What does this equation mean?

which is the reason ring all-reduce is called bandwidth-optimal: in the limit, total traffic per worker approaches twice the buffer size regardless of how many workers participate, rather than growing with p . What does grow with p is the number of sequential steps, 2(p-1) , and each step’s latency is bounded below by the slowest link and the slowest worker in the ring — which is precisely why NCCL switches to a tree algorithm for smaller messages and larger worker counts, trading some bandwidth efficiency for a $\log$ p step count instead of a linear one [ 5 ] . This is an analytical property of the algorithm, not a vendor claim; the tree-versus-ring choice NCCL actually makes at runtime, however, is implementation-specific tuning NVIDIA has published as measured behavior on its own systems, and should be read as a vendor-characterized default rather than a universal law [ 5 ] .