How does PCIe connect everything?
PCIe connects processors with devices such as GPUs, network cards, and SSDs. Follow the whole connection, not just the slot label.
Two roads share one bridge
- Device A + device B
Each requests 20 GB/s: 40 GB/s together
- Shared upstream connection
Assumed sustained capacity: 30 GB/s
- CPU-side destination
Cannot receive the full combined demand through that path
Several devices can share one path
A PCIe link has a generation and a lane width. x16 means 16 lanes, not 16 GPUs. The actual connection may use fewer lanes than the physical slot suggests.
Devices behind a switch can share its upstream path. In the diagram’s invented example, two devices want 40 GB/s together but the shared path carries 30 GB/s.
Inspect without changing
On an authorized NVIDIA Linux host with driver tools:
nvidia-smi topo -mRead its legend to understand device relationships. It is not a bandwidth benchmark. Use the paper exercise on a Mac with an Apple GPU.
Do not change security or isolation settings to chase a faster test without the required review.
Try it yourself
Draw two GPUs and a NIC behind one switch. Label each link with an assumed capacity.
Check your answer
Find the narrowest shared part of the path. Faster endpoints cannot make that connection carry more than its capacity. Actual sharing depends on traffic and arbitration.