossFire

Now, saying that SLi needs driver profiles and CrossFire doesn't isn't entirely true. CrossFire does need profiles depending on the rendering method that's employed - whereas SLi uses each graphics card to render alternate frames, CrossFire can employ one of three different rendering methods. The most basic rendering mode is tiling, which cuts the scene up into loads of squares and splits the load between the cards. Tiling doesn't require any kind of profile, but there's also no guarantee that there will be a performance increase over using a single card. The second rendering mode is scissor, which splits the screen in half, with each card rendering each half of the scene. The third and final method is alternate-frame rendering, which just like SLi allows each card to create alternate frames ' for this method, just like SLi, CrossFire needs a game profile.

I had initially thought that the CrossFire driver would allow the user to choose which rendering method to use, but when I got my hands on the hardware I found out that this wasn't the case. It seems that the Catalyst driver determines the best rendering mode and applies it. Of course, these are still very early drivers, and a manual rendering mode configuration may yet appear in future versions.

Unlike SLi, you can't build a dual-GPU CrossFire system out of any two cards. Instead you have to purchase what ATI calls a master card and marry this up to a normal ATI graphics board. At present, the master cards come in two flavours, Radeon X850 XT and Radeon X800 XL. Although the slave card doesn't have to be an exact match to the master card, I suspect that the system will run at the speed of the slower card. Although ATI has hinted at the fact that cards can run asynchronously, I didn't have time to put this to the test.

In order to put CrossFire through its paces I needed some hardware, and Evesham was kind enough to supply me with some. Evesham built a complete PC based on the CrossFire platform - this consisted of an ATI reference motherboard for Socket 939 AMD Athlon 64, an FX-57 CPU and two Radeon X850 XT cards, one master and one slave.

When the Evesham machine arrived it fired up first time and proved to be rock solid throughout a complete run of benchmarks. However, when I came to actually play some games, things weren't quite so rosy. There seemed to be considerable tearing when panning around a scene, while turning on FSAA resulted in a horrible motion blur effect that produced a near instantaneous headache.

After much work and investigation we were unable to eradicate the tearing problem, although the motion blurring seemed to stop after I removed the graphics cards and re-installed them. Eventually we were supplied with a new driver revision by ATI which smoothed out the tearing and allowed me to get a better idea of what CrossFire is capable of. I re-ran all the benchmarks using the new driver.

CrossFire Doom 3

CrossFire FarCry

CrossFire Half-life

Looking at the benchmark results first, there's no doubt that CrossFire works, and two X850 XT cards definitely produce enough grunt for pretty much any game you're likely to throw at them. Running our custom Far Cry demo turned in a very impressive 85fps at a resolution of 1,600 x 1,200 with 4x FSAA and 8x AF, while in Doom 3, a game that's traditionally nVidia friendly, the CrossFire machine managed 70fps at the same resolution and settings. Obviously I expected Half-Life 2 performance to be good and with a score of 81fps at 1,600 x 1,200 with 4x FSAA and 8x AF, I wasn't disappointed.

CrossFire 3Dmark03

CrossFire 3Dmark05

With scores like that it's clear that you should be able to push the resolution envelope with a CrossFire system, just like you can with a high-end SLi machine, so you're probably wondering why I capped the testing at 1,600 x 1,200. The simple answer is that I didn't cap the resolution, ATI did. It seems that not only is the CrossFire method of connecting up the two graphics cards not quite as elegant as the SLi method, it also brings with it a resolution ceiling.

Because the two graphics cards are connected via a DVI port, the resolution that CrossFire can support is capped by the upper limit of a single-link DVI connector. A single-link DVI port can output a maximum resolution of 1,600 x 1,200 or 1,920 x 1,200 with reduced blanking employed. So, the highest resolution available to a dual-GPU CrossFire system right now is 1,920 x 1,200 if you happen to have a 23in or 24in widescreen TFT. But for most gamers that don't have over a thousand pounds to spend on a screen, 1,600 x 1,200 is the best you're going to get.

CrossFire

The most bizarre thing about this situation is that a single Radeon X850 XT can produce playable frame rates above 1,600 x 1,200, but then if you add a second card that luxury is off the menu. This resolution limitation is a major problem for CrossFire as it stands today, but it shouldn't be an issue in a few weeks. Why? Because that's when we should see ATI's next generation of graphics cards, based on the new R520 core.

The R520 boards will feature dual link DVI connections, which means that the resolution limitation issue seen in the R4xx boards will be a thing of the past. With this in mind, an R520 based CrossFire system should be able to reach dizzying resolution heights, although I'll have to wait until I get my hands on the hardware for conclusive results.

With the resolution limited to 1,600 x 1,200, I figured that I may as well take a look at one of CrossFire's big selling points ' super-high FSAA. In CrossFire mode you can push the anti-aliasing all the way up to 14x and that's exactly what I did. With 14x FSAA enabled, the sweet spot seemed to be 1,280 x 1,024 ' if you pushed up to 1,600 x 1,200 the frame rate became unplayable. It has to be said that playing Half-Life 2 at 1,280 x 1,024 with 14x FSAA looks absolutely awesome!

During the course of testing Valve was kind enough to release the Lost Coast, which gave me the chance to see Shader Model 2.0 High Dynamic Range lighting as well as 14x FSAA. Bizarrely loading up the Lost Coast brought back the motion blurring problem when I employed FSAA, but only when I ran it full screen - running the game in a window showed no problems at all. Even more bizarre was the fact that the next day the motion blurring problem had disappeared once more - clearly there's a little more work to be done with the driver.

Verdict

So, was CrossFire worth the wait? Well in its current guise probably not. In my opinion, ATI should have waited for the R520 to appear before launching CrossFire, then there'd be no resolution limit, and the performance would most likely rival the best that Nvidia has to offer.

Of course, the problem is that ATI has pitched CrossFire as an upgrade path for existing Radeon X8xx users, so the company had to produce said upgrade path. However, I can't see anyone that's running a compatible card upgrading to CrossFire - not only would those users have to buy a master card to pair with their existing graphics card, but they'd also have to buy a new motherboard to house both cards. When you factor in the cost of both motherboard and graphics card, I think that most users would rather buy a single R520, or even switch camps and go for GeForce 7800 - either way, the result will be better performance and no resolution limit.

Evesham has promised to get me an R520 based CrossFire system in the next couple of weeks and I'm looking forward to pitching it head to head with a 7800GTX based SLi rig. Then I'll know definitively whether ATI's dual GPU platform was worth the wait.

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