PCI Express

PCI Express (PCI-E) is the popular high-speed serial bus used to connect GPUs and NICs to the CPU.

The max bus speed per lane has been doubling every generation in PCI-E:

• Gen 1: 2 Gb/s
• Gen 2: 4 Gb/s
• Gen 3: 8 Gb/s, so that is 1 GB/s
• Gen 4: 16 Gb/s, so that is 2 GB/s
• Gen 5: 32 Gb/s, so that is 4 GB/s

If you see a 2-socket system with 2x PCI-E Gen3x16 links for each socket, to get the total bandwidth of the system:

2 sockets, each with 2x of Gen3x16 lanes
= 2 x 2 x 16 x (Gen3 per-lane bandwidth)
= 64 x (Gen3 per-lane bandwidth)
= 64 x 1 GB/s
= 64 GB/s