CrossFire - More Details
There are a few other details that we'd like to point out with regard to the particular Radeon X850 XT CrossFire configuration we'll be looking at here. We mentioned the custom dongle on the previous page, but would also like to point out some issues pertaining to this particular setup.
The external dongle is equipped with a single DVI input, and single DVI output, and a male CrossFire DMS-59 connector. As you can see in the images above, the pins in the DMS-59 connector are quite dense. Having a connector with a multitude of small pins like this one means it is easier to bend pins, or somehow damage the connector. So if you're itching for CrossFire, be careful when setting things up. We wouldn't want to have you RMA the dongle right away. NVIDIA's internal link is definitely more desirable.
The use of an external dongle also means that multiple connections have to be make between your display and graphics card. And if you've got an analog display, an extra DVI-to-DB15 adapter will have to be used as well. We tested CrossFire in this type of configuration, and experienced a couple of issues. If we didn't securely tighten every connector on the dongle -- and we mean tight -- we saw a pronounced screen flicker, especially at lower resolutions. After reseating the dongle and making sure the connections were tight, we didn't see the same flicker, but know that making multiple connections between the display and graphics card doesn't help and makes the circuit more susceptible to noise.
Another issue with our particular CrossFire configuration is expansion. Should retail motherboards be introduced with the same slot configuration as this reference mobo seen here, you'd be lucky to install an additional half-height PCI card with a pair of X850s installed in the rig. The dual-slot coolers encroach on the PCI slots, but there is room at the bottom of the PCB to change the layout.
You may also notice that there is no PCI Express lane configuration card between the PEG slots on this board, like the ones found on most nForce 4 SLI boards. Until newer versions of the Radeon Xpress chipset with 32 PCI Express lanes are introduced, the 16 lanes dedicated to graphics are split in an 8x8 configuration by default. Should you use only a single graphics card however, a small card can be inserted into one of the PEG slots to rededicate all 16 lanes to a single slot.
So, what does all this get you? We explained the ideas behind ATI's CrossFire multi-GPU technology in our initial article back in May, but essentially the benefits are very similar to NVIDIA's SLI, with the addition of another rendering mode. CrossFire offers four different rendering modes in total, three of which are geared towards performance and one for improving image quality. The modes include:
Alternate Frame Rendering (AFR)
In this mode, all even frames are rendered on one VPU, while all odd frames are rendered on the other. The completed frames from both GPUs are sent to the Compositing Engine on the CrossFire Edition board, which then sends them on to the display in turn.
In this mode, each frame to be rendered is divided into a number of tiles in an alternating checkerboard pattern, such that half of the tiles are assigned to each of the two VPUs. Because the tiles are kept fairly small (32x32 pixels), this method does a good job of balancing the workload across each VPU regardless of what is being drawn on the screen, and it does so without any software overhead. This mode is only available when two matched 16-pipe cards are used. When a 12-pipe slave card is coupled with a 16-pipe master card, Supertile mode is not available.
In this mode, each frame is split into two sections, with each section being processed by one VPU. The split can be horizontal or vertical, and it can be even (50/50) or uneven ( such as 60/40 or 70/30 ) The ideal configuration is determined automatically for each application
CrossFire Super AA
The new CrossFire Super AA modes take advantage of the programmable sample capability of SMOOTHVISION HD to provide a higher quality of anti-aliasing on multi-VPU systems. It works by having each VPU render the same frame with anti-aliasing enabled, but uses different sample locations for each. When both versions of the frame are completed, they are blended in the CrossFire Compositing engine. The resulting image has effectively twice the number of samples, so 4x and 6xAA become 8x and 12x Super AA respectively.
ATI's drivers, or more specifically Catalyst AI, automatically determine the optimal rendering mode to use, based on the specific application or game being run. The default rendering mode is dependant on both the hardware configuration and the application being run. The default modes are either Scissor or SuperTiling mode, but AFR will be used most often. Scissor mode is the default mode for OpenGL applications and SuperTiling will be used only on D3D applications. End user's, however, aren't given very much control over the situation. NVIDIA definitely has an advantage here as well, as they make it easy for end users to experiment with different rendering modes.