I'm wondering about the fact, that PCI architecture is hierarchical. Therefore, even if I have two PCIx4 slots, it's possible that I will not be able to fully utilize it, because the slots will connect in one node, which can have bandwidth insufficient to handle 2x PCIex4.
The background for my question is: I'm trying to utilize eight PCIe 1GBit interfaces. I have two cards packed with 2 ports, and one card packed with 4 ports. I'm able to get the maximum on 4 NIC interfaces. After activating 5th port, the performance slightly drops on each five interfaces. The same after activating 6th, 7th and 8ht interface.
Main question is: How to obtain PCIe structure on a machine, "draw" it, see it's nodes and connections, and deduce weakest node in that tree?