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This story was printed from CdrInfo.com,
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Appeared on: Thursday, July 14, 2005
CROSSFIRE vs SLI


1. Introduction - SLI

With SLI getting so much attention and coverage for its high performance and value (it's currently better value and performance to buy two 6600GT's than one single 6800U), ATI was left on the sideline, watching as Nvidia broke one benchmark record after another. Seeing the public's response to such a move, ATi secretly began working on a new Multi-GPU architecture of its own which we now know as CrossFire.

Crossfire is a direct answer to SLI, so in order to better understand the differences, we'll go back a little and take a brief look at SLI.

SLI stands for Scalable Link Interface. In order to play a game in SLI mode, you need:
- An nForce 4 SLI powered motherboard with two PCI-Express 16x slots
- Two, identical in every way, VGA cards linked using an SLI bridge connector.
- SLI support for each game through the NVidia driver.

SLI offers 2 rendering modes: Alternate Frame Rendering (AFR) and Split Frame Rendering (SFR).

SFR is the most commonly used and works by splitting the screen in two horizontal portions, each rendered by a different card. If the driver determines the load is just too much for one card to handle, it changes the position of the separating line to balance the load.

First GPU

Second GPU

Final results of Scalable Link Interface (SLI™)

As a result, SLI benchmarks report quite steady framerates, very consistent to increasing resolution or enabling image quality options like Anti-Aliasing and Anisotropic Filtering. Both cards do the best they can and the performance boost against a single card is no doubt obvious.

With that out of the way, let's move on to what the Crossfire platform can offer.


2. Rendering Modes

Crossfire on the other hand, offers 4 kinds of rendering modes: AFR, Supertile, Scissor and SuperAA.

AFR works by rendering all odd frames with one GPU while all even are left to the other. The method is exactly the same as AFR used by NVidia. The main problem with this method is that if a frame contains data that affects the rendering of the next frame to come, then that data needs to be transferred between the two cards. This overhead is why AFR is not so popular.

Scissor is also similar to NVidia's SFR with a small difference. Each frame is split in two parts and each GPU is assigned with a portion to render. That way, the GPUs can share the number of triangles processed in a scene, reducing their time to present us with the final rendered frame. Scissor can cut the screen in two parts, either horizontal or vertical, (SLI can only split the screen horizontally) and can also adjust the split to be uneven if in one of the two parts the scene gets more complicated than the other.

This is where ATi comes in with its novel method, Supertile. Their idea was to efficiently share the workload among the GPUs without spending time on calculating the GPU workload. Supertile is the default rendering option for Crossfire enabled systems but you'll also be able to pick your preferred method from the ATi Catalyst driver menu.

Supertile splits each frame into small tiles, just 32px by 32px in size and alternates their assignment to the GPUs just like a checkerboard. ATi has found that splitting the screen this way does not overload one GPU over another, as screen objects are usually much larger than the tiles and thus are split in two or more adjacent tiles.


3. SuperAA

With Supertile bringing out the best combined performance from the GPUs, ATI decided to push its technology one step further and use CrossFire to improve visual quality. In our reviews, we've seen that modern displays and CPUs still tend to limit the performance coming from a graphics card. Also, in lots of games, high end cards alone can deliver so many frames per second that the number easily surpasses an average monitor's refresh rate. In these cases, extra GPU power would normally go to waste either because your CPU can't handle that many frames or because you don't actually need that high a performance. So, ATI thought: Why not put the second graphics card to work improving Anti-Aliasing to a much higher level. The result of this research was SuperAA.

A pixel, with it's center and the 6 sample points (6xAA)

Anti-Aliasing (AA) is a well known rendering technique designed to remove jagged edges, shimmering, and pixelisation problems that are common in rendered 3D images.

On modern graphics cards, Anti-Aliasing can go up to 6x. In that case, first the card has to render an ultra high resolution image that will never be output to screen. Then, for each pixel of the final rendered image, the card samples a pattern of 6 points out of the high resolution image. Using the 6 samples, the card determines the final pixel colour. As you can probably understand, downsampling like that causes a serious performance drop. The more samples you use, the less frames you will get rendered per second. Also, as an immediate result of the way AA works, some objects that are close to horizontal or vertical get jagged edges as you can see in the picture below.

Click to enlarge
SuperAA delivers far superior image quality (Click to enlarge)

SuperAA is ATi's innovation that promises to take care of all AA's main problems.

The second GPU is now given the task of rendering the high resolution image from which the samples will be taken. The two cards each having the ability to sample 6 points from that image, use different patterns (actually rotated by 180°) instantly pushing the sampled points up to 12 (12x).

A pixel, with 2 centers and 12 sample points.
The two outer squares mark the two Crossfire cards.

Another brilliant idea implemented by ATi was to use alternate pixel centers for each card, which further increases the effective samples to 14 (14x). In essence, this means that each card will hold a different image of the scene that when combined, will boost image quality to the highest level.

As a result, Crossfire systems will be able to make use of 8x, 10x, 12x and finally 14x. Performance is still untested, but ATi has promised "little to no performance hit".

Another bonus of the alternate pixel centers is that it's going to work well with Anisotropic Filtering. Anisotropic Filtering removes the blurry effect of distant, non-parallel to the screen objects, by applying a number of texture samples on them. The different point of view of the cards for 10x and 14x SuperAA will work at the same time for AF, doubling its effect up to 32x!

The full list of ATi's Anti-Aliasing techniques

4. The mainboards

To enable Crossfire, first of all you need a motherboard with a Crossfire-ready chipset. The first chipset that will really qualify as Crossfire ready is the Radeon Xpress 200 Crossfire Edition that will begin shipping after mid-June. However, unlike NVidia which forces you to buy an Nforce4 SLI board, ATI is going to keep Crossfire an open platform for all vendors. Intel based motherboards with dual PCI-E x16 slots are already confirmed to be compatible and ATI has even offered its help to make the Nforce4 SLI chipset work together with Crossfire.

The Radeon Xpress 200 CE chipset comes in two flavours for both AMD (socket 939) and Intel (LGA775) processors.

- Radeon 200 Crossfire Edition (AMD)

You can see the reference model for the 200 Crossfire Edition for AMD processors in these two photos. The two large black slots at the center of the motherboard are the PCI-Express x16 slots that will hold the Crossfire cards. The board looks a lot like the EQS M56K9-MLF motherboard we tested some time ago.

 

- Radeon 200 Crossfire Edition (Intel)

The reference board for Intel Pentium 4 processors you can see here, follows a similar layout but features one small and one larger PCI-Express x1 slot instead of 2 small ones.


5. The cards

Next thing you need for Crossfire is of course the graphics cards. One major advantage of Crossfire over SLI is that it doesn't require the cards to be identical. All you need is a "Crossfire Edition" card and then any card from any vendor, as long as it belongs to the same family.

The following table lists the Crossfire Edition cards along with their matching pairs.

Crossfire Edition Works with Price
X850
X850 XT PE
X850 XT
X850 PRO
$550
X800 128MB/256MB
X800 XT PE
X800 XT
X800 XL
X800 PRO
X800
All-In-Wonder X800
$250/$300

The price for an X850 Crossfire Edition is almost double that for an X800 CE. That means the X850 CE is built on an X850XT, whereas the X800 CE (unfortunately for X800XT owners), is probably just a 12-piped X800. That would mean, to get the most performance out of Crossfire, one would have to pair the X850 with an X850 XT PE.

The X850 Crossfire Edition

If you decide to match an X800 Crossfire Edition with a 16-pipeline card like the X800XT, then the XT would have its 4 extra pipelines ignored, to match performance with its counterpart. However, apart from the pixel pipelines, there is no restraint on the cards' clock frequencies. ATI claims no card gets underclocked and even Supertile runs problem-free.

The CE cards are pretty much the same as the ordinary cards we've seen before with just two differences. One is that instead of a DVI output, you'll find a new slot called DMS.

DMS
The new DMS slot

DMS looks just like DVI but serves a difference purpose. Along with your CE card, you'll find a cable dongle. This dongle is to be plugged on the DMS slot of the CE and on a DVI slot of your standard VGA card.

Two X850XT ready for action

As you can probably guess, all data is sent through that cable to the CE card. This leads us to the other difference which is the compositing engine located on board. Its sole purpose is to combine data from the two cards and prepare the final image to be output on screen as you can see in the diagram below.

Another interesting fact that differentiates Crossfire from SLI is the use of the system's physical memory. ATi utilizes system memory to hold each card's command buffer but also shared data and some more space to use for non-local memory.

Finally, note that if you unplug the dongle from the secondary card, crossfire gets automatically disabled and you can use the 4 slots along with the onboard VGA slot of the Radeon 200 Crossfire Edition to enable SurroundView for up to 5 monitors!

SurroundView

6. Compatibility

Compatibility is another claimed feature of Crossfire over SLI. The main problem with SLI is that if an application or game is not in the driver profiles, then Split Frame Rendering doesn't work as intended. The driver is unable to dynamically adjust the location of the bar and the screen is split into two even parts, each handled by a different card which causes a serious performance drop.

The NVidia Driver profiles

Although NVidia is working to keep the SLI-supported games list up to date, needing a driver update every time a new game is released or waiting for such a driver is annoying and one of the first things NVidia has to change in future platforms. At the time of the writing this article, Grand Theft Auto : San Andreas is still not supported, a month after the game's release.

On the other hand, ATI with its default rendering mode being Supertile, which statistically shares the load equally across the two cards without the need for load balancing, skips any kind of problems and is by default compatible with any 3D applications and games.

Another thing worth mentioning is that even though Crossfire products have still yet to come, the Catalyst drivers seem to already support them. We only have to wait for the first product samples to see how well they perform.


7. Performance

We should not expect anything extraordinary out of Crossfire, in terms of performance. MultiGPU is of course limited at around 200% performance of a single card and both Crossfire and SLI have reported results close to that limit. However, it will be more than interesting to see how cards with different cores and clock frequencies behave under the Crossfire architecture.

Crossfire VS Single GPU

Apart from the released by ATI charts, you can see below comparisons of Crossfire and SLI performance, while only leaks of 3DMark03 and 3DMark05 scores are available around the web. These confirm that the crossfired X850XT gets a higher 3DMark05 score but can't beat SLI in 3DMark03, which indeed does not appear in ATI's released performance charts.

Crossfire VS SLI

8. The future

So what does the future holds for us? NVidia is surely on edge over Crossfire's features and there's not much that can be done to improve SLI other than reorganize it from the start. That leaves us waiting for SLI2, which currently only exists as a rumour and will not arrive anytime in the near future.

NVidia once again managed to lead the way with the reincarnation of MultiGPU but ATi, judging from SLI's mistakes, seems to be coming up with a more sophisticated solution. Nvidia's only answer to Crossfire was the recent release of the 7800GTX and its SLI benchmarks, breaking all records for yet another time.

The first RADEON X850 CrossFire Edition cards are already in production and will become available around mid-July. The X800 CE will come a little later in the first weeks of August.

The next major thing is going to be the R520 and its still secret specifications. As no Crossfire X800 or X850 will ever reach the 7800GTX SLI performance, the R520 is expected to do this dirty work. The R520 based cards will no doubt support Crossfire and there's going to be another ultra high performance fight with dual cards from both sides.

For more info on Crossfire and extensive benchmark results, the only thing we can do is wait a few more days, to get our hands on the first Crossfire hardware and we'll be back with our findings.



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