B2 finding: TPOT idx peaks at 32k, not 65k — cost migrates to TTFT

The B2 same-worker TPOT p90 idx is non-monotone: 7.89x at 32k drops
to 2.26x at 65k. The naive reading is "interference gets weaker for
huge prefills"; the actual mechanism is a regime shift, and reading
TPOT p90 alone is misleading.

Three superimposed effects:

1. Cost migration TPOT -> TTFT. A 32k prefill is short enough that
   chunked-prefill keeps interleaving decode steps, so overlapping
   decodes trickle tokens out at painful per-token rates. A 65k
   prefill is long enough that overlapping decodes are *fully*
   blocked for ~10s; once they break through, the injection is
   winding down and subsequent iterations run unobstructed. The
   cost lands on the TTFT clock (14s) instead of inflating TPOT.

2. Bimodal TPOT distribution. At 65k overlap, decodes split into
   "blocked entire prefill then normal rate" and "trickled slowly
   through prefill chunks". p99 sits on the second population and
   grows 59 -> 169.5 ms; p90 sits on the first and shrinks.

3. "Clean" stops being clean. With 4x ~10s injections in 60s, the
   110 "clean" decodes at 65k are squeezed into 2-3s recovery
   pockets. TPOT p90 clean rises 6.9 -> 9.6 ms (40%), shrinking
   the denominator of the ratio.

window_1_results.md adds a new B2 subsection laying out the
mechanism with the per-cell data table and the explicit reading
rule: headline interference metric is TTFT idx (monotone); TPOT
p99 is the right tail indicator; TPOT p90 alone is unsafe across
regime shifts. Direct implication: TTFT and TPOT need separate
SLO thresholds under PD-colo, because they measure costs from
different points in the request lifecycle and the cost migration
between them is workload-dependent.

current_results/characterization_claim_matrix.md adds a new
supported claim for the cost migration, listed against the existing
B2 evidence. current_results/reviewer_risk_register.md adds a
low-severity entry warning future readers off TPOT p90 alone.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

analysis/characterization/current_results/characterization_claim_matrix.md

@@ -13,6 +13,7 @@ sweep, B2 PD-colo interference microbench).
 | Cache-aware LMetric leaves a measurable locality gap (22.7 pp). | `supported` | lmetric achieved 56.9% vs intra-session ceiling 79.6%; B3 sweep window_1_results/b3_policy_comparison.json. | — | sticky data shows the gap can be recovered by harder affinity. |
 | Hybrid affinity (`unified`) breaks the locality-vs-latency tradeoff. | `supported` | unified APC 79.4% (97% of intra ceiling) AND TTFT p90 7.24 s (lmetric is 15.6 s). | — | unified concentrates a single very hot worker (engine_4 at 37.7 s p90); hotspot_index 3.35. |
 | Same-worker prefill-decode interference is causal, not correlation. | `supported` | B2 microbench: different-worker control idx 0.92-1.02 across 32× prefill-size variation; same-worker TTFT idx scales 2.15× (2k) → 218× (65k). window_1_results/b2_sweep_summary.json. | — | Synthetic decode load (256-token prompts at 4 req/s) bounds the realism; production behavior is layered on top of B3. |
+| The cost of same-worker prefill interference migrates from TPOT to TTFT as prefill size grows past the chunked-prefill horizon. | `supported` | B2 same-worker TPOT p90 idx peaks at 32k (7.89×) and *drops* at 65k (2.26×), while TTFT idx grows monotonically (94.6× → 218×) and TPOT p99 grows monotonically (59 → 169.5 ms). See window_1_results.md "TPOT idx peaks at 32k, not 65k". | — | SLO thresholds for TTFT and TPOT cannot be the same under PD-colo; this should be reflected in B4 SRR sweep design. |
 | Hard session affinity (`sticky`) inflates same-worker prefill-decode interference. | `supported` | sticky interference_index 13.65 vs lmetric 6.53; sticky's slow-request breakdown 57% same-worker overlap vs lmetric 23%. | — | Confirms the B2 causal claim observed at the system level. |
 | Heavy-tail sessions are a contributor to hot-spot but not the sole cause. | `supported` | Cap-8 trace (37% requests dropped) reduces hotspot_index only 13% (2.24 → 1.94). | Run capped under unified to see whether unified's hotspot also persists. | Reviewer might counter that cap=8 is too soft; a stricter cap could be tried. |
 | SRR per policy under SLO is not yet measured. | `not_yet_supported` | B3 was driven by trace timestamps with strict session sequentiality; saturation is reached but not parameterized. | Run B4 with the A4 open-loop Poisson loadgen, per-class SLO, 5 policies × λ binary search. | Without B4 the paper cannot claim "policy X sustains higher load than Y". |

analysis/characterization/current_results/reviewer_risk_register.md

@@ -13,3 +13,4 @@ Updated 2026-05-25 after Window 1.
 | Trace-timestamp dispatch with strict session sequentiality stretches replay wall time | medium | lmetric's 600s trace dispatched over 49 min; system over-saturates and the dispatch window expands. | Window 2 uses A4 open-loop Poisson loadgen with explicit arrival rate, decoupling load level from trace structure. |
 | Capped cap=8 may be too soft | low | Reviewer might prefer cap=2 or cap=4 to test "no multi-turn" extreme. Cap=8 was chosen to sit between turns/session p90 (1) and p99 (18). | Re-run with a stricter cap if reviewer pushes back; underlying capped script is parameterized. |
 | B2 microbench uses synthetic short-prompt decode load (256 tokens) | low | This bounds the realism of the "decode" workload. Production decode tokens come from prior turns of long context. | The signal magnitude is robust enough that prompt length shouldn't qualitatively change conclusions; B3 sticky's failure breakdown is the production-trace confirmation. |
+| Reading B2 same-worker interference from TPOT p90 alone gives a non-monotone curve | low | TPOT p90 idx peaks at 32k (7.89×) then drops at 65k (2.26×) even though TTFT idx grows monotonically (94.6× → 218×) and TPOT p99 grows monotonically (59 → 169.5 ms). The drop is regime shift (cost migrates from TPOT to TTFT once prefill blocks first-token long enough), not interference relief. | Reports must lead with TTFT idx; TPOT p99 is the right tail indicator for TPOT. See window_1_results.md §"TPOT idx peaks at 32k, not 65k". |