How are PCIE lanes alotted to components?

[Nailezs]

Hey guys. I've been looking into ow PCIE lanes are used by components, and I some questions that I haven't been able to find answers to.

The title doesn't really state my question(s) in a succinct manner, so I have an example.

i5-10400 CPU
supports 16 PCIE Lanes

Z490 Chipset
Processor PCIE config: 1x16, 2x8, 1x8+2x4
PCH PCIE config: 24

RTX-3080
uses 16 PCIE lanes (right?)

M.2 nVME drives
use 4 PCIE lanes


Do the PCIE slots all share the same 16 lanes?

So, if the CPU only supports 16 lanes, does the GPU use up all of the usable lanes on the MB, period?

Then, the M.2 slots have 0 lanes to use, right?

OR, are M.2 lanes (and other peripherals) running off of the "PCH" PCIE lanes?

What are PCH PCIE lanes?

Are PCH PCIE lanes completely separate from CPU PCIE lanes and/or the PCIE slot PCIE lanes?


Thank you!

 

[therealmeep]

Take a look and see if you can find a PCIE layout of your board. I was able to find one for my MSI X99 board that showed exactly all of this. Judging by the spec of your chip it will most likely be running your GPU at 16x, and then your storage off of the PCH's lanes. PCIE slots can go to either, so I would see if you can find the layout for your board. You can verify by using GPU-Z and looking at the bus interface section. It will say something along the lines of "PCIe x16 3.0 @ x16 1.1" Meaning it's negotiated to use up to PCIE 3.0 with 16 lanes.

 

[Sasqui]

PCIe lanes aren't shared. If the chipset and board support a certain number of lanes, that's it. Its the lesser of the two. If the CPU supports 16... then you'll get either 1x16 OR 2x8 OR 1x8+2x4. As long as the motherboard supports at least the same. If you have another device using any lanes, your graphics card will use no more than x8. Good news is it doesn't affect performance terribly much

 

[Nailezs]

@therealmeep Your suggestion of RTFM was apt - according to page 21 of the board in question, the M.2 slots are run off of the PCH, which is separate from the PCI-E Bus the GPUs plug into. Similarly, the 2x PCIE x1 slots also run off of the PCH.

This simple diagram answered all of my questions, hah.

 

[R-T-B]

@therealmeep Your suggestion of RTFM was apt - according to page 21 of the board in question, the M.2 slots are run off of the PCH, which is separate from the PCI-E Bus the GPUs plug into. Similarly, the 2x PCIE x1 slots also run off of the PCH.

This simple diagram answered all of my questions, hah.

The PCH is also further bottlenecked by whatever link it has. On intel, it's usually equivalent to a DMI link equivalent to a PCIe 3 x4 link. So you get 24 lanes competing for x4 worth of bandwidth. Just FYI.