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How does PCIE g4 handle pcie3 devices

Long post/question incoming:

With the new launch of third gen ryzen I find myself in a tough spot. My current workstation has a 1950x and I had thought that I would be able to put a Zen 2 processor in it back when I purchased it. Now with the new socket and AMD dropping 16 core chips from threadripper I’m having a hard time deciding what to do. I don’t really need the more cores that would come with the 3960x but I do think I need the PCIE lanes. I need 2 graphics cards in my workstation for VMs along with 40Gb fiber and a sas controller. To my understanding the 3950x only has 16 pcie lanes now. does anyone know how those lanes would be split when pcie gen 3 cards are inserted? Would a card that normally runs on 8x pcie g3 now only need 4 lanes on a pcie gen 4 system?

I would really like to upgrade since IPCs are a little low on the 1950x and NUMA can sometimes be a headache. Just wondering if anyone might be able to help

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Unfortunately it doesn’t work that way. 8 lanes is 8 lanes, so your pcie 3.0 card would still use 8 lanes, and talk at pcie 3.0 speed. There’s no splitting going on at the slot.

That’s not to say you couldn’t probably find (in a few years?) a riser card that contained a pcie 4.0 switch and multiple 3.0 slots, turning x16 at 4.0 into 4 x8 at 3.0 for example. I would expect to see those risers popping up at the enterprise level soon enough.

Ye old Zen 1 and 1.5 threadrippers are still beastly workhorses, they have a ton of bandwidth but it’s hard finding an single application that takes advantage of that.


Well thank you for confirming what I had suspected. Looks like I’ll just have to stick with my current setup for the time being.


If you are willing to do some work, you can probably wrangle this Mobo to do what you need. (You didn’t mention what else you need but those 4 cards).

That gives you 3 slots, and you can wrangle a fourth x4 slot (probably the SAS controller can “manage” with that if you are fine with “only” 4GB/Sec to your SAS drives, by leveraging either an m.2 or u.2 > x16 slot adapter. Depending what you do on your GPUs (if you only do rendering / compute, usually, they don’t experience a lot of bus traffic, possibly one of the GPUs can go in the x4 slot).

They just found out that the 8-lane wired RX 5500 XT 4GB benefits greatly from increased bandwidth from PCI-E 4.0 because of it’s limited VRAM buffer:

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