Nvidia SLI on 2 × Titan RTX in the test

– CrossFire and SLI could not convince in the last test on BitcoinMinersHashrate.
However, both GPUs were only connected with 8 PCIe lanes each, which was criticized by readers. BitcoinMinersHashrate SLI is therefore testing again with up to two 16 PCIe lanes and for the first time via NVLink. Nothing changes in the conclusion.

SLI with 2 × Nvidia Titan RTX

The last multi-GPU in-game test just over half a year ago showed that AMD CrossFire and Nvidia SLI are no longer worth recommending in games these days. Because no matter whether AMD or Nvidia, CrossFire or SLI, the gaming experience is almost always worse with two graphics cards than with just one – and with this observation it is ignored that most even the halfway current games no longer support a multi-GPU at all.

However, one technical feature can still help the two technologies: The editorial team had carried out the tests on a common consumer platform at the time, so that both graphics cards had only 8 instead of the full 16 PCIe lanes available. This has been criticized by readers. And now the technology has also evolved: With Turing, SLI can now access NVLink as a connection between the two GPUs. The bandwidth at SLI with a Turing graphics card is therefore up to ten times higher than with the old high-bandwith bridge (SLI-HB).

2 × 16 lanes vs. 2 × 8 lanes for SLI

For another test, BitcoinMinersHashrate has therefore once again grabbed the previously used Mifcom computer with two Titan RTX graphics cards, which relies on an X299 mainboard and thus on Intel's Skylake X platform. The installed Core i9-9900X controls both graphics cards with 16 PCIe lanes each. The system must then prove itself against an ordinary Kaby Lake computer that only offers 8 lanes each. One finding: The full 16 lanes per graphics card and NVLink actually help SLI. But that doesn't change the conclusion about SLI (and CrossFire).

Two different test systems for x8 and x16

It should be borne in mind that two different systems are used for the two configurations: an Intel Core i7-8700K with six cores, Z370 mainboard and 32 gigabytes of memory on one and an Intel Core i9-9900X with ten cores, X299 mainboard and 64 gigabytes of memory on the other hand. This is the only way to achieve the desired effect of changing from two 8 PCI lanes with the first system to two 16 PCIe lanes with the second.

All components run with the factory settings. As a result, the Skylake-X system has significantly more CPU cores and thus, with some programs, a much higher processor performance than the Kaby Lake computer, but is slightly disadvantageous in games.

High resolutions for high graphics loads

To compensate for this, all games are tested in the resolutions 3,840 × 2,160 and 5,120 × 2,880, achieved by downsampling (DSR). Therefore, the different computer configuration plays in the tests barely another role. For the highest game performance, it is almost only a question of the graphics cards used, which are identical to two Nvidia Titan RTX, and the PCIe lanes.

The GeForce 417.71 is installed as the graphics card driver. The games use Call of Duty: Black Ops IV, Far Cry 5, Kingdom Come: Deliverance, Middle-earth: Shadow of War, Shadow of the Tomb Raider (DirectX 12) and Star Wars Battlefront II. SLI works in all six titles, otherwise the advantages of the double PCIe lanes could not be shown. The details are maximized, only in Kingdom Come: Deliverance is switched back to the high preset.

Benchmarks in 3,840 × 2,160 and 5,120 × 2,880

The first finding when looking at the rating is: On average across all games tested, the test system with two 16 PCIe lanes in Ultra HD resolution delivers 3 percent less FPS than the system with two 8 PCIe lanes. The two weaker connections do not limit the resolution enough to compensate for the lower gaming performance of the CPU of the X299 system.

The situation is no different with the frame times. At least on average, the double PCIe lanes are useless and are 4 percent behind the x8 system. Regardless of this, there is no advantage in frame times with SLI: a single Titan RTX performs slightly better in the measurement series.

The results with SLI fluctuate massively

A look at the individual titles shows how different the result can be with SLI. There are titles in which SLI works properly. This increases the frame rate in Middle-Earth: Shadow of the war by 55 percent through the use of a second Titan RTX and the frame times still climb by 32 percent. Kingdom Come: Deliverance, on the other hand, is completely different: the FPS only increases by 24 percent and the frame times are even around 35 percent worse than with a single graphics card.

Call of Duty: Black Ops IV and Shadow of the Tomb Raider also show two games in which the system has advantages with Titan RTX connected twice via 16 PCIe lanes. These are 11 or 6 percent in Black Ops IV and 3 or high 25 percent in SotTR.

Even more GPU load improves performance

In 5,120 × 2,880, the SLI system can stand out a little better. Two Titan RTXs then deliver an average of 47 percent more pictures per second, which is a good result. It is the same for x8 and x16 connections, so the latter could catch up. The system with two 16 PCIe lanes is now on average 12 percent better in frame times than with a single graphics card. The system with two 8 PCIe lanes, on the other hand, reaches exactly the level of a single graphics card.

Frametimes with big problems

The "99th percentile" shown does not yet reveal the actual problem, except in the case of Kingdom Come: Deliverance. Because even if the frame times for the Turing generation with NVLink are actually better than for the Pascal predecessor with the classic SLI bridges, they remain bad. This is much more noticeable when you look closely at the course than in the line diagrams.

Frametimes remain an absolute disaster regardless of the SLI system in Kingdom Come: Deliverance. Even those of a single graphics card are not perfect, but with two Titan RTX the title is simply not playable. The outliers are very large with only two x8 lanes, but there are also serious problems with two 16 PCIe lanes. The two graphics cards perform best in Call of Duty: Black Ops IV. Although the x8 system shows a large and clearly noticeable hanger, it does not exist with two x16 lanes. Black Ops IV runs quite well.

All other games are somewhere between the two extreme cases. However, all three titles have in common that the frame times are worse than with a single Titan RTX. In every game there are big outliers that are always noticeable. As a result, a game on two graphics cards may feel better than one for most of the time, but then the picture suddenly freezes.

In addition, the diagrams show that in most cases, SLI with twice 16 PCIe lanes offers slightly better frame times than with twice 8 PCIe lanes. However, this does not always apply, because there are also titles where the results are the other way round. For example, a Skylake-X system with more lanes for SLI (and CrossFire) is not always better than a faster Kaby Lake system with only two times eight lanes.

Conclusion

"You have to say it so tough: Both CrossFire and SLI leave a simply catastrophic impression in modern games“Was the conclusion in the last multi-GPU-in-game test in 2018. Too few games offered the appropriate support and those who could handle it rarely showed a satisfactory result.

At that time, however, older Pascal graphics cards with classic SLI bridges were used in two PCIe slots, each with only eight PCIe lanes, which was criticized by the community. BitcoinMinersHashrate therefore repeated the test with the Turing architecture, which for the first time relies on NVLink as the connection between two GPUs at GeForce. And the difference between 2 × 8 and 2 × 16 PCIe lanes was examined again. But that doesn't change the conclusion.

Although SLI on Turing runs a little better than SLI on Pascal, which is primarily due to the higher bandwidth over the NVLink, and the double PCIe lanes per GPU also bring a little boost in some games. But that doesn't change the fact that most new games – and this also applies to Triple-A titles – no longer support multi-GPU at all. And the titles that can handle it often show very bad frame times. So the game feeling remains worse than with a single graphics card.

Multi-GPU: There are usually problems

Out of ten games that still support CrossFire and SLI, maybe two are ok. The other titles show small to very big problems. Enthusiasts should also keep their distance from SLI or CrossFire. You have to know very well in which games or benchmarks the technology is still an advantage in order to be able to justify the purchase of a second graphics card using these titles.

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