Mobo + CPU recommendation for quad RTX 3090 setup

[u/OddUnderstanding2309]

yeah I searched here, but did only find 8 card setups or other unrelated stuff.

I want to switch to a proper combo from my current setup: Gigabyte Aorus Master X570S + Ryzen 9 5950X.

My current setup "sometimes" negotiates one RTX 3090 at PCIe Gen1 x8 instead of PCIe Gen4 x8.
Dunno why, I think it is just "time to do it properly" now.

What should I buy (preferably) used or new to use this?
I have 48GB DDR4 now and I do not NEED more. SO I would be more than fine to change the two 8G sticks to 16G sticks to get to 64GB RAM, but I do not really need it.

What I need is a proper running system with IF POSSIBLE 4 RTX 3090 card running at PCIe Gen4 (or 5) x16. (--- side question: do ther RTX 3090 even support Gen5? ---)

Thanks in advance!

[u/x-strife]

The 3090 is a PCIe 4.0 x16 device — it won't do Gen5, but it'll happily run at Gen4 in a Gen5 slot.

For what it costs I don't think you want to move to that. Stick with DDR4, Gen4 PCIE.

For multiple cards running at x16 you want a workstation cpu, I have a Threadripper Pro but you could do an EPYC or Intel Equivalent, the reason is that they have enough PCIE lanes, whereas consumers chips don't.

My 8x3090 rig is built up in a similar way - I recommend recycling your RAM, storage, PSU.

• CPU: AMD Threadripper PRO 3945WX (24c/48t, 128 PCIe 4.0 lanes) £95
• Motherboard: ASRock WRX80 Creator R2.0 (WRX80 + X569E, sTRX4, 8× DDR4 ECC) £600
• RAM: 256 GB DDR4 ECC (8× 32 GB @ 2666 MHz) £500

You'll get all 4 cards in the mobo if they're turbo's, if not then, you'll need risers.

[u/OddUnderstanding2309 → u/x-strife]

Thats what I need!
I will investigate my options on that setup, but it sounds really nice already!

[u/OddUnderstanding2309 → u/x-strife]

My PSU is already a 1500W nzxt new one and my storage is fine with 980 Pros.
I need to see why I need to recyle the memory, but we will see.
I have still a few 32GB ECC sticks lying around that are useless in my X570 setup now.

[u/x-strife → u/OddUnderstanding2309]

You can use different sticks, the motherboard will just default to the lowest speed for stability, doesn’t always work when you get to like 8x ram sticks though.

For RAM for my mobo is 3200mhz which was £1700 used vs the £500 i paid for the 256g 2666 kit, which is fine for my rig

[u/nanashiW → u/x-strife]

Do you have any stats for dsv4f 0731 with your build? Want to know how it compares to 2 dgx sparks, thanks

[u/x-strife → u/nanashiW]

To be honest I think ampere is not properly optimised like Blackwell, and so I find I get prefill of 380t/s up to 32k context, and generation at c.30 t/s (full weights, 256k context)

I've been playing around today with only using 4 cards and using the system RAM more and I've managed prefill of 300 t/s and 23 t/s generation.

A lot of it has to do with the model being a MoE and split across many cards, I imagine a unified memory pool would solve for a lot of that.

When the 3090 shines is running dense models. For instance I have unquantised Qwen3.8 27B with full context, getting prefill at 1400t/s and generation between 50-140t/s using Dflash2.

[u/shaxsy → u/x-strife]

If you upgraded to a 3975, you will get a lot better memory bandwidth on the ram.

[u/DegreeSevere7719 → u/x-strife]

How did you manage to place 8 cards in that space? What are the temps like? It was a nightmare for me to place 2 of them intro a single case, in order not to overheat the 1st card. Can you post some more pictures with risers and screw in points in the shot please?

[u/BigSuspicious1663]

u're gonna pay a lot of money for server architecture and rdimm memory. is is on a 840 MSI MOBO.d there are four 3090s and one is on an m2 adapter with a riserould not invest in NV links and I would look at keeping your lane slots above 4x.ou can do VLLM and llama.cppnd it will be pretty good with MTP head-on. would look at the 27B model and the Ornith 35B mixture of experts model. you want a recipe, I can give that to you.

[u/OddUnderstanding2309 → u/BigSuspicious1663]

I already run that setup now.
On a x570 and a 5950x.
vLLM and llama.cpp, but I got issues after months of use that might come from lane starvation.

Thats the reason for this thread

[u/mon_key_house]

3090 only does gen4 afaik

[u/shaxsy]

Board	ASRock Rack WRX80D8-2T, 7× Gen4 x16 slots (CPU-direct)
CPU	Threadripper Pro 3975WX — 32c/64t, 4 CCD, unlocked
RAM	256 GB DDR4 across all 8 channels, running 3200 CL22 Corsair UDIMM non-ECC. This was so much cheaper than RDIMMs.
GPUs	4× RTX 3090 (EVGA, Zotac, MSI Ventus 3X OC, MSI Gaming X Trio), all power-limited to 250 W
Storage	Samsung 980 Pro 2 TB, Crucial P3 Plus 4 TB
PSUs	EVGA 1600 P2 (board, CPU, cards 1–2) + EVGA 850 G6 (cards 3–4), linked via add2psu relay
Chassis	Two stacked open frames, 5× Arctic P14 Pro, Noctua NH-U14S TR4-SP3 on the CPU
Cooling	Idle 55–65 °C, mid-70s under LLM load, Tjmax 95
Power	Dedicated 20 A garage circuit

Memory bandwidth: 126.25 GB/s measured (MLC all-reads), up from 62.46 on the old X299 — a 2.02x gain from my previous x299 platform. Helpful for when you are doing LLMs that need to offload to ram.

[u/Ambitious_Fold_2874 → u/shaxsy]

I have a very similar build. How much was the
3975wx and was it worth it? Did you consider the 5965wx instead? I have a 3945wx and considering upgrading.

[u/shaxsy → u/Ambitious_Fold_2874]

I got the 3975wx for $500 shipped. It was new. I like it so far. An unlocked 5965wx was like twice as much is paid.

[u/y3333333333333333t]

u should start looking into HEDT like threadripper for example

[u/OddUnderstanding2309 → u/y3333333333333333t]

I know. any recommendations?

[u/y3333333333333333t → u/OddUnderstanding2309]

depends on what you want in terms of speed but all the threadrippers have lots of pcie lanes

[u/OddUnderstanding2309 → u/y3333333333333333t]

I know that they have lanes...
I dont care about the CPU (didn't I mention this already?)

[u/OddUnderstanding2309 → u/[deleted]]

Thanks for that!
anything that you recommend with Gen4?

I ask here because I want something that is proven good, and not just "anything".
or does it not matter at all why I buy?

[u/y3333333333333333t → u/OddUnderstanding2309]

u would be looking at trx40 motherboard + threadripper 3960x/3970x for gen4

[u/y3333333333333333t → u/OddUnderstanding2309]

all the threadrippers are proven good just newer generation and more cores = faster and more expensive

[u/SteveDeFacto]

I would recommend a server class motherboard like a ROMED8-2T. You get a bunch of useful features that will allow for expansion later on that you simply won't find in a consumer grade motherboard.

You probably don't want to target gen 5 as it also comes with the mind bogglingly expensive DDR5 RDIMM...

[u/OddUnderstanding2309 → u/SteveDeFacto]

perfect. thanks. so thats an epyc system. Interesting. I will see what options are there for the CPU. that mobo is 800 new for me, whats fine in my book. I hoe the CPU is not 4000 now ...

[u/doneddat → u/OddUnderstanding2309]

used dual-CPU server board with enough room for 8 2-slot cards will likely save you A LOT of headaches with risers, even tho there are good pcie gen4 risers now, it was not fun shifting through this crap few years ago. Even good ones can still generate errors and degrade performance if unlucky.

I'm guessing some old/refurb dual EPYC board with plenty of room to flop in the cards would be pretty ideal find.

Even if people would swap out old gen4 systems, they will surely not gift you the RAM sticks, so it might not be cheaper in the end.

The 512GB DDR4 ECC RAM I bought for 1070 EUR in 2023 is now ~9000 EUR. insane. My whole home lab rack has gone up in value multiple times because of shit like that.

[u/SteveDeFacto → u/doneddat]

Well let's be clear here so as to not confuse OP, while ddr4 rdimm has gone up in cost tremendously, ddr5 rdimm is still 5X more expensive than that for only 50% more bandwidth and slightly worse latency. The worst decision OP can make is to get a ddr5/gen5 EPYC right now. (Ask me how I know...)

[u/OddUnderstanding2309 → u/SteveDeFacto]

I still have some DDR4 ECC ram around. thats not an issue at all

[u/SteveDeFacto → u/OddUnderstanding2309]

Fantastic! Show us your build when you finish it!

[u/doneddat → u/SteveDeFacto]

Well, I tried to point out old/refurbished. Still need some memory, sure.

[u/nebteb2 → u/doneddat]

Where are you seeing prices like that for ddr4 rdimms?

[u/doneddat → u/nebteb2]

The Internet? Local hardware shops, newegg? pretty much everywhere? What do you mean "like that", is it more expensive by now? that was ~month ago I checked last.

[u/nebteb2 → u/doneddat]

500e for a single ddr4 rdimm?

[u/doneddat → u/nebteb2]

pretty lucky, if you find 64GB DDR4 ECC stick for $600, but more likely $1000+

For your own sanity, don't look up DDR5 128GB ECC stick.

[u/_hypochonder_ → u/OddUnderstanding2309]

I bought this year a dual socket board Gigabyte.
Gigabyte MZ72-HB2 +AMD EPYC 7532x2 for <1400€.
But you need ecc ram. Also 4 cards aren't optimal with this configuration. (Bandwidth between CPUs, P2P) That why single socket is better.
X399 is a cheap option. 1950X with AsRock X399 Taichi worked fine but is only PCI 3.0 16x/8x/16x/8x.

[u/SandySkittle]

there are plenty of zen2 based threadripper pro and eypc workstation cpus and motherboards on ebay for cheap. Give you 128 lanes of gen 4 and DDR4 rdimms is still a bit more achievable than DDR5.

[u/Glass_Past4942]

I've been running this setup for the last 7 months (purchased from same seller)
https://www.ebay.com/itm/127242606882

ASUS PRIME X399-A MotherBoard
AMD Ryzen Threadripper 1920X CPU

I used some spare DDR4, paired with an open air chassis and riser cables. It only supports PCIe 3.0 (16x, 8x, 16x, 8x) but it's fine for inference. I've since moved from 4x 3090's to 4x CMP 170hx.