I am looking into building a new machine soon. I figured why not go for x570 motherboard. My biggest question is with the video cards, and PCIe on x570 motherboards.
The specs seem to suggest that with two GPUs, the PCIe slots will run at 4.0 8x/8x at best with the current x570 boards. This is where my question comes in.
The current generation of video cards only support PCIe 3.0. So what would be the speeds of the PCIe slots with 3.0 video cards installed? My my guess is that they will run at 3.0 x8/x8, but I would like to know if that is not the case.
I might choose different video cards if it does, in fact, run at x8/x8. Mostly because there is no point spending the extra cash to have a RTX 2080ti that will pobably take 2-3% performance hit at 3.0 x8.
They will not be running in SLI. I plan on doing VFIO and doing PCI passthrough. That’s why I am asking specifically how the PCIe slots would function, and not about the GPUs.
By installing a PCIe3 GPU into a PCIe4 slot, you are limiting the slot to the older spec, yes. And if you do that twice, that should end up as 8x each. … Maybe I’m still not quite getting what you are asking but I agree with you that that is what should happen.
Thank you, that’s what I was trying to figure out. I was unsure of how pcie lanes worked. I posted what my understanding of it was from what I researched this morning, but I was hoping someone could confirm my understanding.
I don’t have a problem spending money to get a higher performing part, but there is no point in buying the part if it is bottlenecked by the bandwidth of the PCI Lanes due to my configuration. It’s better to buy 2 video cards that won’t out-perform the available bandwidth.
That is what I was originally hoping to. But the pcie3 devices will not be able to utilize the 4.0 protocol, so it won’t be able to benefit from the bandwidth enhancements.
PCIe is only as fast as the oldest generation in the chain for better or for worse.
In theory it might be possible to run two PCIe 3.0 x16 cards through PCIe 4.0 x16 using something like a complex PLX chip (think of a VRM phase doubler, except digital IO), but I’m pretty sure something like that, if it did exist, would add quite a bit of $$$ to the BOM for the board.
You could get a board that has x4 4.0 ( x16 mechanically ) from chipset and hook Radeon 5700 (XT) there. Bandwith should be enough.
But at that point you dont want to connect much else through chipset.
The only alternative is HEDT from Intel or AMD ( wait for TR 3000 or deal with NUMA ).
Hello, it is not a problem to run your gpu’s at 3.0 @ x8 speeds. GPU’s cannot saturate a 3.0 x8 bus anyway let alone a 4.0 x8
Your VFIO setup will run just fine with two cards at x8 without any noticable performance loss.
Be aware that many mainboards go 8x/4x when dual cards are installed.
If you wan’t a really good motherboard that provides x8/x8/x8 for 3 GPU’s and or capture cards, there is only one board on the market for that config and that is the ASUS Pro WS X570 ACE
The downside is that it “only” has 2 m.2 connectors and 4 SATA ports. However, you can turn the onboard U.2 slot into a m.2 OR you can install a breakout cable and turn it into 4x SATA
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