Question Z790 Motherboards and Lane Sharing

[Midnight822]

As I've been researching Z790 motherboards for a new build, I'm seeing a common issue of the primary M2 slot and the primary GPU slot share the same lanes. So if you use your 5.0 M2 in the slot connected to the CPU, your GPU in it's top spot will go from 16 lanes to 8 lanes. Is this a common problem the entire z790 platform shares, or do I need to widen my search on potential options? I've been looking for an E-ATX, so I've been looking at a small pool of potential options.

 

[kanewolf]

As I've been researching Z790 motherboards for a new build, I'm seeing a common issue of the primary M2 slot and the primary GPU slot share the same lanes. So if you use your 5.0 M2 in the slot connected to the CPU, your GPU in it's top spot will go from 16 lanes to 8 lanes. Is this a common problem the entire z790 platform shares, or do I need to widen my search on potential options? I've been looking for an E-ATX, so I've been looking at a small pool of potential options.

That is not typical. The M.2 and GPU slot have separate PCIe lanes.

 

[HarryDrews]

That is not typical. The M.2 and GPU slot have separate PCIe lanes.

Does that mean I would be able to add USB expansion card to PCIe lanes direct to the CPU? I'm using MSI Pro Z790-P, but having difficulty finding a resolution for creating a USB port direct to CPU.

 

[kanewolf]

Does that mean I would be able to add USB expansion card to PCIe lanes direct to the CPU? I'm using MSI Pro Z790-P, but having difficulty finding a resolution for creating a USB port direct to CPU.

If you used the slot(s) that have PCIe mapped to the CPU. But then your graphics card would have fewer PCIe lanes.

 

[HarryDrews]

If you used the slot(s) that have PCIe mapped to the CPU. But then your graphics card would have fewer PCIe lanes.

Interesting.. I'm running a 4080 Super and do 99% gaming, so I don't want to do that. Is there any other alternative?

 

[kanewolf]

Interesting.. I'm running a 4080 Super and do 99% gaming, so I don't want to do that. Is there any other alternative?

Get a CPU with a lot more PCIe lanes. Threadripper for example.

 

[HarryDrews]

Get a CPU with a lot more PCIe lanes. Threadripper for example.

I was warned, and I should've done more research. I should've gone with the 7800X3D but I gotta live with my decision

 

[35below0]

As I've been researching Z790 motherboards for a new build, I'm seeing a common issue of the primary M2 slot and the primary GPU slot share the same lanes. So if you use your 5.0 M2 in the slot connected to the CPU, your GPU in it's top spot will go from 16 lanes to 8 lanes. Is this a common problem the entire z790 platform shares, or do I need to widen my search on potential options? I've been looking for an E-ATX, so I've been looking at a small pool of potential options.

The primary PCIe 4.0 x16 slot will downgrade to 8x if you use a NVMe drive in the M.2 5.0 slot.

This is not as bad as you might think. GPU performance will not be halved for example. Depending on GPU you might lose 2-5% performance.

You can avoid this downgrade, if it's important to you you to use full x16, by not using the M.2 5.0 slot. Instead, stick your drive into one of the other M.2 4.0 slots. Difference in speed between 5.0 and 4.0 (and 3.0) is negligible. You COULD notice it if you transferred a billion files every day, but in normal use there is effectively no difference.
So yeah, use the chipset connected M.2 slots.

Or buy a motherboard that has a CPU connected M.2 slot that doesn't downgrade the PCIe 4.0 16x slot. A few exist. The M.2 slot on those motherboards is not 5.0 but 4.0 instead however that makes little difference. The main thing is it doesn't downgrade the PCIe slot.

But the simplest thing you could do? Forget your fears. There is hardly any penalty to this lane-sharing issue.

 

[thestryker]

Intel only has 16 lanes of PCIe 5.0 on current client platforms so using a PCIe 5.0 M.2 slot will always drop the lanes in the primary PCIe slot to 8. It's a weird design choice (from motherboard manufacturers), and doubly so when manufacturers like MSI make it the only option for PCIe M.2 from the CPU. I'm sure the reason they did it is "bigger numbers" despite the fact that it's barely useful. PCIe 5.0 drives don't improve shortcomings over PCIe 4.0 drives so unless there's a need for the higher speed read/write it gains nothing.