PCIe lanes confusion

[WinterLFG]

I can't find anything on this so I wanted to know more a bout how PCIe lanes work.

My cpu has 24 lanes supported, 4 are taken for other tasks so that leaves 20 open.  x16 4.0 for the graphics card and x4 3.0 for a capture card.

Is the 3.0 and 4.0 interface considered separate from each other?  Or do they both take up the same lane?  The motherboard I'm getting soon has a 4.0 and 3.0 m.2 slot and I wanted to get one but I as you would expect I don't want my graphics card to go down to x8 just for the m.2.  How do all the parts interact with the different versions of PCIe if that is the case?

 

Parts for ref:

X570 Crosshair VIII Hero Mobo
RTX 3080

Elgato 4k60

Any 4.0/3.0 m.2 that I find.

 

Thanks!

[Helpful Tech Witch]

All slots are PCIE gen 4. Just some cards, like the capture card, do not push gen 4 amounts of speed through.

[WinterLFG]

6 minutes ago, HelpfulTechWizard said:

All slots are PCIE gen 4. Just some cards, like the capture card, do not push gen 4 amounts of speed through.

Okay, so it doesn't have any dedicated lanes for things?

[Fasauceome]

9 minutes ago, WinterLFG said:

My cpu has 24 lanes supported, 4 are taken for other tasks so that leaves 20 open.  x16 4.0 for the graphics card and x4 3.0 for a capture card.

Your CPU has 4 lanes dedicated for NVMe storage, and 4 for the chipset. Then, it has 16 for the GPU. The capture card will pull lanes from the chipset, not from the CPU. 

 

You can add an NVMe drive and still have x16 for your GPU.

[minibois]

7 minutes ago, WinterLFG said:

Is the 3.0 and 4.0 interface considered separate from each other?  Or do they both take up the same lane?

Same lanes.

7 minutes ago, WinterLFG said:

I wanted to get one but I as you would expect I don't want my graphics card to go down to x8 just for the m.2.

The GPU goes to x8 because of the capture card, more on that later.

8 minutes ago, WinterLFG said:

How do all the parts interact with the different versions of PCIe if that is the case?

They will run at the lowest PCIE version between the slot and device. The slots are all 4.0, so if the device is 4.0 (like the RTX 3080 and possibly the M.2), the slot will run at 4.0. Otherwise it will just switch to PCIE 3.0.

9 minutes ago, WinterLFG said:

Parts for ref:

X570 Crosshair VIII Hero Mobo
RTX 3080

Elgato 4k60

Any 4.0/3.0 m.2 that I find.

This would be the applicable PCIE mode:

That would mean x8 for the videocard, x8 for the capture card (but the Elgato 4K60 Pro only uses 4 of those lanes) and the M.2 can run at up to PCIE gen 4.0, x4.

I am unsure if you can just install the videocard in the top slot, run it at x16, skip PCIE x16_2 and just use PCIE x16_3 at x4. That would be the perfect mix of all slots.

 

By the way, Nvidia suggests older PCIE versions will not impact the videocard performance, at least not as much as a CPU choice will:

Quote

Q: Will customers find a performance degradation on PCIE 3.0?

System performance is impacted by many factors and the impact varies between applications. The impact is typically less than a few percent going from a x16 PCIE 4.0 to x16 PCIE 3.0. CPU selection often has a larger impact on performance. We look forward to new platforms that can fully take advantage of Gen4 capabilities for potential performance increases. Note also, all benchmarks and performance data on our website and presented by Jensen was from a PCIE 3.0 platform.

https://www.nvidia.com/en-us/geforce/news/rtx-30-series-community-qa/

PCIE 4.0 x8 is equal to PCIE 3.0 x16.

[WinterLFG]

2 minutes ago, Fasauceome said:

Your CPU has 4 lanes dedicated for NVMe storage, and 4 for the chipset. Then, it has 16 for the GPU. The capture card will pull lanes from the chipset, not from the CPU. 

 

You can add an NVMe drive and still have x16 for your GPU.

Okay so that's great to know.  I was getting worried that I couldn't have all 3 without ditching GPU lanes.

 

Thanks!

[WinterLFG]

11 minutes ago, minibois said:

I am unsure if you can just install the videocard in the top slot, run it at x16, skip PCIE x16_2 and just use PCIE x16_3 at x4. That would be the perfect mix of all slots.

 

So that as far I do know is possible.  I am running it on my current Mobo that has the same config as the X570 Crosshair VIII Hero with the 4k60 in the 3rd slot and it reserves 16 for the GPU then pulls 4 from the chipset.  As for the m.2 @Fasauceome says it reserves 4 for it.  So, 16x for the GPU (1st slot), 4x for the 4k60 (3rd slot), 4x reserved for m.2 (m.2 slot).  Does that sound right?

[Moonzy]

PCIE4.0 and 3.0 is just the protocol, lanes are still lanes.

you have a total of 20 lanes, and those lanes can run on either pcie 4.0 or 3.0 protocol

 

your CPU has 20 free lanes, so you can use 16 for GPU, and 4 for NVME/capture card

 

if you'd like to keep your GPU at x16, then you need to connect your capture card thru the chipset lanes

which slot to use can be found in the manual.

 

edit:

Spoiler

GPU in the CPU x16 slot, capture card into the slot from chipset

 

 

connect M.2 to the M.2_1 to use the 4 lanes from the CPU

[WinterLFG]

Thanks all for the quick responses!  I know how my mobo will operate with it all.