Background nvidia-smi poller runs at 1 Hz for all 4 GPUs throughout
the sweep, writing CSV to $OUTPUT/gpu_util.csv. Captures:
timestamp_iso, gpu_index, util_pct, mem_used_MiB, mem_total_MiB,
sm_clock_MHz, power_W, temperature_C
Sampler is started before benchmark-live and torn down via trap on
EXIT/INT/TERM so it always cleans up even if the run is killed.
This data lets us plot time-windowed wall-clock GPU utilization
(per-card) so we can answer "is concurrency the bottleneck or is
each D's per-session decode the bottleneck" — a question that
came up during E4-v3 / v5 analysis.
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