ring

Two phases, n minus one steps each, one slice per step: the derivation that gives 2(n−1)/n times the buffer per rank, the argument that no algorithm can beat it on bytes, and the latency term that makes NCCL switch to trees at scale.

Before the first byte moves, the library has already discovered the machine's topology, built rings and trees over it, split the work into parallel channels and chosen a wire protocol by message size. What each of those four decisions is for, and the environment variables that let you see and override them.