This processing arrangement removes
the overhead of synchronizing the two processors from the PCI-Express bus,
allowing improved performance. The video bus link itself apparently runs at up
to 10GB/s, though we doubt that this bandwidth is fully utilized.
Currently, the only nVIDIA SLI-compatible video processors are the
Geforce 6600GT, 6800, 6800GT and 6800 Ultra. The graphical processors in each video card must
be identical, as must the video BIOS revisions, though the cards can run
at separate speeds; the SLI system will assume the lowest clock
speeds for both cards.
This means that it is going to be pretty much essential
to have two identical cards from the same manufacturer to get SLI working
correctly. nVIDIA has introduced a certification program to ensure that users
can find compatible products.
SLI rendering process uses one of two modes: Alternate Frame Rendering (AFR) and
Split Frame Rendering (SFR). AFR has each video card render a separate
frame, while SFR, the method that has gotten more publicity, uses each GPU to render
part of one frame.