The Bridge

Introduction

Anyone who has followed PC gaming over the years knows that to achieve the highest possible framerates, running dual video cards has been an attractive solution. It allows you to experience next-gen performance today, at the cost of additional power use, heat... and money, of course! But dual-cards systems have always had their detractors, both for the reasons mentioned, and also due to perceived deficits in SLI performance, whether due to poor scaling or jumpy framerate pacing. AMD got into hot water a few years ago regarding its Crossfire system, which offered high framerates in benchmarking tools available at the time, but was revealed to be producing far fewer usable rendered frames than was originally thought. AMD addressed the issue with an updated specification for Crossfire that used bandwidth available on the PCIe bus, ending the use of modular connectors. Nvidia had gotten around that by using a frame-pacing technology, so that only usable frames were rendered, but it still had an issue with the bandwidth available on its modular SLI bridges, released back in 2004.

So, for the first time in years, Nvidia has released an official update to the SLI specification. With the Pascal generation of video cards, Nvidia has now enabled dual-link SLI. While previous Nvidia GeForce video cards that supported SLI have typically included dual SLI "fingers", attaching links to both had no effect. The only purpose of the dual fingers was to enable three-way SLI, wherein you'd attach cards #1 and #2 using one set of fingers, and cards #2 and #3 using the other set of fingers. Well, with Pascal, Nvidia giveth and taketh away. SLI is no longer supported for more than two cards (with minor exceptions for benchmarking applications used by competitive overclockers), but Nvidia has found a new and perhaps better use for those dual SLI fingers. Because modern GeForce cards pass so much more data in between each other in SLI mode due to their higher performance versus older cards, the SLI link had become a bottleneck. So Nvidia simply enabled both fingers to be used by the same pair of cards. It claims that this is at least as good as using shared bandwidth on the PCIe bus, as AMD does with Crossfire. 

Test Setup

For our testing we used two EVGA GeForce GTX 1080 SC 8GB models purchased at retail. These feature a factory overclock of 100MHz on the core, which equates to around 5% when boost is taken into account. Note that TBG has a policy of purchasing all CPU and video card-related gear at retail, rather than requesting press samples, as wel feel this is the only way to avoid potential conflicts of interest when it comes to reporting on the performance of these products. If you would like to support our efforts, please feel free to use any of the product links provided in our articles the next time you're in the market for tech gear!

Bridge Back

As we stated above, Nvidia has updated the SLI spec to use dual links. But Nvidia actually went a step beyond that, introducing a special "HB" SLI bridge, which can be ordered only at GeForce.com. It's a solid connector featuring two embedded SLI bridges working in tandem. In fact, each SLI works at a higher speed, 650MHz, than the original flexible connector, which operated at 400MHz. That means the total link speed has increased from 400MHz to 1300MHz. Clearly, there's a lot more bandwidth on the table with this new link design.

To test how this all works out in practice, we're using an EVGA SLI HB Bridge, purchased at retail, along with two standard-issue flexible SLI bridges, included with Asus motherboards (we have several, and each includes one). We actually ordered an Nvidia-branded HB SLI connector in early August 2016 for purposes of this article, but two months later, it still hadn't arrived (despite appearing in stock on Nvidia's website, mind you). So we gave up on Nvidia and went with EVGA's own HB SLI bridge, which can be seen above. Just like the Nvidia connector, it's a non-flexible bridge featuring dual embedded links. This model must be purchased to match the PCIe x16 slot spacing on your motherboard, which on modern boards would be slots 1 and 4, or alternatively 2 and 5. This is what EVGA calls 1-slot spacing, confusingly, as there's one slot in between the mounted cards, while Nvidia and other board partners refer to it more accurately as 3-slot spacing. Ah well, it's just semantics after all, but we know some consumers are going to fall into the trap of ordering the wrong (expensive) HB SLI Bridge and wonder why all this couldn't be a bit simpler.

The Bench

Here are the specs (and a pic) of the system we used for benchmarking:

  1. CPU: Intel Core i7-6900K, overclocked to 4.4GHz
  2. Motherboard: Asus X99-Pro/USB3.1
  3. Video Cards: 2x EVGA GeForce GTX 1080 SC 8GB
  4. Standard SLI Bridges: 2x Asus Flexible SLI Bridge
  5. High-Bandwidth SLI Bridge: EVGA PRO SLI Bridge HB, 1 Slot Spacing (100-2W-0026-LR)
  6. RAM: Corsair 4x8GB Vengeance LPX DDR4-3200
  7. SSD #1: Samsung 950 Pro M.2 512GB 
  8. SSD #2: Samsung 850 Evo 1TB
  9. Case: SilverStone Primera PM01 
  10. Power Supply: EVGA Supernova 1000 PS 
  11. CPU Cooler: Corsair Hydro H100i v2 
  12. Operating System: Windows 10

To eliminate bottlenecks as much as possible, we chose to use our X99-based benchmarking system, which we found in our previous look at SLI performance on the X99 and Z170 platforms can increase scaling by as much as 10%. We also overclocked our Core i7-6900K processor to 4.3GHz, which is at the stability limit for most Broadwell-E chips. Our RAM ran at DDR4-3200, also close to the limit for the X99 platform, but with quad channels available, memory bandwidth is most definitely not an issue, and in fact game performance really doesn't scale beyond DDR4-2666, as we found in our DDR4 Gaming Analysis.

For our testing, we're using one benchmark test and five games: 3DMark Time Spy, Crysis 3, Battlefield 4, Far Cry 4, The Witcher 3, and Rise of the Tomb Raider. As you can see below, Nvidia markets the HB SLI Bridge for use with a number of demanding formats, including 2560x1440 gaming at 120Hz, as well as 4K gaming. To test whether it really makes a difference at those settings, we chose game settings that brought us as close to the suggested standards as possible. That meant Crysis 3, The Witcher 3, and Rise of the Tomb Raider, along with the Time Spy benchmark, were run at 2560x1440, which translated to performance very close to 120fps. The reamining games (Battlefield 4 and Far Cry 4) were run at 4K, as their 1440p performance was off the charts (close to 200fps), while their 4K performance was right around 120fps, making for a nice even comparison with the other games. 

Nvidia marketing

OK, now that we've explained the method to our madness, it's time to show you the results! 

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