kzlin c5519066de docs(kvc): add TTFT probability density figure (KVC v2 vs 4DP) ...

Adds a two-panel TTFT PDF comparison plot inserted as a new V2_DEEP_ANALYSIS
§3.4 ("TTFT 概率密度对比: bimodal vs unimodal"). Single-percentile numbers
(p50 / p99) hide the qualitative difference between the two distributions;
the figure makes it visible at a glance.

Left panel (linear x in [0, 0.6]s, body):
  KVC has a sharp peak at ~40ms (the direct-to-D fast path).
  DP has a broad peak around 50-200ms (full prefill per request).
  Annotated with p50 and p90 markers for each side.

Right panel (log x in [10ms, 10s], full range):
  KVC is visibly bimodal: a tall fast-path peak plus a small reseed tail
  around 1-5s.
  DP is unimodal: a single broad peak with shorter tail.
  Annotated with p99 callouts pointing to each tail.

KDE: scipy.stats.gaussian_kde, bandwidth=0.15 for the body (Scott's rule
oversmooths the sharp fast-path peak), log10-transformed for the full-range
panel so the bimodal structure is visible.

Bundled:
- scripts/analysis/plot_ttft_pdf.py -- rerunable when v2 / DP data change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-11 17:46:27 +08:00

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