Indeed 
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Some recommendations from Lambda Labs (source)
RTX 3090
- 1x – No concerns.
- 2x – Use standard, non-blower GPUs, large case fans, an air shroud, and have an empty channel that’s at least 2x PCIe slots wide between the GPUs.
- 3x – Not yet recommended. Liquid cooling may work, but this needs testing.
- 4x – Not practical due to the power constraints discussed above.
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For aircooling:
So we need a motherboard with at least 4 GPU slots to cool them, actually more than 4 slots would be ideal since the first 3090 would cover 2.5-3 slots and if we insert the second one into the 4th slot…that leaves us with just 1 slot spaced in between 
If I wanted more than one 30 series card, and the cards were not going to be liquidcooled, I would get an open air case with some pcie risers like the crypto miners use.
And if Dettmers is correct in that pci lanes dont matter too much, maybe go get 8 “cheaper” cards if you can parallel them correctly.
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If you’re not going to use these 3080 TI’s all the time, you might consider just one 3080 TI, and then supplement with TPUs on the cloud. Of course, you should calculate the cost depending on your usage.
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8x gen3 or 4x gen4 lanes per card are sufficient (up tp 4 cards), but that setup will be a nightmare for the dust.
Dust is what makes the fan bearings fail, and reduce the efficiency of the heatsinks.
One should buy one of these mining rigs and make a “case” around them, possibly with fans and HEPA filters.
There are many 2000W ATX units around (Fortron, SuperFlower).
As long as your power outlet has sufficient capacity.
2000W doesn’t work so well on the NA power grid, Europe no problem. 1800W is the max you can draw without tripping at standard 15A breaker, hence 1600W generally being the largest you find here outside of server equipment that is setup for 20A or higher circuits.
You can do the miner trick and synchronize multiple PSU. You can buy a little relay that attaches to one of the output of the lead supply that triggers the ATX on for the follower and you can use the follower to power some of the GPU.
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You could use a PCIe X16 extension cable. In that convenient way, it will be sufficient for the mb to have just 2 slots, no matter the layout.
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Of course the following are just rumors. But still…:
Supposing that that behemoth will be priced twice a 3090 (but I expect more), I would still prefer two 3090, given the flexibility such a solution would offer, not to mention the superior processing power (I really doubt that Titan Ampere will integrate 20K cuda cores).
Of course, if you have money to buy two Titans… 
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@balnazzar
Btw, the channel mentioned as source in this video is usually disliked on reddit so not sure how reliable the rumour will be…
Sorry I had never looked at using an extender, I wasn’t sure how is the GPU “mounted” with the same.
I’ll go back to my homework 
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Can someone recommend a CPU / Board for a 2 x 3090 Setup?
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Well, if your budget can handle 2x 3090, why not get MSI Creator TRX40 paired with an AMD Ryzen Threadripper 3970X?
Uh… It’s just my personal opinion, but I won’t advise such a setup.
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Note that the board will not be able to support two 3-slot cards out of the box: indeed, the only usable slots will be the first and the last. But if you use the last, you’ll cover a lot of connectors, including the front panel’s. Moreover, you will need a case with sufficient space below the last slot.
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That little chipset fan will be loud as hell.
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The threadrippers cannot use RDIMMs, and ECC UDIMMs are expensive.
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I think that a 3970X alone costs more than a 3090. At this point, even if he can afford it, I’d go for a cheaper processor and add a third 3090. For example, the Xeons E5-v3/v4 are crap cheap and provide RDIMM support and a sufficient number of lanes. Their mobos do have better layouts and IPMI.
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The AMD processors, while awesome, are still slow when it comes to MKL. Examples (source: phoronix):
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The first reviews for the RTX 3080 FE are out and it seems that in the 1x layout the GPU thermals are stable at 75 degrees Celsius at full load.
I don’t think any review for the RTX 3090 FE is out yet.
Do you know in general how long it takes to see reviews for 2x layouts?
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78C, to be fair. But what worries me is the maximum power draw. See https://www.techpowerup.com/review/nvidia-geforce-rtx-3080-founders-edition/31.html
370W for a card rated ~320W. I really hope the 3090 won’t crush the 400W wall.
You will start to see 3090 reviews around ~27 sept.
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I’m working on compiling the best advices, and trying to interview people on these topics along with working with my vendor to build a PC and a video on how it was built on this topic. Will keep everyone posted 
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I doubt you will see any 2x reviews for a while as SLI is only enabled on the 3090. For DL applications, it would require a user to get their hands on 2 cards, which judging by the quick sellout yesterday, may be a long ways away.
On a side note, there was an article that made reference to a 20GB 3080 card. That one may be worth holding out for depending upon the price. That may be the sweet spot for DL practitioners like “us” on a price/performance, etc stand point.
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Update on my efforts:
Tim Dettmers has kindly agreed to be on my podcast (for the second time), meanwhile I’m trying to get Lamdalabs folks as well to share how to make things work w 2x Setups.
For Tim, he has agreed to accept any questions, please send them my way
Will keep you all posted
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That should be the alleged 3080ti (or “super”). Some think that AMD ‘big navi’ will have performances in between the 3080 and the 3090, so a beefed 3080 would counter that big navi.
Maybe not. The sellout is due to gamers. I don’t think gamers will adopt the 3090 so early, given that it is overkill and its price point. But I may be wrong about that.
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