ATI's Multi-GPU Solution: CrossFire

Final Words

Anyone need a quick recap?

On the hardware side, ATI is launching a multi-GPU solution called CrossFire that can be added to any existing Radeon X800 or X850 graphics card when run in (to be verified) any motherboard with 2 physical x16 PCI Express slots. Also being announced is ATI's push further into the high end chipset space with their Radeon Xpress 200 CrossFire-Edition. This chipset can be used on motherboards for Intel or AMD solutions and will provide 2 x16 physical, x8 electrical PCI Express slots for CrossFire support. ATI CrossFire-Edition motherboards could also support NVIDIA's SLI cards if NVIDIA's drivers were properly adapted.

Setting up CrossFire on a system that uses selector ICs to allow BIOS control of PCI Express slots makes hardware installation easier than SLI. All that is required is to insert the graphics cards and then connect them with the external dongle. Of course, not all ATI solutions will include this feature. Using SLI, a bridge must be installed inside the case. This is a more elegant solution than a dongle, as it is out of sight, but switching from CrossFire mode to a 4-monitor setup is as easy as changing the way cables are plugged into the back of the computer. In order to use multi-monitor configurations on an NVIDIA SLI board, the SLI bridge must be removed. We don't consider the SLI selector card on the motherboard to be an advantage or disadvantage, as some of ATI's partners will be implementing selector cards or terminator cards rather than the BIOS configurable selector ICs. Really, both companies have pluses and minuses, and we leave it to the end user to decide whether the internal bridge or external dongle fits their needs better.

As far as software support goes, CrossFire will offer AFR, split frame, and supertiling rendering modes. Two new types of AA (called Super AA modes) will also be enabled by CrossFire: 8x/10x and 12x/14x with combine MSAA (the latter two modes including SSAA). We expect the same games (or types of games) to run using one of the three performance modes (AFR, split frame, or supertiling) and run well under SLI, while all games will be accelerated under any Super AA mode.

Had enough yet? Our initial performance test on prerelease hardware and drivers shows roughly 50 to 85 percent improved performance under Doom 3 from CrossFire. This indicates that we could see very good performance from CrossFire when it is finally released. Our initial tests aren't enough to draw any firm conclusions, especially in comparison to SLI performance, but we are looking forward to running a full suite of tests on the hardware.

The down sides of CrossFire mainly stem from the motherboard chipset. Either adding cost through the selector ICs or limiting convenience with a terminator card or SLI Style selector card is a tough call for vendors. Supporting two x8 electrical PCI Express slots does limit potential bandwidth and therefore, possibilities open to software developers. This isn't any better than what NVIDIA has to offer, so ATI need not worry much here. With vendors either using a ULi southbridge, or the (currently) buggy ATI southbridge, we may want to pay close attention to whose hardware is on the board. As far as the CrossFire card itself, we would prefer to see the 16 pipe CrossFire card not drop to 12 pipes when paired with a 12 pipe card (at least in split frame rendering). ATI's thinking is that the 16 pipe card would always be waiting on the 12 pipe, but in split frame rendering, giving more work to the 16 pipe card would help balance the performance. We just believe in people getting what they pay for.

If ATI can get CrossFire out to the market in good volume (for its potential demand), we could have an excellent alternative to SLI on our hands. ATI is also working on licensing CrossFire to SiS, so we may see SiS based boards with CrossFire support early next year as well. Exhaustive performance tests remain to be run, but from a feature standpoint, CrossFire looks good. We would like to see CrossFire offerings for Radeon cards slower than the X800, but other than that, we will have to sit back and wait for hardware to draw more conclusions.