[SOLVED] Will Z790 boards hold back a 4090 by stealing away PCIe lanes for the NVMe SSDs?

[K1llrzzZ]

Hey!

So I read this article by TechPowerUp: Nvidia Geforce RTX 4090 PCI-Express scaling
It has a sentence that had me worried a bit:
"Intel is now allowing motherboard designers to add Gen 5 M.2 NVMe slots on their upcoming 700-series motherboards, but they're going about this by stealing PCIe lanes from the x16 PEG slot. This means that when 13th Gen Core users install an SSD in that M.2 slot, it will redirect eight lanes that were used by the x16 graphics slot, and use those on the M.2 slot. So those building a bleeding-edge Core i9-13900K + RTX 4090 gaming PC with a Gen 5 NVMe SSD will have to content with the RTX 4090 running at PCI-Express x8 bandwidth."
So I'm planning to get an Asus Rog Strix Z790-F Gaming mobo with a 13900K and a 4090, I have a single Samsung 970 Evo Plus 500Gb PCIe 3.0 SSD, if I install that on this mobo will that cut back the GPU performance? This is only an issue if I run out of PCIe lanes right? How many NVMe SSDs do I have to install in order for the GPU to lose PCIe bandwidth? Also the mobo supports PCIe 5.0, the CPU does to, however the GPU only supports PCIe 4.0 but it supposed to be backwards compatible, so does this mean that if my PCIe speeds drop down to x8 it will be PCI 5.0 x8 so the same bandwidth as PCIe 4.0 x16 or will it drop to PCIe 4.0 x8 because the GPU doesn't support 5.0 which will mean I will lose performance?

Thanks for the help!

 

[Darkbreeze]

It specifically says this can only be a problem IF you install a Gen 5 NVME SSD. So no, a 970 EVO Plus, which is Gen 3, would not incur this penalty.

Also, nothing installed on that board will run at PCIe 5.0 speeds UNLESS it is a PCIe 5.0 device, of which there are currently none, so there is nothing to worry about at all at this point. The only part of that card that supports anything relevant to PCIe 5.0 is the power architecture, not the PCI express architecture which governs lane usage.

 

[hotaru.hino]

Installing an M.2 slot on the one tied to the CPU PCIe lanes will cut the video card slot's lanes. Most of these boards come with another M.2 slot that routes to the chipset instead. This will not cut into the CPU's lanes. And which "version" the PCIe lanes use depends on the lowest version in the chain. So no, you won't get PCIe 5.0 unless you use a PCIe 5.0 device.

As far as performance goes, even PCIe 4.0 x8 only cuts about 2-3%, which for most people is within the margin of error. Go look at the summary in the end: https://www.techpowerup.com/review/nvidia-geforce-rtx-4090-pci-express-scaling/28.html

 

[Darkbreeze]

Installing an M.2 slot on the one tied to the CPU PCIe lanes will cut the video card slot's lanes.

No it won't. Not according to the article that was linked to. It SPECIFICALLY states that it will do so ONLY if a PCIe Gen 5 NVME drive is installed. It does not say it will do this (In fact, it says it indicates it won't) if a Gen 4 or Gen 3 device is installed. If you have links to information that is contrary to that I'd be appreciative of you sharing it so we can all look at it.

 

[hotaru.hino]

No it won't. Not according to the article that was linked to. It SPECIFICALLY states that it will do so ONLY if a PCIe Gen 5 NVME drive is installed. It does not say it will do this (In fact, it says it indicates it won't) if a Gen 4 or Gen 3 device is installed. If you have links to information that is contrary to that I'd be appreciative of you sharing it so we can all look at it.

I originally looked at the CPU page on Intel's site and it said 12th gen CPUs have 20 lanes, and simply assumed they're doing the thing they did in previous generations where they included the DMI lanes as part of this count

Looked at a block diagram of the platform and realized they actually went the AMD route and dedicated 4 lanes to an NVMe SSD and these lanes are PCIe 4.0

However, they described the PCIe 5.0 distribution supporting graphics and/or an SSD. To me this implies that if there's a PCIe 5.0 NVMe slot, it's tied to the PCIe 5.0 side only because it wouldn't make sense to add complexity to see if the NVMe SSD is 5.0 or 4.0 and route the lines appropriately.

So I'm led to believe you can still eat into the GPU's lanes if you plug an SSD into the wrong slot.

EDIT: Had a look at the ASUS Z960 ROG Maximus Extreme and it has two M.2 slots specifically tagged as tied to the CPU. One uses 5.0 and the other 4.0 and both are 4 lanes. So I'm pretty sure if you plug in anything into the 5.0 slot, you'll lose 8 lanes for the GPU (because nobody supports 12 lanes)

 

[DRagor]

I like the theoretical discussion you have here guys. However I would like to point out that the motherboard OP linked according to both ASUS page and manual does not have any PCIe 5 M.2 slots - so the whole discussion is moot
OP you will not lose GPU lanes on this board. The article you quoted does not apply in your situation.

 

[hotaru.hino]

I like the theoretical discussion you have here guys. However I would like to point out that the motherboard OP linked according to both ASUS page and manual does not have any PCIe 5 M.2 slots - so the whole discussion is moot
OP you will not lose GPU lanes on this board. The article you quoted does not apply in your situation.

Well first we had to establish that the 5.0 M.2 slots use the 5.0 lanes regardless of what the NVMe SSD uses.

People will think otherwise "oh, it's PCIE 4.0 and there's 4.0 dedicated to SSDs so maybe it uses that instead"

 

[Zerk2012]

Hey!

So I read this article by TechPowerUp: Nvidia Geforce RTX 4090 PCI-Express scaling
It has a sentence that had me worried a bit:
"Intel is now allowing motherboard designers to add Gen 5 M.2 NVMe slots on their upcoming 700-series motherboards, but they're going about this by stealing PCIe lanes from the x16 PEG slot. This means that when 13th Gen Core users install an SSD in that M.2 slot, it will redirect eight lanes that were used by the x16 graphics slot, and use those on the M.2 slot. So those building a bleeding-edge Core i9-13900K + RTX 4090 gaming PC with a Gen 5 NVMe SSD will have to content with the RTX 4090 running at PCI-Express x8 bandwidth."
So I'm planning to get an Asus Rog Strix Z790-F Gaming mobo with a 13900K and a 4090, I have a single Samsung 970 Evo Plus 500Gb PCIe 3.0 SSD, if I install that on this mobo will that cut back the GPU performance? This is only an issue if I run out of PCIe lanes right? How many NVMe SSDs do I have to install in order for the GPU to lose PCIe bandwidth? Also the mobo supports PCIe 5.0, the CPU does to, however the GPU only supports PCIe 4.0 but it supposed to be backwards compatible, so does this mean that if my PCIe speeds drop down to x8 it will be PCI 5.0 x8 so the same bandwidth as PCIe 4.0 x16 or will it drop to PCIe 4.0 x8 because the GPU doesn't support 5.0 which will mean I will lose performance?

Thanks for the help!

The board your looking at really says none of that.


Total supports 4 x M.2 slots and 4 x SATA 6Gb/s ports*

Intel® 13th & 12th Gen Processors
M.2_1 slot (Key M), type 2242/2260/2280/22110 (supports PCIe 4.0 x4 mode)

Intel® Z790 Chipset
M.2_2 slot (Key M), type 2242/2260/2280 (supports PCIe 4.0 x4 mode)
M.2_3 slot (Key M), type 2242/2260/2280/22110 (supports PCIe 4.0 x4 mode)
M.2_4 slot (Key M), type 2242/2260/2280 (supports PCIe 4.0 x4 & SATA modes)
4 x SATA 6Gb/s ports

The top one runs off the PCI-E lanes of the processor All the 13xxx processors I have seen have 20 lanes.

 

[boju]

That article is confusing. Or im confused. They talk of m2 slot but think they mean pcie expansion cards. Well if that's the case, taxing 8 lanes from cpu for additional storage isn't anything new. This motherboard has pcie 5 gen m2 slot running at x4 https://rog.asus.com/motherboards/rog-maximus/rog-maximus-z790-extreme-model/spec/

Pcie 5 m2 nvme drives being released are x4. Maybe in future we see single 8x nvme? Where pcie slot is used to take advantage of, but until then, i don't really know what that article in Op is about, it doesn't make sense.

 

[DRagor]

That article is confusing. Or im confused. They talk of m2 slot but think they mean pcie expansion cards. Well if that's the case, taxing 8 lanes from cpu for additional storage isn't anything new. This motherboard has pcie 5 gen m2 slot running at x4

This motherboard has gen 5 M.2 slot that is linked to second x16 slot. Its nothing new except this kind of sharing so far was done on PCH lanes, while on this board second x16 slot is linked to CPU, so (in this case) putting any M.2 into this slot will cut 8 lanes from top GPU slot exactly same way as if you would put second GPU into that second x16 slot.

 

[boju]

This motherboard has gen 5 M.2 slot that is linked to second x16 slot. Its nothing new except this kind of sharing so far was done on PCH lanes, while on this board second x16 slot is linked to CPU, so (in this case) putting any M.2 into this slot will cut 8 lanes from top GPU slot exactly same way as if you would put second GPU into that second x16 slot.

This way of doing things supposed to benefit direct storage or something? With that setup, what's the point in having 20 lanes from cpu if you're still going to be penalised?

 

[DRagor]

With that setup, what's the point in having 20 lanes from cpu if you're still going to be penalised?

Because that's not true 20 lanes? Its only 16 gen 5 lanes plus 4 gen 4 lanes. So the board in question still has gen 4 M.2 fed from CPU (as well as two M.2 fed from PCH) and you can use that no problem. But if you want to use gen 5 M.2 that's different story.

 

[boju]

Because that's not true 20 lanes? Its only 16 gen 5 lanes plus 4 gen 4 lanes. So the board in question still has gen 4 M.2 fed from CPU (as well as two M.2 fed from PCH) and you can use that no problem. But if you want to use gen 5 M.2 that's different story.

That clears that up then, i thought all lanes were pcie5. Thanks

 

[K1llrzzZ]

Thank you all for the replies! I figured it should be alright, I mean my current Z390 board can handle an NVMe SSD without cutting the GPU lanes in half, I just wanted to be sure before I buy.