At the end of September we published our on Tweakers review of the RTX 3090. But what’s thicker than the fastest video card of the moment? Exactly: twice the fastest video card of the moment. Armed with two GeForce RTX 3090 Founders Editions and Nvidia’s new NVLink bridge, in short, almost EUR 3,200 in GPU hardware, we are investigating the performance of this monster combination.
SLI in short
The term SLI stands for scalable link interface and was first used by GPU manufacturer 3dfx in the late 1990s. Nvidia acquired the brand name through a takeover and breathed new life into it for a technique to combine the computing power of multiple video cards. In recent years, this has usually been done on the basis of alternate frame rendering, whereby the video cards alternately render an image. An inherent disadvantage of this is that the time between images (the frame time) can vary because not every frame is ready at the same time. You perceive that as ‘stuttering’.
With the arrival of DirectX 12, everything would be different – and above all better. This api contains a technique that explicit multi-adapter has been baptized and thus a game can control multiple video cards itself. It is even possible to mix completely different video cards from AMD and Nvidia. However, the vast majority of DX12 games do not apply this technique, while Nvidia officially does this summer pulled the plug out of the ‘old’ way with SLI profiles in the driver.
The SLI bridge that you used to get with every nice motherboard has not been used for a long time. After a high bandwidthversion of that bridge, Nvidia switched the RTX 2000 series to NVLink, a much faster interconnect that originated in the data center world. The same technology is used for the RTX 3000 series, but due to the changed connectors you still need a new bridge.
The controlled death of SLI
In the past you could put just about any video card in SLI. AMD’s counterpart CrossFire was even more flexible on that point: you could even mix different GPUs. If we look at the last four GeForce generations, it is noticeable that support for SLI has been phased out further and further. In the past, for example, you could still easily combine two GTX 960s with each other, with the 10-series you needed at least a GTX 1070 for this, and with the previous generation it was only possible from the RTX 2080. In the new series, only the RTX 3090 offers support. for SLI.
| RTX 3000 | RTX 2000 | GTX 1000 | GTX 900 | |
| **90 / Titan | And | And | And | And |
| **80 | No | And | And | And |
| **70 | No | No | And | And |
| **60 | No | No | No | And |
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SLI with the RTX 3090
If you continue the line, the next generation will end the story for SLI. Now ‘being an SLI enthusiast’ is in any case comparable to ‘being a Feyenoord fan’: over the past three generations, functionality has become increasingly limited while the required hardware has increased in price. The NVLink bridge required for the RTX 3000 series uses different connectors than the previous one for the third consecutive generation. Moreover, it costs no less than 85 euros.
For this test we built a different test system than in our regular GPU reviews. Where PCI Express 4.0 according to our tests adds little, the mainstream platforms of AMD and Intel reduce the GPU bandwidth by half again. This is due to the limited number of PCIe lanes. To prevent this and also minimize the expected CPU bottleneck with so much GPU power, we performed these tests on a HEDT platform, about which more on the next page.
This test was made possible by the tweakers Shroudster in Rsnubje, who kindly made their 3,090 FEs available on a temporary basis. Thanks!
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