Zen2 pci-e lanes question

[Trypt]

Could someone please explain how pci lanes work on Zen2? My system is in the sig.

I am confused as to how many total lanes I have, and how they can be used. How many for the cpu and the ones for the chipset, are they added to it? And how are they used? Can they be split, so that I can use 4x x2 on a 16x slot and use only 8 lanes there?

My mobo has 3x pci-e x16 slots but I'm guessing it'll never be actually able to fill those slots with 16 lanes each, so how exactly does it work?

For example, I am running a x16 video card, even though it's really x8 as it's last generation, I'm guessing it's taking all 16 lanes?

Now, for my bottom slot, my plan is to split it into 4x4 lanes and run a m.2 nvme riser, I believe my mobo supports bifurcation, but I'm not even sure of that.

However, I have read that my cpu doesn't support 32 lanes, so right there I'm stuck, but then I really don't need 4x nvme card, only a dual, since my mobo has two m.2 slots.

So ideally, I'd like to run my video card, and eventually it's going to be a gen4 amd card, and a wifi card in one of the pci-e x1 slots, and a dual x16 nvme m.2 riser card for two x4 nvmes, for a total of 8 lanes on a secondary x16 slot. Can I do that?

 

[wingman99]

Here is your board specifications. https://www.asus.com/Motherboards/PRIME-X570-PRO/specifications/

 

[MaddMutt]

Here is your board specifications. https://www.asus.com/Motherboards/PRIME-X570-PRO/specifications/

I think what has him confused and me is this :

*3rd Gen AMD Ryzen™ Processors
2 x PCIe 4.0 x16 (x16 or dual x8)
*2nd Gen AMD Ryzen™ Processors
2 x PCIe 3.0 x16 (x16 or dual x8)
*2nd and 1st Gen AMD Ryzen™ with Radeon™ Vega Graphics Processors
1 x PCIe 3.0 x16 (x8 mode)
*AMD X570 chipset
1 x PCIe 4.0 x16 (max at x4 mode)
3 x PCIe 4.0 x1

 

[The Coolest]

The CPU has a total of 24 PCI-E lanes. 16 for graphics, 4 for nvme and 4 are reserved and are used as the communication link with the chipset.
So you can have either a single x16 PCI-E slot or 2 x8 PCI-E slots, connected directly to the CPU. And a x4 NVME slot which is connected directly to the CPU, it's usually the "first" (or only) NVME slot on the motherboard, and normally positioned closer to the processor.
The rest of the lanes are provided by the x570 chipset, and eventually connect to the CPU over the dedicated x4 connection.

APUs only have 16 PCI-E lanes (12 usable), that's why you see "1 x PCIe 3.0 x16 (x8 mode)" for those chips. 8 lanes for graphics and 4 for NVME storage.

With the X570 chipset there are different ways in which the lanes can be split between providing SATA ports, PCI-E slots and connectivity with peripherals (such as NICs, audio devices, etc.)
The specific setup depends on how the motherboard manufacturer decides to do it.

 

[Johan45]

This is taken from my launch review and generalizes CPU/chipset lane breakdown with X570

 

[Trypt]

Umm, the picture does help indeed, but it's still not exactly clear. On the left it's the CPU, so when going by that, my cpu is pretty much maxed already, using a 16x video card, a 4x nvme and usb 3.2.

That leaves the chipset. So, on the right, the blue boxes, all three apply? So, 8x pci lanes plus pick one from each of the big blue boxes? So, by that, I should be able to add another 4 4x nvmes to the chipset lanes right, if I don't put any SATA drives in there?

If that is so, do I have to tell the mobo in bios how to do that or is it all automatic? For example, on the chipset it says you can run 4x SATA plus you can also pick how to use remaining lanes (blue boxes), but is that automatic? And bifurcation, I do have the pro version of the mobo, I'd be very disapointed if it wasn't included.

And another issue here, if I add a video card to my rig, a 5700XT, but want to keep my RX580 just for mining for example, that will take 8 lanes from the chipset right (the top blue box)? Then I still have the choice of the "pick one" from both of the blue boxes below correct, in other words, I'd still have 2x 4 pcie lanes to run my dual riser nvme?

EDIT: I think I'm getting it now, the blue boxes that say "pick one" are not for me, the end user, but for the motherboard manufacturer, for them to decide how to build the board? So in other words, I have to check how Asus decided to "pick one" for my particular board. Well, I know they didn't use all the lanes for usb/sata, cuz I have less, and have an extra x16 slot, so hopefully their setup on the Pro board will allow me to do what I want to do.

 

[EarthDog]

Worst case is your AIC for M.2 drives goes in the second, full-length cpu fed pcie slot. This breaks them down to x8/x8...and you may lose 1% performance on the GPU.

 

[wingman99]

Worst case is your AIC for M.2 drives goes in the second, full-length cpu fed pcie slot. This breaks them down to x8/x8...and you may lose 1% performance on the GPU.

@EarthDog when you use both 16X slots does the PCI-E bus reduce from PCIe 4.0 to PCIe 3.0?

 

[EarthDog]

@EarthDog when you use both 16X slots does the PCI-E bus reduce from PCIe 4.0 to PCIe 3.0?

I don't believe so. Good point...which means there wouldn't be any losses as that is 3.0 x16 anyway.

 

[wingman99]

I don't believe so. Good point...which means there wouldn't be any losses as that is 3.0 x16 anyway.

I was thinking more like Crossfire two 16x slots and the top 16x slot is 8x PCIe 4.0 and the second 16x slot is 8x PCIe 3.0, so then does the video cards train at PCIe 3.0 for both slots?

 

[The Coolest]

It says right there in the specs it's pci-e gen4 x16 or 2x8.
I'm actually using the two SSDs in my system in a similar configuration. The Evo 970 is in the NVME slot and the EX920 is sitting in the 2nd PCI-E slot. The GPU is running at x8.
SSD benches are identical to the normal NVME slot.

 

[EarthDog]

I was thinking more like Crossfire two 16x slots and the top 16x slot is 8x PCIe 4.0 and the second 16x slot is 8x PCIe 3.0, so then does the video cards train at PCIe 3.0 for both slots?

It says right there in the specs it's pci-e gen4 x16 or 2x8.
I'm actually using the two SSDs in my system in a similar configuration. The Evo 970 is in the NVME slot and the EX920 is sitting in the 2nd PCI-E slot. The GPU is running at x8.
SSD benches are identical to the normal NVME slot.

 

[Trypt]

So after all this time, I found this card: https://www.sybausa.com/index.php?route=product/product&product_id=992

What you guys think about this? I know it's pci-e 3.0, but I figure that is exactly what I need, but I'll wait for a pci-e 4.0 version which can hopefully use only 8 lanes in total for two nvmes. Now, this card is probably expensive as it is designed to work on non-bifurcation boards, but will it also work on a board with bifurcation and bypass it's own chip?

Not easy, figuring this out.

 

[The Coolest]

If you only have 2 NVME SSDs, then install one into the NVME slot on the mobo, and install another one into the 2nd PCI-E x16 slot on your motherboard using an adapter card like this: https://www.ebay.com/itm/273426170383

 

[Trypt]

I have two already, both on the motherboard. I want two more, and I want them using pci-e x4 each, while sharing a x16 slot, since I have the lanes available and that will be ultra fast no matter how I send files, it's something I'm looking forward to. Getting a 2TB or higher nvme is impossible due to cost, but getting 4 1TB nvmes is definitely possible and within budget. If there are plans for 2TB or bigger nvmes that are reasonable, I'd wait, but I don't see any of those plans, plus I have pci-e slots and lanes to spare which is why I started this thread, to understand completely how they work and how they can be used/split.

 

[The Coolest]

Really depends on what you're looking for in an NVME drive, but from what I've been seeing, 2TB drives are not that much more more expensive in $/GB than 1TB ones.