NVLink: A Bridge To…Anywhere?
TU102 and TU104 are Nvidia’s first desktop GPUs rocking the NVLink interconnect rather than a Multiple Input/Output (MIO) interface for SLI support. The former makes two x8 links available, while the latter is limited to one. Each link facilitates up to 50 GB/s of bidirectional bandwidth. So, GeForce RTX 2080 Ti is capable of up to 100 GB/s between cards and RTX 2080 can do half of that.
One link or two, though, SLI over NVLink only works across a pair of GeForce RTX boards with at least one empty slot between them for airflow. Officially, Pascal-era GPUs endured the same two-card maximum. Technically, however, as many as four top-end GeForce GTXes could be made to work together in a handful of benchmarks. These days, you’ll also have to purchase your own GeForce RTX NVLink Bridge for multi-GPU connectivity. Three- and four-slot sizes are both available for $80 from Nvidia’s website.
Some of the trouble last generation was caused by bandwidth constraints between SLI bridges. Compared to the original SLI interface’s 1 GB/s MIO link, Pascal’s implementation drove ~4 GB/s. That was fast enough to get the second card’s rendered frame back to the primary board in time for smooth output to a 4K monitor at 60 Hz. But it wouldn’t have been able to keep up at 120 Hz and higher, which is where today’s highest-end gaming displays operate.
Even in a single-link configuration, NVLink can move data so quickly that SLI on an 8K screen is possible. Driving three 4K monitors at 144 Hz in Surround mode is no problem at all. Two x8 links have the throughput needed for 8K displays in Surround.
Really, the question is: who cares anymore? AMD and Nvidia did such a good job of pumping the brakes on multi-GPU configurations that Tom’s Hardware readers rarely, if ever, ask for benchmark results from an SLI setup. Back in the day, value-minded gamers used SLI to match the performance of higher-end cards. Nvidia put a stop to that by removing support from lower-end models in its product stack. Now, even GeForce RTX 2070 lacks an NVLink connector. Older DirectX 11-based games still run well across two cards, and a handful of DirectX 12-based titles do exploit the API’s explicit multi-adapter control. But the fact that developers like EA DICE are pouring time into taxing features like real-time ray tracing and ignoring multi-GPU says a lot about SLI’s current importance.
We’ve heard Nvidia representatives say they’ll have more to discuss on this front in the future. For now, NVLink support on GeForce RTX 2080 Ti and 2080 is a novelty, particularly as we’re able to focus on playable frame rates at 4K and G-Sync technology to keep the action smooth.
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