ATI's Multi-GPU Solution: CrossFire
by Anand Lal Shimpi & Derek Wilson on May 30, 2005 9:00 PM EST- Posted in
- GPUs
ATI's Answer to SLI: CrossFire (The Card)
ATI does not have dedicated silicon on their GPUs for chip-to-chip communications as NVIDIA does. ATI bills this as a positive aspect of their solution, as their CrossFire solution is capable of running cards with two different (and even different speed) GPUs. NVIDIA's SLI solution is restricted to running on not only the same model of card, but cards with the same video BIOS. Timing is crucial when hooking the two GPUs together for SLI. In fact, an out-of-spec SLI bridge can even cause problems. We will only be able to see which solution performs better when we get hardware in our hands, but if all things are equal, ATI will have the advantage here.
In order to make up for their lack of a chip-to-chip interconnect, ATI includes a Compositing Engine chip on their CrossFire card. Because of this, the CrossFire card can be paired with any Radeon X800 or X850 (there will be a CrossFire card for each flavor). The driver controls clock speeds of each card automatically and manages synchronization as necessary. Synchronizing boards can be done on a general scale and doesn't need to be clock for clock. All CrossFire cards have 16 pixel pipelines, but disable 4 pipelines when running in tandem with a 12 pipe Radeon. This is what allows ATI to provide a limited number of CrossFire cards to work with multiple Radeons. Each card does need its own x16 PCI Express slot, and the boards communicate through an external cable.
This may look more like the older 3dfx SLI solution, but in reality, the Radeon X800 or X850 sends its data digitally from the DVI output to the input on the CrossFire card, which then handles the data and forwards the final frame to the display device. Under alternate frame rendering (AFR), the data is simply sent on unchanged, but the Compositing Engine handles the combination of a split or supertiled frame, as well as the final rendering of ATI's super AA modes (more on these later).
In order to function properly, the standard and CrossFire cards share some system RAM. This allows each card access to all necessary data that doesn't need to be unique for each frame. ATI's driver handles splitting the workload and configuring a unique command queue for each card based on the application and the rendering mode selected. Rendering modes are not user selectable, and are predetermined through Catalyst AI. Each card also has access to its own system memory as usual.
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yacoub - Tuesday, May 31, 2005 - link
Is it just me or do several things about this scream "bottleneck" and "latency"? The 2PCI-E x8 slots instead of x16 slots. The extra Compositing Engine chip. The ability to pair different cards such that it will drop clock speeds and/or pipelines to sync them up. The lack of direct chip-to-chip interconnect.I'm curious to know just how much performance gain is realized if you pair, say, an X800XL and an X850-something, over just the X850-something. And also how much bottleneck and latency there is in this implementation over the NVidia offering of SLI.
The only upside I can see is cost/upgrade since a user can own an X800-based card (assuming they have a Crossfire compatible motherboard) and go out and buy an X850-based card later and use BOTH cards together (assuming they are both Crossfire-capable cards). Then again with those assumptions I'm not sure it's truly any more cost-effective. =\
LoneWolf15 - Tuesday, May 31, 2005 - link
As usual, the fanboys of both sides come to the show to spout their comments.For everyone saying "Man, you have to buy a Crossfire that matches your card, and throw it away when you upgrade"...umm, don't you have to buy two of the exact same matching card for running nVidia SLI, and if you wish to upgrade, you have to sell both? Doesn't sound that different to me. One thing I think a lot of current ATI owners will be happy about is that they won't have to get rid of a card they already own and buy two of a new one; they can just buy a single Crossfire card (and of course a mainboard).
On the other hand, to those thinking ATI has now "0wned" nVidia, it is WAY too early to tell. The solution looks promising, but if you have to sacrifice mainboard performance (i.e., SATA hard disks, memory bandwidth, etc.) it may be a hard sell. Benchmarks in Doom 3 are also not the end-all be-all. We'll have to wait for a more comprehensive performance review, including DirectX benches, and performance/quality with older games using this new AA method, as well as game compatibility reports. We'll also need to know what TRUE pricing is (we've seen claimed pricing vary quite a bit from what it has turned out to be at product release in the past two years).
Do I hope it will beat nVidia's solution? You bet. I like ATI, but even more I like competition that drives the industry. Do we proclaim ATI the winner/loser on this one? Heck no, it isn't even a purchaseable product yet.
ElMoIsEviL - Tuesday, May 31, 2005 - link
23 - They ran Doom3.It's not an ATi game at all as we all know. And it still does REALLY well. And it's not in release stages yet.
;)
ElMoIsEviL - Tuesday, May 31, 2005 - link
hehehehe.. it's better then SLi... heheheheFigures, all the NV on here prolly aren't too happy today.
I can't wait to test out the new AA modes.. :)
vertigo1 - Tuesday, May 31, 2005 - link
This is insane, who on earth will buy this?!JarredWalton - Tuesday, May 31, 2005 - link
30 - Yes. The PCIe bus likely provides slower performance, as it is used for lots of other things (like communication between the CPU, RAM, and GPUs). I believe NVIDIA SLI works without the dongle but at slower speeds - at least, I heard that somewhere, but I haven't ever had an SLI board so I can't say for sure. Anyway, since DVI is a digital signle, using DVI in/out seems about as good as the SLI bridge - at least in theory. Now we just need to wait and see how theories pan out. :)Jalf - Tuesday, May 31, 2005 - link
I was under the impression they were going to use the PCI-E bus for transferring data between the cards. Is the external dongle going to handle that instead?Murst - Tuesday, May 31, 2005 - link
I really don't see how the xfire is better than sli based on hardware compatibility. Sure, you don't need the exact same cards, but you will likely buy only one x850 type card per x850 xfire. It would be extremely unlikely that someone upgrades from x850 xt pro -> x850 xt pe.Basically, in the end, you will buy a specific xfire tailored to your gfx card, and throw it away with the next generation of cards.
gxsaurav - Tuesday, May 31, 2005 - link
Great this just means more heat, man, even a single 6800 nU playes everygame fine, while running coolViRGE - Tuesday, May 31, 2005 - link
#21, yes it is. This is what hurts ATI the most, Nvidia already had 4 release cycles of experience with motherboards(2 of those being highly popular, highly recommended boards) before attempting SLI. ATI has a previous launch for a board almost universally ignored. I would not use an ATI board at this time, so I would also not consider CrossFire. ATI needs to get CrossFire working on Nvidia's boards to have a fighting chance this round.