PCI Express

1 reply [Last post]
Rakes
Offline
Joined: 12/19/2010
Posts: 4

How much are the preformance differences between the 16x, 8x, 4x PCI express modes and how do they effect you?
Some general information about PCIE would be nice as well.

hnkftalnot
Offline
Joined: 10/14/2010
Posts: 262

As you probably know PCIe is used as a communication interface for expansion cards. The amount (x16, x8, x4, x1) stands for the number of lanes that can be used, which determines the bandwidth of the data stream. Think of it as a highway; the more lanes you have, the more traffic can move side by side at the same time.

PCIe 2.0 runs at 5 GHz so that means that each lane can move 5 gigabits of data per second (5Gb/s or 500MB/s). Multiply that with the amount of lanes and you'll get the total maximum bandwidth: a x16 slot can move 80Gb/s or 8GB/s. This is of course an enormous amount of bandwidth and almost no expansion card today uses all of it. For example soundcards, network cards, SATA cards, don't even saturate one lane (x1) and as a result only have a very short physical connector. The x4 and x8 connectors are a bit longer and the x16 is the full size you see on graphics cards. So the lanes for the data transfer are literally next to each other.

This difference in size can also be seen on the slots on the motherboard. Generally the longer the slot is, the more lanes it has, but there are some exceptions; some full sized x16 slots only support x8 in some situations. It might seem weird to have different lengths of connectors and slots, but it's actually quite convenient because they all work together, it's just that the actual bandwidth available is limited by the amount of pins (lanes) that are connected: You can run any card (x1,x4,x8,x16) in an x16 slot, but also the other way around: Any card can be connected to an x1 slot, only it will run at the x1 speed because the extra pins are not used.

Again, today there are no cards that really utilize the bandwidth of x16. Even in high-end graphics card setups you see that whether they are run at x8 or x16 barely impacts performance. Ironically, PCIe 3.0 is already announced and promises to even double the bandwidth, which makes sure that PCIe will stick around for a very long time.