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(I'm not sure if this is the correct place to ask this, please let me know if should post this somewhere else)

For a long time, using SLI for gaming has been dying out and for good reason. From what I've read, there are a few main reasons why:

  1. It's difficult to support.
  2. Buying 2 graphics cards of the most current generation is very expensive and doesn't have a good price-to-performance ratio.
  3. Buying a current generation card gives you new features and almost always gives similar performance when compared to 2 previous generation cards using SLI.

It seems to me that these reasons should have also applied back when SLI was introduced. If that's the case, then why was SLI for gaming introduced and supported in the first place? Why did it last so long? Finally, what changed to cause it to fall out of favor?

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    Could you please define SLI for those who don't know that abbreviation?
    – DrSheldon
    Sep 4 '20 at 6:32
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    @DrSheldon I suspect he avoided doing so because it's actually two different things, the more modern contorted into the same initials to give it the same name as the older. 3dfx's 1998 take was 'scan line interleaving', in which two cards with completely independent RAM, GPU, etc, coordinate to produce a single video output so that each of them has to render only half the pixels. Nvidia repurposed the initials in 2004 to mean 'scalable link interface' in which up to four cards each render subsections of a frame and composite that into a single framebuffer digitally.
    – Tommy
    Sep 4 '20 at 14:26
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Your second and third points are disadvantages from the user’s perspective. Looking at things from the manufacturers’ perspective, SLI has one significant advantage: it raises the maximum number of graphics cards in a single system, which automatically raises the market’s ability to envision buying more graphics cards.

SLI started with 3dfx, where it was based on alternate scan lines (Scan-Line Interleave). 3dfx designed SLI into their hardware starting at least with the Voodoo Graphics (even though SLI was only marketed starting with the Voodoo²), probably before it was realised that the performance benefit wasn’t as great as initially hoped. Regardless of what 3dfx engineers knew, the Voodoo² was still marketed as providing double the performance when paired in an SLI setup. Even if game support and performance wasn’t as good as users would have wanted, a non-negligible portion of the 3D-accelerator-buying community at the time was driven by bragging rights rather than economic sense, and thus SLI Voodoo² was perceived as “the thing to have”. That might not have translated into all that many actual purchases though; when 3dfx Voodoo² cards were common on the second-hand market, SLI connectors weren’t. 3dfx’s SLI did have one significant advantage: it raised the maximum resolution from 800×600 to 1024×768.

Much of the above applies to Nvidia’s SLI (Scalable Link Interface) and AMD’s CrossFire: it presumably cost less to develop than the missed opportunities in the market resulting from its absence (especially for AMD). The bragging-rights-driven nature of the GPU marketplace continued long after the Voodoo², helped by the advent of 3DMark and the rise of e-sports celebrities and their endorsements; thus for any given generation, an SLI setup would always be more desirable.

As to SLI’s falling out of favour, the cynical answer would be that manufacturers no longer need it to sell multiple graphics cards; they can sell more cards per system for computational workloads nowadays (and at a higher price). SLI is also less visible because, at least in some configurations, it happens over the PCI Express bus rather than a dedicated bridge (XDMA on Radeon).

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It's all about the top end performance.

What can be done if the performance needed is greater than the best card available can offer? Right, using two of them, and that's the whole idea of SLI, adding another card to satisfy demand. Quite the same reason why in the mid 90s dual socket workstations kept popping up to satisfy the demand for power, despite the inability of most software to use more than one core.

It's difficult to support.

No. It isn't. Or better, not more complex than any advanced feature.

Buying 2 graphics cards of the most current generation is very expensive and doesn't have a good price-to-performance ratio.

Of course not. Buying at the top end performance never gives a good performance per money ratio. This is already true when buying a graphics card alone. A top end NVIDIA RTX 3090 is about three times as expensive as a RTX 3070, but gives only about 50% more. Bang per Buck ratio is never good at the top end. Much like with processors, cars, or anything else. Anyone buying for an optimal BpB ratio, will always end at a middle class CPU/GPU/car. If one needs/wants top end performance, the price will skyrocket in a way that buying two cards is quite within reason.

Buying a current generation card gives you new features and almost always gives similar performance when compared to 2 previous generation cards using SLI.

Sure, if one got time to wait for the next generation (like a few years) to play a game at the resolution his screen can provide and in maximum details, fine - but I'm not sure how many gamer there ever were who would wait a few years before being able to play the newest game.

A fair comparison is only possible within a generation - in fact only within a current offering. Looking at that you'll note that low to mid range models (*1) will never offer SLI, AFR, Crossfire or alike (*2), as there is more to gain from using a top end card. Only when a top end card can not supply a required performance coupling them makes sense.

It feels as if the question is written in hindsight with all past options available and no real need for improvement. It hasn't always been that.

It seems to me that these reasons should have also applied back when SLI was introduced.

I dare you to get some Voodoo or early GeForce card and install a 2000s. Compared to these games Lego bricks are high resolution. Graphic cards were far from being able to display what game designers wanted in simple triangles - not to mention higher functions.

And that's why 3dfx developed SLI - coupling multiple chips (2 or 4) on a card to increase performance to satisfy needs by games. Since then development has gone a long way and GPUs did speed up way past a point that is needed to generate even most complex scenes.

If that's the case, then why was SLI for gaming introduced and supported in the first place?

Triangles, Triangles, Triangles.

It allowed more detailed content.

Why did it last so long?

Because there was such a long way for GPUs to deliver a performance sufficient to give a level of detail that is sufficient to display content in a way that closes in to natural.

Finally, what changed to cause it to fall out of favor?

In the game area mostly because, as said, detail level reached a point were adding more doesn't result in much better rendering, so requirements from game designers no longer have to be cut down to fit existing cards.

On a technical level SLI became obsolete with the availability of (local) busses fast enough to exchange abstract data between GPUs instead of linking and joining partitial rendered screens.


*1 - If they did, it's usually to support buyers interested more in bragging not top performance.

*2 - SLI being either 3dfx Scan Line Interleave (~1998) or Nvidia's Scalable Link Interface (~2005). AFR is alternate Frame Rendering of ATI, while CrossFire is it's later (meaningless) name to have GPU chips working on the same result. Nowadays it got replaced by high speed links.

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    Two CPU machines were also desirable because a CPU-heavy process could make the system unresponsible on a single CPU machine. Sep 3 '20 at 7:39
  • True, that' why I had already a dual pentium pro in 1996. Then again, this is about games, which didn't use multiple CPU/cores until rather recent.
    – Raffzahn
    Sep 3 '20 at 7:44

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