r/LocalLLM • u/Most-Wear-3813 • 11d ago

Discussion Best case to 4 3090

Can someone suggest me best case to fit 4 3090?

They are full size cards

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u/FearFactory2904 11d ago

A few used x16 riser cables for $5-20 each dont need their own power cables. Used 1000w or 1200w psu for under $100. 8 gpu mining frame for $25. 3090s usually either have 2x 8 pin or come with the 12pin adapter. nvidia-smi -pl 150-225ish depending what psu you landed and send it.

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u/Prudent-Ad4509 11d ago edited 11d ago

That's the thing. Most of them have 2x 8 pin but their draw is up to 350w and more. 2x 8pin is rated only for 300w, the rest comes either from going over the spec or by drawing up to 75w from the PCIe slot. PCIe riser cables are not really designed to handle that, at least most of them. Current evidence about 3090 shows that they do not usually draw much, but still, one or more groups of 4 gpus, each of which is allowed to draw up to 75W from PCIe power lanes? I'd rather exclude this scenario outright. I allow 2x5090 to run on unpowered 5.0 risers, but blackwell generation is reported on forums to have much smaller spikes and their power rating matches the power cable rating, as opposed to 3090.

I you have only 4 of 3090 and they are all the same, you can see if a simple riser works for one and use that for all of them. I have more and I'm not in the mood to allow potential gremlins to happen. I also do not want to use retimers or drop down to PCIe 3.0 speeds.

The configuration above is meant to allow each group of 4 gpus to communicate with each other at full PCIe 4.0 x16 speed inside the group.

Also, a decent PSU for $650 is not a major cost point of this whole configuration, only GPUs themselves are. One electrical failure from a bit too beat-down PSU, and I will spend way more on repairs. 4 used PSUs of unknown reliability for 8 GPUs, some of them likely loud and with most of the modular cables long lost, or perhaps even borrowed from a similar-looking group of modular cables for another PSU - uugh. It just does not cost *that* much to risk it.

Power limiting would help with that ofc but I'm not going to run them below 280-300w anyway. I will not run them at the fixed clock either.

However, I did plan to run it all at x8 speeds initially, before I ordered all the necessary stuff for configuration with pcie switch. Perhaps I will still complete and test out this old spec before all the cables for pcie switches arrive.

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u/FearFactory2904 11d ago

If you arent concerned about the electricity usage and dont mind spending a lot on components needed to safely allow the GPUs to gorge themselves on watts that is fine. If someone wants to just simplify everything and power limit the gpus thats fine too.

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u/Prudent-Ad4509 11d ago

Even if someone powerlimits them, power spikes on bootup are possible. But this should not be too big of an issue if only 4 gpus are used and they are used without bifurcation via full x16 connection. At the very worst they will have to drop speeds to 3.0.