Port upstream sgl-project/sglang #24416 (staging-free). Our transfer metrics
were computed from the full prompt length, so total_mb / speed_gb_s were
systematically over-reported on every prefix-cache hit and every CP shared-KV
per-rank page filter. Now each sender accumulates the actually-sent KV/state
indices and reports bytes = sent_pages * per-page item bytes via a new
KVTransferMetric returned by get_transfer_metric(); prefill consumes that
instead of estimating from len(origin_input_ids), and skips fake-bootstrap and
dummy-CP-rank senders (which transfer nothing).
Also route convert_to_duration() phase deltas through duration_between(), which
returns 0 when a phase timestamp is uninitialized, fixing nonsensical negative
durations in the time-stats log.
Unit-tested (test_kv_transfer_metrics.py, 10 cases) in the CUDA-13 container.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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