[mlyle]

> FWIW, I was also able to iperf3 around 3.7Gbps on a X520-DA2 connected to an RPi4's single-lane PCIe 2.0.

Yah, I said forwarding-- so each packet goes over the PCIe link twice.

So, to use your test-- 3.7 x 4 / 2 = 7.4gbps. I said around 7. 7.4gbps would be a bit more than I'd expect, but I'd not be shocked.

[q3k]

But PCIe is full duplex! With PCIe 2.0 x4 there's 4 lanes in each direction [1], so when 'forwarding' over a single 10G link you can expect to send and receive simultaneously at the speed I mentioned earlier.

[1] - https://en.wikipedia.org/wiki/PCI_Express#Pinout

[mlyle]

Yah, I guess dividing by 2 isn't fair. But transmitting does impact receiving and vice-versa: when you're reading DMA descriptors, you need to wait/hold for posted completions, etc. It's not fully uncontended between send and receive, but more uncontended than a naive division by 2 would imply.