3090 SLI NVLINK HELP

[whargokus]

Hey guys , is there anyone here skilled enough to make old sli profiles ( nvidia inspector ) work on the 3090's ?

[Sweepi]

Hi,

the RTX 3090 does not support SLI (in a Gaming context) at all.
It only supports (as well as any other DX 12 GPU) native Multi-GPU-Titles.

These include: (via https://www.computerbase.de/2020-09/nvidia-sli-unterstuetzung-beendet-2021/)
DirectX-12
Shadow of the Tomb Raider
Civilization VI
Sniper Elite 4
Gears of War 4
Ashes of the Singularity: Escalation
Strange Brigade
Rise of the Tomb Raider
Zombie Army 4: Dead War
Hitman
Deus Ex: Mankind Divided
Battlefield 1
Halo Wars 2

Vulkan
Red Dead Redemption 2
Quake 2 RTX
Ashes of the Singularity: Escalation
Strange Brigade
Zombie Army 4: Dead War

Ashes of the Singularity supports native Multi-GPU since 2016, including support for combining GPUs from different vendors:
https://www.computerbase.de/2016-02/ashes-of-the-singularity-directx-12-amd-nvidia/4/#abschnitt_multigpu_amd_und_nvidia_in_einem_rechner_sind_kein_problem
https://www.forum-3dcenter.org/vbulletin/attachment.php?attachmentid=74398&stc=1&d=1614590886

74398

[whargokus]

I know about DX12 , I was wondering about DX11 ... old style sli with nvidia inspector.
There must be some way to make them work...

[Sweepi]

[i will abbreviate "old SLI" / "DX 11 SLI" / "driver-side Multi GPU support" as SLI in the following text]

well, I have never seen any GPU driver source code, but my guess is as follows:

There is a part of the driver which checks if all prerequisites for SLI are present, which will return false when using 2 3090s. This need to be patched. (comparatively easy)
I strongly assume there is architecture (pascal/turing/ampere) -, perhaps even model (1070/2080/2080 Ti) specific part in the driver which implements details like how the workload shall be split/synced between the GPUs. This part maybe exist for Ampere but is dormant in the official nVidia driver (if you are lucky), only exists inside nVidia's HQ or does not exist to this day, since to my knowledge there is no Ampere product which supports SLI. Therefore, you either need to activate this part or write it from scratch (good luck!). Once you are done with this, you maybe get into trouble because since Pascal Maxwell, nVidia GPUs refuse to clock higher than idle clocks when using a driver which is not signed by nVidia (but after going to all the previous trouble regarding implementing SLI for Ampere, maybe there is even a way around that after all)
You need to convince the 3090s to communicate over the NVLink bridge via the SLI protocol. This might be easiest part, since I remember reading someone did that (for whatever reason) during the the Ampere launch reviews (Does anybody else remember? Link?). Alternatively, it maybe turns out SLI communication over NVLink does not work due to some hardware limitations, rendering 1) & 2) pointless (maybe you can get it work using communication via PCIe).



If someone has better insight than my wild guesses, especially regarding 2), I would be pleased to read it :)

[whargokus]

I would be pleased to use my 3090's :) There has to be a way

[whargokus]

I'm willing to turn this into a paid project if anyone is up for the task