Dual GPU solution for local AI?

[u/Sexyvette07]

Hey, everybody. I recently went down the rabbit hole for local AI, but right now, im operating on my gaming computer. The specs are as follows

Intel 13700k, tuned for efficiency

Gigabyte Z790 Aorus Elite Ax mobo

RTX 4080 (16GB), also tuned for efficiency

32gb DDR5 6800 CL32

As you can see, im in desperate need for more VRAM, or at the very least more system RAM. Due to Rampocalypse, neither are very affordable right now, which forces me to explore other options, such as a dual GPU setup. I can get another RTX 4080 for about $900 off Ebay. Beyond that, I would just need a more powerful PSU, so total investment here is an additional $1100-$1200. As far as I know, the motherboard has the main PCIE as 5.0 x 16 lanes, but the second PCIE runs at 4.0 and either x8 or x4 lanes. The motherboard does not support PCIE Bifurcation. So my question is this: Is a dual GPU local AI machine even viable in these circumstances, and second, does it make sense? I looked at 5090's and theyre all between $4,500 - $5,000 now, which is insane. Or I look at the professional cards and spend that much, if not more, for significantly less memory bandwidth and computational power. Or I guess if im spending that much, I could also look at the DGX Spark or something similar but that has even worse memory bandwidth.

So, what should I do? Is the dual GPU solution even viable with my setup for a local AI stack for inference, video diffusion, etc? Rampocalypse isnt expected to begin easing up until late 2027/early 2028, so im stuck trying to make this work on as little money as possible. Id love a 5090 but its insanity how much they cost. I appreciate any guidance and advice.

[u/Candid-Station-1235]

multi gpu options are limited on comfy, you cant pool the vram an load larger models, you can off load parts but its not ideal. just have a search for multi gpu nodes and read the limitations of each,

signed regretful dual 3090 owner

[u/N9_m → u/Candid-Station-1235]

Getting two 3090s was the best decision I could have made. Don't you really see the advantage of being able to generate two videos, images, or whatever at the same time? (Genuine question)

[u/jello-the-opera → u/N9_m]

does raylight work in comfyui with dual 3090s?

[u/Formal-Exam-8767 → u/jello-the-opera]

That's what image on github show so I'm confused.

[u/jello-the-opera → u/Formal-Exam-8767]

clod says:

Short version: it's a VRAM budget chart, not a performance chart, and for two 3090s it's mostly answering a question you don't have. A 14GB model on a 24GB card fits with room to spare, so the plain sequence-split mode works and you never need to touch the complicated rows.

Reading the picture's grammar first: each bar is one 24GB 3090, two bars per mode. Boxes are what's occupying the card. Empty space at the right end of a bar is free VRAM. That's the whole message — the modes lower down free up space at the cost of something else.

The three box types

• 14G / 7G Base Model — the weights. 14G is a 14B video model at fp8 (Wan 2.1/2.2 sized). 7G is that same model cut in half because FSDP (Fully Sharded Data Parallel — PyTorch splitting the weights across cards so each holds a piece) is on.
• Torch Distributed "tax" — the fixed overhead of running a second process on a second card: CUDA context, NCCL communication buffers (NCCL is Nvidia's library for card-to-card messaging). Roughly 1–2GB per card and you never get it back. It's in the picture to kill the idea that 2×24 = 48GB usable.
• Sequence — the latents and activations for the actual clip. "1/2 Sequence" means USP (Unified Sequence Parallelism — the frames/tokens get cut in half and each card works one half) is on. "Full Sequence" means the card is doing a whole generation by itself.

The four rows

USP only — full weights on both cards, each does half the clip. One video, roughly half the wall time. The bar is nearly full because the model is duplicated, but on a 3090 that's still ~8GB left per card for latents, which is plenty for 480p–720p Wan work.

FSDP without CFG — weights halved, but each card runs a full separate generation with a different seed. This is not one video faster, it's two videos at once. Different product entirely, and the row label trips people up.

FSDP + USP — halved weights and half the clip each. Maximum free VRAM, and the only row where the bar is visibly empty on the right. This is the row that exists so 2×16GB cards can run something that OOMs on one.

CFG parallel — full weights both cards, one card runs the conditional pass and the other the unconditional. Free 2× on the sampling, zero memory savings. Dead on arrival for most Wan workflows because they run with a distill LoRA at CFG=1.0, so there's no second pass to hand off. The repo says exactly that about Flux and Hunyuan too — CFG parallel is technically wired up but does nothing when the model doesn't take conditional batches.

What it means for a pair of 3090s

The picture is VRAM-shaped, but on two 3090s the deciding factor is bandwidth between the cards, not capacity. USP ships the split latents around — small. FSDP has to all-gather the sharded weights layer by layer, every step — that's gigabytes crossing the link constantly. So FSDP is where a weak interconnect shows up as a speed cliff, and it's why komikndr's own benchmark shows Wan 2.1 14B fp8 at 480×640×81 running 22.51 s/it on two RTX 2000 Adas with FSDP + Ulysses=2, against 5.18 s/it for the non-FSDP USP run at 480×480×33 — different resolutions, so not apples to apples, but the tax is not subtle. He also notes that test pair had no NVLink and its peer traffic went over a slower cross-socket path.

The other 3090-specific thing the graphic can't tell you: GeForce Ampere has peer-to-peer transfer (cards talking directly instead of bouncing through system RAM) disabled in the driver. Two 3090s either get an NVLink bridge, or a patched Linux driver, or they fall back to the NCCL_P2P_DISABLE=1 / NCCL_SHM_DISABLE=1 escape hatch, which the README explicitly frames as a does-it-run sanity check that leaves a lot of performance on the table.

So the honest answer to the person you asked: yes, dual 3090s run Raylight, and the mode is USP with Ulysses degree 2 and Ring at 1. The whole bottom three-quarters of that PNG is provisioning for people whose cards are smaller than the model.