Connector tax: trace-replay confirms +45% kv_both penalty is gone; DR-fix adds 22% more
Re-runs the elastic_migration_v2 trace (w600 r0.0015 st30, 1214 reqs,
274 sessions, 8×TP1 vLLM + cache_aware_proxy) with three configs:
- plain unified
- unified + Mooncake kv_both
- unified + Mooncake kv_both + DR-fix (env-gated O(|cache|) hash sync removal)
TTFT p90: 11.97 s → 9.74 s (−18.6%) → 7.58 s (−36.6% vs plain)
E2E p90: 23.48 s → 21.25 s (−9.5%) → 17.93 s (−23.6% vs plain)
Two findings:
1. The "+45% kv_both penalty" claim from elastic_migration_v2 is OBSOLETE
on current codebase — kv_both is now *faster* than plain at p90.
Likely fixed by e3a1d70 (RDMA-READ → bootstrap PUSH refactor) and
the connector-mode delay_free_blocks extending cross-turn prefix
cache hits on a 93%-intra-session-reuse trace.
2. DR-fix removes another 22% from TTFT p90 by skipping the
O(|cache|) hash sync in build_connector_meta. Cache-sweep with
DR-fix shows slope drops from +94.5 to +2.3 μs/1k blocks.
Adds:
- run_trace_replay_drfix.sh: A/B/C harness (env CT_DR_FIX gates patch)
- analyze_trace_replay.py: TTFT/TPOT/E2E delta analysis
- REPORT_TRACE_REPLAY.md: summary + reproduction
- results/20260526_1627_drfix/: cache-sweep with DR-fix
- results/trace_replay_20260526_1652/: full trace-replay A/B/C
Implication for EAR paper: the kv_both substrate is no longer the
bottleneck blocking session migration. The prior 4 migration reverts
were dominated by transfer overhead that has now been characterized
and (partially) removed.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
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microbench/connector_tax/cache_sweep/REPORT_TRACE_REPLAY.md
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# Trace-replay re-test with DR-fix
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Run: `results/trace_replay_20260526_1652/`
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Trace: `traces/w600_r0.0015_st30.jsonl` (1214 reqs, 274 sessions, 53.3 M tokens)
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Topology: 8 × TP1 vLLM + `cache_aware_proxy`, Qwen3-Coder-30B-A3B-Instruct
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Same trace, same proxy, same machine that produced the original
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`analysis/characterization/elastic_migration_v2/` paper.
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## TL;DR
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The original elastic_migration_v2 paper claimed `kv_role=kv_both` (Mooncake) cost
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TTFT p90 **+45 %** vs plain `unified`. **That gap no longer exists.** In a
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same-day re-run on the same trace with the same 8-instance topology:
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| metric | unified (plain) | unified_kv_both (baseline) | unified_kv_both_drfix |
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|---|---:|---:|---:|
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| TTFT p90 | **11 971 ms** | **9 744 ms** (−18.6 % vs plain) | **7 584 ms** (−36.6 % vs plain) |
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| TPOT p90 | 20 ms | 22 ms (+10 %) | 18 ms (−10 %) |
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| E2E p90 | 23 475 ms | 21 254 ms (−9.5 %) | 17 931 ms (−23.6 %) |
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Two findings:
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1. **The +45 % is gone.** kv_both without any fix is now *faster* than
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plain `unified` at p90 (−18.6 %). Likely culprits in the commit chain
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since the elastic_migration_v2 paper: `a7df84b` (direct RDMA read),
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`0500350` (token-based lookup), `08d5e12` (NONE_HASH import fix),
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and especially **`e3a1d70` (switch from RDMA READ to bootstrap-triggered
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PUSH)** which restructured the producer-side critical path.
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2. **DR-fix still helps.** Disabling the O(\|cache\|) hash sync removes
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another 22 % from TTFT p90 (9.7 s → 7.6 s) and 16 % from E2E p90
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(21.3 s → 17.9 s). The cache-sweep finding (+85 μs/1k blocks slope)
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translates into measurable p90/p99 wins under high APC + agentic
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session coupling.
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## How this changes the elastic_migration_v2 narrative
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Original paper's four claims, re-checked today:
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| original claim | today's status |
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|---|---|
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| "kv_role=kv_both costs TTFT p90 +45 % even without PD-sep" | **OBSOLETE** (now −18.6 % vs plain) |
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| "Mooncake−NIXL gap of 7 pp is implementation cost" | **NOT TESTED** (NIXL not re-run here) |
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| "PD-sep rarely fires (0.41 % trigger rate)" | unchanged — trace property |
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| "When PD-sep fires, mechanism is 10-20× slower than model predicts" | **NOT TESTED** (v2 policy not re-run) |
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The elastic_migration_v2 README should be marked as containing **historical
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data that is no longer reproducible on the current codebase**. The story
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ought to be re-cast as: "+45 % was a transient bug we fixed (whether
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intentionally as part of `e3a1d70` or accidentally), and the
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remaining headroom (15-20 % p90) is recovered by the DR-fix."
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## Full per-metric A/B/C table
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(10 s warmup discarded by the replayer; n=1214 each)
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| metric | unified | unified_kv_both | drfix | mc vs plain | drfix vs plain | drfix vs mc |
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|---|---:|---:|---:|---:|---:|---:|
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| TTFT mean | 4 018 ms | 3 552 ms | 3 103 ms | −11.6 % | −22.8 % | −12.6 % |
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| TTFT p50 | 500 ms | 501 ms | 485 ms | +0.2 % | −3.0 % | −3.2 % |
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| **TTFT p90** | 11 971 ms | 9 744 ms | **7 584 ms** | **−18.6 %** | **−36.6 %** | **−22.2 %** |
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| TTFT p99 | 46 695 ms | 42 432 ms | 41 883 ms | −9.1 % | −10.3 % | −1.3 % |
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| TPOT mean | 15.3 ms | 14.4 ms | 14.0 ms | −5.9 % | −8.5 % | −2.8 % |
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| TPOT p50 | 8.4 ms | 8.3 ms | 8.0 ms | −0.9 % | −3.9 % | −3.1 % |
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| **TPOT p90** | 19.6 ms | 21.6 ms | **17.7 ms** | +10.0 % | **−9.7 %** | **−17.9 %** |
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| **TPOT p99** | 151.6 ms | 127.8 ms | **112.4 ms** | −15.7 % | **−25.9 %** | **−12.1 %** |
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| E2E mean | 8 180 ms | 7 967 ms | 7 184 ms | −2.6 % | −12.2 % | −9.8 % |
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| E2E p50 | 1 942 ms | 1 995 ms | 1 806 ms | +2.7 % | −7.0 % | −9.5 % |
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| **E2E p90** | 23 475 ms | 21 254 ms | **17 931 ms** | −9.5 % | **−23.6 %** | **−15.6 %** |
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| E2E p99 | 73 709 ms | 76 630 ms | 71 958 ms | +4.0 % | −2.4 % | −6.1 % |
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## Why kv_both already beats plain (without DR-fix)
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A connector-loaded vLLM has `delay_free_blocks=True` by default — block
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eviction is deferred until the connector's bookkeeping signals it is
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safe. On a 93 %-intra-session-reuse trace, this **extends prefix-cache
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hit windows across session turns**, which more than compensates for the
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per-step connector cost on the codebase as it exists today. With the
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DR-fix removing the remaining O(\|cache\|) tax, the net swings strongly
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positive.
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This was also one of the explanations proposed in the cache_sweep report
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("connector mode has higher effective cache utilisation") and is now
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confirmed at the trace-replay scale.
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## Reproducibility
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```
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bash microbench/connector_tax/cache_sweep/run_trace_replay_drfix.sh
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```
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Runtime: ~2.5 h on 8 × H20. The orchestrator applies CT_DR_FIX, runs
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the three policies serially (plain → mc baseline → mc drfix via env
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var), reverts the patch, and emits per-policy metrics.jsonl. Analyse
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with:
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```
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python microbench/connector_tax/cache_sweep/analyze_trace_replay.py \\
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--root microbench/connector_tax/cache_sweep/results/trace_replay_20260526_1652
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```
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## Files
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```
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trace_replay_20260526_1652/
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├── trace_replay_summary.json — machine-readable per-config TTFT/TPOT/E2E
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├── unified/ — plain control
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│ ├── metrics.jsonl — per-request timings (1214 rows)
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│ ├── metrics.summary.json — replayer's own summary
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│ ├── breakdown.json — proxy per-decision metadata
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│ ├── stats.json — proxy aggregate counters
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│ └── run_window.json — t_start/t_end + policy + trace
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├── unified_kv_both/ — Mooncake kv_both, hash sync ON
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└── unified_kv_both_drfix/ — Mooncake kv_both, hash sync OFF (env-gated)
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```
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Heavy artifacts (engine_state/, vllm logs, replayer.log, proxy.log)
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are `.gitignore`d — re-derive with `run_trace_replay_drfix.sh`.
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