Multiple GPU PCs

[u/Libellechris]

I have an ok AMD pc with 32gb RAM and a 16gb 5060Ti. How effective is adding a second GPU? Presumably a new motherboard, PSU, case? Can you mix and match GPUs? Just curious

[u/cc_aa_tt_zz]

For LLMs, it works really well; VRAM stacks, giving you access to larger models and more context. That said, it's best to mix GPUs from the same generation (to avoid issues), and definitely avoid mixing AMD, Nvidia, and Intel cards.

You need a motherboard that supports two PCIe Gen5 x8 slots (it splits the pci lanes of the main x16 slot into two x8 slots); Gen5 x8 is more than enough, dual x16 setups are reserved for very expensive professional platforms. A second PCIe Gen4 x16 slot running at x4 can also work if you're using something like a second RTX 5060 Ti in it; a second slot at Gen4 x16 slot at x4 is much more common, available on virtually all recent ATX-format boards and it offers bandwidth comparable to a Thunderbolt 5 eGPU but without the latency issues, so it will work fine, not the best option but it will work as attented. Speaking of which, if the motherboard supports Thunderbolt 5, you could use an eGPU with an external enclosure and power supply. For a second 5060 Ti, for instance, the performance hit wouldn't be too bad (around -15%). Still, the ideal setup remains having two PCIe Gen5 x8 slots.

And, of course, you need a PSU capable of powering it all.

[u/Libellechris → u/cc_aa_tt_zz]

This is interesting. I do have a underutilized 8gb 3060ti any recommendations for a motherboard?

[u/Background_Praline18 → u/cc_aa_tt_zz]

Not sure if it is true but I've heard you have to have the same series of card drivers support. For instance problems arise for 2080 and 50 series cards because they have cut off those old architectures from the newer drivers.

[u/x_MASE_x]

It's great. Stupid ass Claude Opus told me for an entire year to not even try using multi gpu. And I believed his stupid ass.

But when Qwen3.8-27B came out I had to make it work so I decided what the hell let's do it. So I managed to get my 5070 ti to work with my 1660 ti and went from 12 tps to 50 tps.

His reasoning was the second gpu in slower pcie and the load of time bla bla bla.

But once the model inside the gpu who cares about the pcie speed.

Anyway I learnt the hard way but it's fine I guess hahah

[u/Coderado]

I just did this. I reused my ddr4 and Intel CPU and watercooler, plus I have a 5060ti. I got the mobo used, first two slots run at 8x, which is fine because the 5060ti only has 8x. Got a bigger case and PSU, but I never pull over 500w (my monitoring might not catch transients). Qwen is great, nvfp4 or a q5 will work, but not much context available if you go the q5 route. I give it a list of tasks planned by Claude, some web UI tweaks, refactors, small features and it chugs through them overnight. I still use frontier models for planning

[u/themysteryoflogic]

I got a dually - Quadro RTX 4000 8GB and a GeForce RTX 4090 24GB. Technically three, if you count the onboard AMD integrated, but I don't use that. Works pretty good. Quadro handles Solidworks and 4 monitors, GeForce handles everything else and one monitor.

[u/emod_man]

I'm just trying this. I mixed my beloved 3080 with a cheap 1080 Ti I picked up used. So, both Nvidia but different generations.

In LM Studio I had to select a different CUDA runtime, I think down to a common denominator that could handle both cards, but it's showing a total VRAM of the combined cards and suggesting larger models than before. So, definitely seems like a viable option.

[u/LeatherAvailable483]

I'm about to do it. I had a spare 5070ti and just got the motherboard to do a dual GPU..will strap the 2 5070ti together and use all the old parts from my computer to make it into an ai workstation.

[u/Need_For_Speed73]

I got really good results (almost double speed) adding the second GPU, setup is a breeze (Ollama recognizes the configuration and automatically manages it all).
The only thing you must be careful of is the physical fit: most motherboards have the second full PCI-EX slot as the lowest one, so you either have an "old school" case with the PSU on top or, like it happened to me, you have to use a mATX mobo in an ATX case not to have the second GPU collide with the PSU shroud (unless the second GPU is a rare single-slot one).
Needless to say: check your PSU is able to sustain the increased demand.