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A two-node deployment loads weights, reports healthy, then hangs on the first request. Debug NCCL and the engine.

A hang gives you no error, no log line and no metric. The answer shows how to make every rank say where it is stuck, why a wrong network transport makes things slow but rarely hangs them, and the order that finds the cause in minutes.

Updated Oct 2026 · Worked answers on inference mechanisms, with calculations, implementation details, and follow-up questions.

A hang gives you no error, no log line and no metric. The answer shows how to make every rank say where it is stuck, why a wrong network transport makes things slow but rarely hangs them, and the order that finds the cause in minutes.

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Next in this trackAfter turning on speculative decoding, p50 latency improved and p99 got worse. Explain it, and decide what to ship.Next in this trackAn 8B model on one H100 stops scaling at high request rates and the GPU is not saturated. Prove the CPU is the bottleneck, then fix it.Next in this trackMemory on an inference server grows slowly all day until the engine OOMs and restarts. Find the leak.
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