ASUS ROG STRIX X870-I GAMING WIFI (w/AMD Ryzen 9 9950X) - UNDERSTANDING PCI LANE INFO

[HBSound]

It's not a bad idea, it's just waste of resources. They are not overwhelmed, they are badly distributed IMO.

You have 16 lanes for GPU, 4 lanes for main M.2 slot and another 4 general purpose lanes. Then another 4 lanes to communicate with chipset. 28 lanes in total from CPU.
On USB 4.0 boards (X870, X870E) GPU slot will switch to x8 mode as soon as you occupy 2nd M.2 slot. (Unless there is special feature for it in BIOS ...)

So you end up with having 8x lanes for GPU, 2x 4 lanes for 2x M.2 and another 4 lanes are consumed by two USB 4.0 ports (two lanes per port). That's 8 + 2x4 + 4 = 20 lanes + 4 lanes to chipset.
It's 24 in use. Another 4 lanes remain totally unused. And that is bad IMO, it's a waste of resources. PCIe lanes are scarce. You can't really do anything about it in this situation.

This is why I advised that motherboard from MSI in my previous post, as it has a PCIe switch and option to run USB 4.0 ports in 4.0 or 3.2 mode.
Thus, GPU will remain to have full x16 lanes, only 2nd M.2 port will get half the bandwidth. This is much better approach IMO because if you have motherboard
that have 2x PCIe x16 slots (first x16, second x8) you can use 8 lanes for GPU and another 8 lanes for other purpose (M.2 bifurcation card for RAID, etc).


That is absolutely pointless. Choose with your wallet and per your requirements. All NVMe drives are mostly handled by Microsoft's universal NVMe driver. Samsung's NVMe driver used to be a bit better but Samsung stopped developing it long time ago. As far as a drive and driver both support certain standard, like NVMe 1.3 or NVMe 1.4, there's no need to install anything. It's same as with SATA (AHCI).

You should, however, use exactly same drives for RAID configuration, it's advised for all drives in same array to be of same speed.


Don't thank me. Thank dude who brought up that MSI motherboard in linked thread.

So if you occupy 2nd M.2 slot, it will run in x2 mode (that's like x4 mode in PCIe Gen 4.0. It will also degrade USB 4.0 ports to just USB 3.2 ports. GPU port remains x16.

Thanks for sharing the information.

I wish MSI offered their X8 motherboards in ITX for the AMD5 chip.

Have a great day!

 

[Speedyblupi]

Why is 5.0 x8 a bad idea in your opinion. What exactly are you losing? The data rate is the same as 4.0x16. Are you suggesting the PCie lanes on the MB are being overwhelmed?

PCIe 5.0 x8 has the same bandwidth as PCIe 4.0 x16, but current graphics cards don't support PCIe 5.0, so it would run at PCIe 4.0 x8 unless you use a next-gen GPU with PCIe 5.0 support.
But the performance loss would only be about 5-10% in a worst-case scenario, at least with current GPUs. This can happen with an RTX 4090 in bandwidth-intensive tasks, and potentially on some weaker PCIe 4.0 x16 graphics cards if they run out of VRAM; but most games don't need much PCIe bandwidth (though a few do, such as DOOM Eternal), and you should generally try to avoid running out of VRAM regardless of your PCIe bandwidth.
Most new low-end graphics cards (e.g. RTX 4060 Ti, RTX 4060, RX 7600 XT, RX 6650 XT, Arc A380) only have 8 PCIe lanes anyway, so you may as well use the extra lanes for M.2 slots if you aren't going to use a high-end card.
If you have a 16-lane GPU, it's usually best to give your GPU all 16 lanes if you have the choice, in order to ensure that you're getting the best possible performance. But the difference is negligible in most situations, so you shouldn't worry much about it if you really need/want to use the lanes for something else.

 

[kapone32]

PCIe 5.0 x8 has the same bandwidth as PCIe 4.0 x16, but current graphics cards don't support PCIe 5.0, so it would run at PCIe 4.0 x8 unless you use a next-gen GPU with PCIe 5.0 support.
But the performance loss would only be about 5-10% in an worst-case scenario, at least with current GPUs. This can happen with an RTX 4090 in bandwidth-intensive tasks, and potentially on some weaker PCIe 4.0 x16 graphics cards if they run out of VRAM; but most games don't need much PCIe bandwidth (though a few do, such as DOOM Eternal), and you should generally try to avoid running out of VRAM regardless of your PCIe bandwidth.
Most new low-end graphics cards (e.g. RTX 4060 Ti, RTX 4060, RX 7600 XT, RX 6650 XT, Arc A380) only have 8 PCIe lanes anyway, so you may as well use the extra lanes for M.2 slots if you aren't going to use a high-end card.
If you have a 16-lane GPU, it's usually best to give your GPU all 16 lanes if you have the choice, in order to ensure that you're getting the best possible performance. But the difference is negligible in most situations, so you shouldn't worry much about it if you really need/want to use the lanes for something else.

Well I tested my 7900XT at X16 and x8 and noticed at the most 3% improvement. There are still 16 physical lanes connected to the CPU and GPU electrically. Even if it is running at x8.

 

[LittleBro]

Well I tested my 7900XT at X16 and x8 and noticed at the most 3% improvement. There are still 16 physical lanes connected to the CPU and GPU electrically. Even if it is running at x8.

How did you test it? By putting GPU into 2nd PCIE_16 x8 slot or by changing bandwidth mode for primary x16 slot to older generation?

They are still connected but when you occupy both PCIE_16 x16 and PCIE_16 x8 slot at the same time, PCIe switch will cut (virtually) 8 lanes from primary slot, though they are physically 16+8 in total.

 

[Speedyblupi]

There are still 16 physical lanes connected to the CPU and GPU electrically. Even if it is running at x8.

Well yeah. The physical traces in the motherboard and graphics card for the other 8 lanes don't somehow stop existing just because they aren't being used. I'm not sure what you're trying to say here, other than stating the obvious.

 

[kapone32]

How did you test it? By putting GPU into 2nd PCIE_16 x8 slot or by changing bandwidth mode for primary x16 slot to older generation?

They are still connected but when you occupy both PCIE_16 x16 and PCIE_16 x8 slot at the same time, PCIe switch will cut (virtually) 8 lanes from primary slot, though they are physically 16+8 in total.

By not populating the 2nd PCIe slot and setting the PCIe setting to x16 and then PCIe RAID mode.