A weird pcie question.

theflyingfoo · February 11, 2021

This is my first time on the forum and I tried looking through the subsections but still couldn't figure out where to post this so I'm hoping I'm in the right place!

I'm struggling to figure out if I've already saturated all of my pcie lanes. I've listed my parts below and eventually I wanted to add in an Elgato 4k60 pro and maybe a sound card down the line. Eventually I will also upgrade the gpu and I don't know if that's a factor. Am I already out of lanes? If I add anything else will my gpu suffer because of it? Thank you in advance!

P.S. can a cpu output both PCIe 4.0 and 3.0 lanes simultaneously? I want to make sure my boot drive is using PCIe Gen 4 speeds.

My current build is a Ryzen 9 3900x on an Asus ROG Strix X570-e Motherboard air-cooled by a BeQuiet Dark Rock Pro 4. I have 32 gb of Trident Z DDR4 3600 MHz ram, a PNY Xlr8 OC RTX 2060 Super, a Tiergrade pcie 4x usb add in card, a 1tb Sabrent Rocket Gen 4x4 NVME SSD boot drive, a 1tb Samsung 860 Evo Sata SSD, 2 random 2.5 in Sata hard drives (1 tb each), and it's all bundled in a Meshify C case by fractal design.

mariushm · February 11, 2021

You have 24 pci-e lanes coming from cpu : 4 go to first m.2 , 16 go to video cards (either one x16 or two slots x8, depends on motherboard maker) and 4 go to chipset.

The chipset creates 8 extra pci-e lanes, which are typically arranged in a second m.2 connector (2 or 4 pci-e lanes) and/OR a pci-e x16 slot that has only 4 pci-e lanes in it, and some pci-e x1 slots

Your board has 3 pci-e x16 slots ... the first two come from the cpu and use those 16 pci-e lanes from cpu. If you have a single video card in first slot, it gets all 16 pci-e lanes. If you plug something in that second pci-e slot, both slots become pci-e x8. Will you feel a performance loss in some games? Yeah probably, but it's gonna be in the 1-2% maximum , so a few fps at best.

The bottom pci-e x16 slot has 4 pci-e lanes which come from the chipset.

It shares bandwidth with the pci-e x1 slot right above it, so you probably wouldn't want a sound card installed in that slot, if you install the capture card in the bottom pci-e slot (because it may affect the transfer speeds in the capture card, if it captures raw video) but it's worth testing ... sound cards don't use a lot of bandwidth.

The M.2 connector will stay at pci-e 4.0 speed if the ssd supports pci-e 4.0

The two pci-e x16 slots will both switch to pci-e 3.0 if your video card only supports pci-e 3.0

theflyingfoo · February 11, 2021

4 hours ago, mariushm said:

You have 24 pci-e lanes coming from cpu : 4 go to first m.2 , 16 go to video cards (either one x16 or two slots x8, depends on motherboard maker) and 4 go to chipset.

The chipset creates 8 extra pci-e lanes, which are typically arranged in a second m.2 connector (2 or 4 pci-e lanes) and/OR a pci-e x16 slot that has only 4 pci-e lanes in it, and some pci-e x1 slots

Your board has 3 pci-e x16 slots ... the first two come from the cpu and use those 16 pci-e lanes from cpu. If you have a single video card in first slot, it gets all 16 pci-e lanes. If you plug something in that second pci-e slot, both slots become pci-e x8. Will you feel a performance loss in some games? Yeah probably, but it's gonna be in the 1-2% maximum , so a few fps at best.

The bottom pci-e x16 slot has 4 pci-e lanes which come from the chipset.

It shares bandwidth with the pci-e x1 slot right above it, so you probably wouldn't want a sound card installed in that slot, if you install the capture card in the bottom pci-e slot (because it may affect the transfer speeds in the capture card, if it captures raw video) but it's worth testing ... sound cards don't use a lot of bandwidth.

The M.2 connector will stay at pci-e 4.0 speed if the ssd supports pci-e 4.0

The two pci-e x16 slots will both switch to pci-e 3.0 if your video card only supports pci-e 3.0