[SOLVED] What reasons are there as to why Multi-GPU setups don't scale well?

[joshua123033]

Since many people say that the reason why multi-GPU setups don't scale well in some games is because there is no code for the multi-GPUs. But could another reason be driver support (for SLI and Crossfire).
So TL;DR, what are the main reasons why multi-GPUs don't scale well?

 

[jojesa]

Yes, there are several reasons why they don't scale accordingly.
This Techquickie video explains it in a few minutes.

 

[Barty1884]

The main reason is game developers don't actively code it into games, as the userbase utilizing multi-GPUs for gaming is <1% of the consumer base.

Sometimes SLI/CFx support is baked into the engine, so the option is theoretically there by default. However, without the developers actively pursuing it, the scaling is mediocre at best.

As for drivers.... I don't think any of it could be pinned on drivers* at all, really. If the drivers limited anything, it wouldn't be by design
In titles that actively support SLI &/or CFx, you can see scaling of up to +75% in the best scenarios. You're never going to get a perfect 2x the performance for a variety of reasons, so +75% would be considered an awesome result.

Remember, drivers are from NVidia or AMD. They have no motivation to hinder multi-GPU configurations*. Infact, actively supporting it is in their best interests to move more GPUs.

*However, only at the upper end. The days of SLI'ing two mid-low tier GPUs to rival a single higher end GPU for less money are long gone. Nvidia cut SLI support from the x60 cards as of the 900 series.
IF you're going to go with Multi-GPU's, NVidia or AMD would definitely prefer you bought two of the higher tier cards.