What is SLI?

NVIDIA’s Scaleable Link Interface

NVIDIA's SLI isn't 3dfx scan line interleave technology reincarnated six years later. Instead, SLI is short for Scaleable Link Interface. When NVIDIA bought 3dfx’s assets in late 2000, they acquired the rights to the Voodoo brand, including SLI. With the enormous success of 3dfx SLI, NVIDIA’s marketing team felt it made perfect sense to adopt the name to familiarize consumers with their multi-GPU technology.

SLI works similarly to the rendering techniques employed by Alienware for their X2 video array system and Metabyte’s PGC technology in the sense that the graphics cards split the workload horizontally across the screen. One card takes the upper portion while the second card takes the lower segment.

It’s important to note that the screen isn’t necessarily split exactly in half. As we discussed earlier, in some scenes the lower portion of the screen may be more complex than the upper portion, or vice versa. SLI is designed to take this into account, splitting the work properly between both cards.

One GPU handles the upper portion in 3DMark, while the second GPU gets the bottom

Notice it isn't a perfect 50/50 split

Sounds simple, but exactly how does it work?

Once two PCI Express cards are installed in the system, NVIDIA SLI kicks in. Since Intel’s Tumwater Xeon chipset (the only known Intel chipset with plans for dual PCI Express motherboards) only provides for a total of 24 lanes, one card operates in the full x16 PCI Express configuration, this card is designated as the “master”, while the second PCI Express card runs in x8 mode, this card becomes the “slave”.

System configured with one PCI-E GeForce 6800 Ultra card

Now a second card is added

The graphics driver then determines the workload for both cards depending on the scene. NVIDIA has developed their own patent-pending dynamic load balancing algorithms. These algorithms are crucial to ensuring that the workload is split appropriately among both cards, with the master card getting a little more work than the slave board. The master card is also responsible for outputting the final image to your screen.

Cards are now linked together for SLI

SLI system from opposite angle

Both cards are linked together via the multi-purpose I/O (MIO) port, and physically connected to each other via an SLI connector. Unlike 3dfx’s pass through cable which was analog, NVIDIA’s SLI connection is completely digital. This ensures that the data sent between the two cards is consistent; sometimes color data didn’t match correctly with Voodoo2 SLI, tearing would also occur occasionally. We weren’t given a number for the peak bandwidth of the link between the two cards, but the figure must be massive to ensure optimal performance.

The SLI connector

Bottom of the connector

NVIDIA’s SLI isn’t as simple as developing a special SLI connector and throwing a driver around it all either. NVIDIA had to come up with their own inter-GPU communication protocol and dedicated scalability logic as well. You can see this in the following die shot, taken from an NV40 (GeForce 6800) GPU:

NV40 die shot