No error, no hang, the loss still moves, but the run is suddenly half as fast and burning the same dollars. The triage that separates a straggler from a fabric problem from broken comm/compute overlap, using the signals nvidia-smi can't give you.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
No error, no hang, the loss still moves, but the run is suddenly half as fast and burning the same dollars. The triage that separates a straggler from a fabric problem from broken comm/compute overlap, using the signals nvidia-smi can't give you.
This screens for someone who debugs goodput, not correctness. The trap is staring at nvidia-smi (still 100%) and concluding everything is fine. Strong candidates reach for per-rank step-time distributions and a profiler trace to split compute vs comm, then bisect to a straggler (one slow rank), a fabric regression (all ranks waiting), or broken overlap (comm no longer hidden behind backward).
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