Switch-Hitter
Flexible as it is, PCIe has its drawbacks. Being point-topoint means any unused bandwidth stays unused; it can’t be re-assigned once the initial links are assigned. Also, adding PCIe lanes without changing motherboards and/or processors is next to impossible. Enter the PCI Express switch chip, found on some high-end boards. It sits on a PCIe link between devices and the controller, turning one end point into multiple end points, and acting as a network switcher, routing data as devices require it. For example, you could split one x4 link on your motherboard’s PCIe controller into two x4 M.2 ports, each with an SSD drive. Both drives “think” they have a x4 link. Bandwidth is split between the drives as required—x4 is plenty for an SSD anyway. One PCIe link can be fanned out across a range of ports and connections, all of which act as if they had been assigned exclusive PCIe links.
The ability to fool devices into thinking they have more lanes than are physically available also means you can use configurations that would otherwise be impossible. High-end gaming boards can use a switcher chip, such as PLX’s PEX 8747, to run three or four graphics cards at x8 from a single x16 link PCIe controller on the CPU. Or you can fool two cards into acting as if they both had x16 links. Obviously, the total bandwidth is fixed by the PCIe link your switcher is connected to, but you can make full use of the bandwidth across multiple devices. In this case, the cards dynamically share a x16 link— if one wants to run at over x8 performance, it can, as long as there is slack on the second card. It’s a neat solution. Apart from a dollar cost, it does add a little latency (100ns or more), which adds a small performance overhead.