# PCIe vs NVLink vs InfiniBand: The Invisible Impact of Bandwidth on Training

> Source: https://sukruyusufkaya.com/en/learn/fine-tuning-cookbook/ftc-pcie-nvlink-infiniband-bandwidth
> Updated: 2026-05-14T14:42:49.820Z
> Category: Fine-Tuning Cookbook (Model-by-Model)
> Module: Part I — Hardware & Memory Engineering
**TLDR:** Bandwidth is invisible on a single 4090 but at scale-out it alone can slow training. PCIe 4.0/5.0 lane math, NVLink (and why 4090 doesn't have it), NVSwitch topology, InfiniBand 400G, threshold where NCCL all-reduce becomes network-bound, p2p_access detection, GPU-direct.