Single-node fine and two-node halved is a specific signature, and it points at one thing before anything else. Why the copy path costs exactly about half, the five causes that produce it, and the one log line that confirms which.
Updated Sep 2026 · Grounded in real AI infrastructure interview loops and written to a senior-engineer editorial bar, with every number worked and every diagram hand-built.
Single-node fine and two-node halved is a specific signature, and it points at one thing before anything else. Why the copy path costs exactly about half, the five causes that produce it, and the one log line that confirms which.
Ranked by how closely each one overlaps this question's topic, so the first card is the thing to read if the answer above moved too fast.
Scored on reading the signature rather than starting a general investigation, on knowing why the host bounce roughly halves bandwidth, and on the NCCL log line and module checks that identify the cause.
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