SLI Revisions / The Enthusiast Key, An Ecosystem Locked
SLI has been synonymous with NVIDIA for more than a decade now but things are changing and that’s a good thing since this standard has been a bit stagnant for a good amount of time now. However, let’s start this section off by preemptively stating that NVIDIA isn’t killing triple and quad SLI support. Rather they are being somewhat forced towards some changes by prevailing market conditions and the future of graphics APIs.
In order to see where NVIDIA is going with everything I’m going to describe below, its important to understand how the data paths work between more than three cards. In a three and four card setup, the information from the lower GPUs had to be passed through the “higher” ones in a kind of Rube Goldbergian-style setup since the topmost “master” GPU was the one outputting the rendered frames towards the display.
Since Pascal is capable of processing such a high amount of information and with the advent of UHD HDR, 5K displays and high resolution multi monitor setups, its entire SLI interface needed an overhaul. The PCI-E interface couldn’t be used since even in its 3.0 iteration there was a very real chance of saturating it with both CPU calls and inter-GPU communication. Meanwhile, the existing interconnect on Maxwell and previous generations just couldn’t keep up with the requirements.
Naturally NVIDIA needed a solution so they simply upgraded their existing setup to the point where it is now capable of doubling the legacy connection’s bandwidth. Thus the so-called High Bandwidth SLI (or SLI HB) bridge was created. It is capable of interfacing with two GPUs across a dual link parallelized interconnect running at 650MHz compared to the single channel 400MHz of flexible SLI bridges. On the positive side of things, backwards compatibility of a sort was retained (more on this below) with existing bridges but that doesn’t mean there wasn’t any collateral damage.
With the advent of UHD HDR, 5K displays and high resolution multi monitor setups that high bandwidth connection will be necessary to deliver an optimal gaming experience on the GTX 1080. The older SLI bridges are still fully compatible but they may encounter understandable bottlenecks when trying to operate in scenarios where massive amounts of information are passed back and forth. Meanwhile, the so-called present generation rigid LED bridges are recommended (again, not required) for anything up to 4K resolution gaming since when used with a Pascal card, their effective clock speed will be increased to 650MHz though only they will still operate in single channel mode.
This brings us to the HB SLI Bridge which is built for next generation display standards or higher resolution surround setups. The graph above shows one usage scenario with a pair of cards running the new / old SLI bridges while gaming on an 11520x2160 (triple 4K) surround setup. While this situation is truly niche and I doubt even a pair of GTX 1080 cards would achieve acceptable framerates without a significant downgrade in image quality, there is a difference between the two results. The older SLI bridge exhibits quite a few frametime spikes while the newer HB SLI connection does without most of the drama. Honestly though, the possibility you will notice the difference between 5ms spikes and ones which extend to about 10ms is debatable though it is completely chartable as evidenced by NVIDIA’s results above.
The Enthusiast Key - A 3/4-Way SLI Ecosystem Locked Away
Now we come to the crux of the matter: with a rigid 2-way connector, what the hell is NVIDIA doing with triple and quad card SLI? Like the interface changes above this will likely require a relatively long-winded explanation.
Now for the longer explanation and for that I’m going to have to get into the current situation with multiple GPUs, of which there are three modes present in the changes Microsoft has made to DX12: MDA, LDA Implicit and LDA Explicit. MDA or Multi Display Adapter was implemented to give developers more control over how and where GPU resources are made available, though each GPU’s memory pool remains independent. It also allows for different GPUs to be utilized with one another since they aren’t directly linked together with a bridge. Rather they communicate with one another through a secondary interface like the PCIe bus. NVIDIA is fully supporting this standard going forward. Since MDA’s functions and its interaction between the application and GPU is fully controlled by the developer it could represent a glimmer of hope for anyone who hopes to use more than two Pascal GPUs.
LDA Implicit is where the current iteration of SLI resides. Within its walled garden, the GPU memory pools can interact with one another so they appear to be one large pool of memory to the OS and associated applications. Unlike MDA, Linked Display Adapter Implicit gives the display adapter’s driver stack much of the control over GPU functions and this is where NVIDIA believes they can best affect the experiences of their users. Meanwhile, they don’t support Explicit LDA since like MDA it grants the developer more control over the GPUs’ functions but requires there be a direct link in place.
While NVIDIA would love to sell you more than two $700 GPUs, they’ve found current processors and game technology just can’t keep up with what Pascal has to offer. As a result, positive performance scaling improvements past two cards will be extremely rare and will certainly not happen as consistently as users would expect.
With all of this taken into account, NVIDIA still allows for more three or four to be used in SLI but they will no longer be recommending gamers use more than two cards, nor will they supporting larger setups on Pascal cards. Legacy products in the Maxwell and Kepler products will still be supported though.
If you do want to run more than two Pascal-based cards over SLI (as stated above, MDA is another matter), you’ll need to request something called an Enthusiast Key from NVIDIA. In order to do so, there will be a few steps. No, really.
The first step will be to run an app locally to generate a “signature” for your GPU and then use that to request an Enthusiast Key from the NVIDIA Enthusiast Key website. That key can then be used to unlock 3 and 4-way SLI functionality for Pascal cards (again, older products won’t need to go through this process). Trust me, you aren’t the only one throwing your hands up in the air right now since I also think this is patently ridiculous.
So is NVIDIA blocking or stopping 3+ card SLI? Yes and no. On one hand they won’t be actively developing driver patches in a fruitless effort to optimize triple and quad card performance on upcoming games that are actively working to dissuade such setups. However, NVIDIA taking a hands off approach shouldn’t be taken as a bout of ambivalence either. There may be good reason for this new stance: with DX12’s unique Explicit Multiadapter feature allowing for more multi-GPU flexibility developers now have the ability to build additional functionality for the likes triple-SLI directly into their appications. It has me wondering whether maybe, just maybe NVIDIA is doing the right thing here by focusing resources on their core ecosystem while putting the onus on developers to properly implement support for higher levels of multi card support. Time will tell I guess…..
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