Quad SLI Explained: PCI Express x48 Bridge Improves the Bandwidth
Certainly, to develop a system that uses four graphics processors to render a frame is not easy and involves both hardware and software technological expertise. One of the problems that developers of multi-GPU technologies usually face is bandwidth between graphics chips. The processors should communicate between each other in various cases, especially in “scissor” mode also known as “split frame rendering”, when the card that is going to display a frame should get the rest of it from another board as rapidly as possible.
In order to ensure that there is plenty of bandwidth between the chips and, perhaps, provide its GPUs some new capabilities, Nvidia has invented a special PCI Express x48 bridge, which, basically speaking, allows two GPUs to communicate with each other using full-speed PCI Express x16 bus and also to have a “shared” PCI Express x16 lane to the rest of the system.
Such an approach allows the company not only to ensure relatively speedy chip-to-chip interconnect, but also to allow such dual-chip graphics cards to work on mainboards that do not officially support the SLI technology, for instance, on Intel’s platforms. In the situation of its 4-way SLI Nvidia ensures that the bandwidth provided to each chip is be optimal.
It is not clear whether such method with additional chip is efficient in terms of performance compared to methods with no bridges, but it may have a tangible advantage if the chip (or driver) can determine priority for each data stream, something which may reduce data latencies. Moreover, Nvidia has been using its PCI Express <=> AGP bridge for several years now without a problem.