# Claude Characterization Work Plan

Status: planning, awaiting dash0 idle
Date: 2026-05-25
Owner: Claude (not interns)
Source of requirements: `analysis/characterization_todo_for_interns.md`

## Scope

This plan covers the four hard gates and the B2–B5 GPU experiments that the
intern TODO marks as `NOT DONE` / `protocol DONE`. The B0 analyzer, the
B1 trace-shape statistics, and the B6 audit scaffold are already done; this
plan does **not** re-do them, only refreshes their inputs.

The work is split into:

- **Phase A (CPU-only)** — instrumentation + analyzer extensions. Can run
  on the local dev box; does **not** need dash0. Must finish before any
  GPU run.
- **Phase B (dash0 GPU)** — controlled microbench + routing sweep + SRR
  sweep + failure attribution.
- **Phase C (CPU-only)** — final audit package refresh.

## Phase A: Instrumentation + Analyzer (CPU-only, before dash0)

### A1. Replayer instrumentation — close Gate 1 + Gate 2

File: `replayer/metrics.py`, `replayer/replay.py`

Add these fields to `RequestMetrics`:

```text
t_dispatch_unix     float   # absolute wall-clock when POST starts
t_first_token_unix  float   # absolute wall-clock at first stream chunk
t_finish_unix       float   # absolute wall-clock at stream done or error
proxy_request_id    str     # value sent in X-Request-Id (matches breakdown)
endpoint_url        str     # which proxy/instance the request hit
trace_hash_ids      list[int]  # carried from trace for reuse joins
```

Change `_dispatch_request` to:

- send a deterministic `X-Request-Id: <session_id>:<turn_id>` header (so
  proxy breakdown can be joined to metrics by exact key);
- record `time.time()` (unix) at dispatch, first token, finish; keep
  `perf_counter` for the latency arithmetic.

Acceptance: a 30-request smoke run produces `metrics.jsonl` where every
row has those fields; `breakdown.json` rows from the proxy have the same
`request_id` keys.

Effort: 1 small PR. Pure CPU.

### A2. Proxy instrumentation — close Gate 1 + Gate 3 + Gate 4

File: `scripts/cache_aware_proxy.py`

Changes:

1. Honor incoming `X-Request-Id`: if header present, use it instead of
   generating a new uuid. Falls back to uuid otherwise.
2. Record on every breakdown row:
   - `session_id` (already on header, not currently stored)
   - `input_length`
   - `estimated_new_tokens` (already produced by router)
   - `candidate_scores` (list of `{url, p_tokens_score, cache_score, bs,
     occupancy}`)
   - `chosen_score`
3. At route decision time, snapshot per-worker state:
   - `pending_prefill_tokens` per worker
   - `running_decode_requests` per worker
   - `kv_blocks_used` / `kv_blocks_total` per worker
   - `apc_hits` / `apc_queries` cumulative per worker
   Write to a separate `worker_state.jsonl` (one line per route decision)
   with `(t_decision_unix, request_id, per_worker_state)`.
4. New endpoint `GET /worker_state` returns the latest snapshot per worker
   (for sanity / live debugging).

Acceptance: smoke run produces `breakdown.json` with new fields and a
non-empty `worker_state.jsonl` that joins to breakdown by `request_id`.

Effort: 1 medium PR. Pure CPU + light proxy work.

### A3. Engine-side step timestamps — close Gate 3 for B2

vLLM 0.18.1 already exposes:

- `vllm:request_prefill_time_seconds` (histogram, per-request)
- `vllm:request_decode_time_seconds`
- `vllm:time_per_output_token_seconds`
- step-level scheduler stats via `engine.async_step` logging

For B2 we need decode-step and prefill-chunk timestamps with worker id.
Plan:

1. Inspect whether the vLLM proxy can be polled at high rate (e.g.
   100 Hz) for per-engine scheduler counters
   (`num_running`, `num_waiting`, `gpu_cache_usage`,
   `prefix_cache_queries`, `prefix_cache_hits`). If yes, sample
   into `engine_state.jsonl` during runs.
2. If finer step-level data is needed, patch one vLLM file
   (`vllm/engine/async_llm_engine.py` step loop or
   `vllm/v1/core/sched/scheduler.py`) to emit a JSONL line per
   scheduler step with `(t_unix, worker_id, num_prefill_tokens_scheduled,
   num_decode_steps, running_request_ids)`. Patch goes under `patches/`
   so it can be applied/reverted cleanly.
3. Worker id mapping: when running TP1xDP8 or similar, each engine
   listens on a distinct port; `worker_id == endpoint_url`.

Acceptance: a single 10-minute run produces `engine_state.jsonl` from
which a decode step at time T on worker W can be classified as
"overlapping a same-worker prefill chunk" or not.

Effort: 1 medium investigation (decide poll vs patch) + 1 medium PR.

### A4. Open-loop session-causal loadgen for B4

File: `replayer/replay.py` (new mode) or new `replayer/srr_loadgen.py`

Current replayer dispatches by trace timestamps. SRR sweep needs:

- pool of session templates (each = ordered list of turns from the
  trace);
- Poisson arrivals of new sessions at rate `lambda`;
- within a session: strict sequentiality (turn N+1 waits for turn N
  finish);
- per-run warmup window (e.g. 60s) + steady-state window (e.g. 300s);
- attempted / completed / error counters per window.

Add a new mode `--mode srr --arrival-rate <lambda>
--warmup-s 60 --steady-s 300 --session-pool-size N`. The trace
file becomes the pool; sessions are drawn with replacement.

Acceptance: at `lambda = 0.5 sess/s`, the run shows exponential inter-
arrival times and per-session sequentiality in `metrics.jsonl`. A
`window_summary.json` lists warmup vs steady-state attempted/completed.

Effort: 1 medium PR.

### A5. Analyzer extensions

File: `analysis/characterization/analyze.py` (extend, do not rewrite)

Add:

1. **Joined-record builder.** Given `--metrics metrics.jsonl
   --breakdown breakdown.json --worker-state worker_state.jsonl
   --engine-state engine_state.jsonl`, produce
   `joined.jsonl` keyed on `request_id` with all fields merged.
2. **Reuse decomposition (real).** Using joined records that carry
   `session_id` + `hash_ids` + `cached_tokens`, compute
   `intra_session` / `cross_session` / `shared_prefix` /
   `unclassified` cached-token mass. Replaces the current
   `status: unavailable` placeholder when fields are present.
3. **Interference index.** Per decode step, label "overlap same-
   worker prefill" using `engine_state.jsonl`. Compute
   `TPOT_p90(overlap) / TPOT_p90(no_overlap)`.
4. **Hotspot index.** Per worker queue delay p90, output
   `max_worker_q_p90 / median_worker_q_p90`.
5. **Failure label.** For each slow / SLO-violating request, assign
   one of: `same_worker_prefill_overlap`, `hot_worker_queue`,
   `high_kv_occupancy`, `cache_miss_large_append`, `transfer_wait`,
   `p_queue_wait`, `d_admission_wait`, `unknown`.
6. **Window summary.** For SRR runs, compute attempted/completed/
   error/goodput plus latency percentiles on the steady-state
   window only.

Acceptance: re-run analyzer on smoke output and confirm `reuse_decomposition`
no longer says `unavailable`; `interference_index.json` produced when
engine state present; `failure_breakdown.json` populated when
labels assigned.

Effort: 1 large PR. CPU-only.

## Phase B: GPU experiments (needs dash0)

### B1' Workload characterization closure

Inputs: instrumented replayer + small smoke trace (≤500 req).

Steps:

1. Pick `kv_bytes_per_token` for the production model. For
   Qwen3-Coder TP1 the value depends on layer/head config; compute
   from `vllm.config` once at run start and record in manifest.
2. Re-run analyzer on full GLM-5.1 trace with `--kv-bytes-per-token`.
   Output: KV footprint p50/p90/p99 in `kv_footprint_summary.json`.
3. Run a 1k-request session-causal smoke replay with instrumented
   proxy. Use the joined records to populate real reuse decomposition
   for the small sample. (Full-trace replay is too expensive; sample
   is acceptable for the decomposition claim.)

Wall-clock: ~30 min GPU. Produces 2 figures: KV footprint CDF, reuse
decomposition stacked bar.

### B2 PD-colo interference microbench

Setup: 1 combined instance on TP1. Two synthetic load generators:

1. **Decode-only steady load** — short-prompt sessions at fixed
   per-second arrival, designed to saturate decode without prefill
   contention.
2. **Prefill injector** — single-shot long-prompt requests at
   controlled cadence; same worker (target the decode worker) vs
   different worker (route to a paired idle instance).

Sweep `uncached_prefill_tokens ∈ {2k, 8k, 16k, 32k, 64k}` × `{same,
different} worker`.

Outputs: `interference_microbench_summary.json`,
`decode_step_timeseries.csv` (from `engine_state.jsonl`),
`prefill_overlap_events.jsonl`, `interference_index.json`,
TPOT-with-overlay figure, interference-index-vs-prefill-size figure.

Wall-clock: ~2–3 h GPU including warm-up between sweeps.

### B3 Routing sweep on session-causal trace

Setup: 8 combined instances (TP1 × DP8) with the cache-aware proxy.

Run the same session-causal trace (e.g. r=0.0015 st=30 850-req config
from auto-mem `feedback-bench-config.md`) under five policies:

1. corrected LMetric / cache-aware (`--policy lmetric`)
2. load-only (new policy `--policy load_only` — picks min running)
3. hard sticky (new policy `--policy sticky` — once a session lands
   on a worker, never moves)
4. current Unified hybrid (`--policy unified`)
5. session-mass capped replay (filter the trace so no session exceeds
   `cap_turns` or `cap_input_tokens`; rerun policy 1)

Per run, collect: replayer metrics, proxy breakdown, worker_state,
engine_state. Compute per-worker queue delay, GPU util, KV occupancy,
APC, session-to-worker map.

Outputs: `worker_balance_summary.json`, `session_to_worker_map.json`,
`session_mass_summary.json`, `routing_policy_comparison.json`,
`hotspot_index.json`, `capped_session_replay_summary.json`,
8 figures from the TODO list (§5.figures).

Wall-clock: 5 runs × ~13 min ≈ 1.5 h GPU.

Implementation note: `load_only` and `sticky` are small additions to
`scripts/cache_aware_proxy.py` — they reuse existing affinity / score
machinery.

### B4 Sustainable Request Rate sweep

Setup: same 8 instances. Use Phase-A `--mode srr` loadgen.

SLO (locked per-class):

```text
TTFT_p90 <= 2.0 s
TPOT_p90 <= 0.15 s
error_rate <= 0.5%
queue length stable (no monotone growth over steady window)
KV occupancy stable
E2E_p90 <= T_class[c]  for each output-length decile c
```

`T_class[c]` is derived from a low-load reference run as
`E2E_p90_low_load(c) * 2` (factor configurable). The reference run
is done once and cached as `analysis/characterization/srr/slo_classes.json`.

Per policy sweep `lambda` from low (clearly safe) to high (clearly
broken) using a bisection-ish search:

```
λ_low  = 0.1 sess/s
λ_high = doubling until first SLO violation
binary-search λ_low .. λ_high for max sustainable λ
```

Policies covered: LMetric, static PD-disagg, Unified, hard sticky,
load-only.

Outputs: `srr_curve.json`, `lambda_runs/<lambda>/summary.json`,
`slo_violation_reason.json`, `goodput_vs_arrival_rate.json`,
`stability_summary.json`, all 8 figures from §6.figures.

Wall-clock: this is the most expensive batch. With binary search,
~6 lambda points × 5 policies × ~8 min (warmup + steady) ≈ 4 h GPU.

### B5 Failure attribution near SRR boundary

For each policy: pick `λ ∈ {0.9, 1.0, 1.1} × SRR`, run with full
instrumentation, then run the analyzer's failure-label step.

Outputs: `slow_request_attribution.jsonl`, `failure_breakdown.json`,
`case_studies.md`, `worker_failure_windows.json`, 5 figures from §7.

Wall-clock: 3 lambdas × 5 policies × 8 min ≈ 2 h GPU.

## Phase C: Audit package refresh (CPU)

Re-run `summarize_runs.py` and `plot_current_results.py` after each
GPU batch. Final pass after B5: refresh `claim_matrix`, `risk_register`,
`allowed_runs`, regenerate all figures, update
`reproduction_commands.sh`.

Effort: ~1 h CPU.

## Sequencing & rough timeline

```text
Phase A (CPU, before dash0):
  A1 + A2     (parallel)               ~half day CPU
  A3 patch    (scheduler.py)           ~half day CPU
  A4 SRR loadgen                       ~half day CPU
  A5 analyzer extensions               ~1 day   CPU

Window 1 on dash0 (B2 + B3 only, ~5 h GPU):
  smoke validation of A1–A4            ~30 min GPU
  B1' KV footprint + reuse decomp      ~30 min GPU
  B2 interference microbench           ~3 h    GPU
  B3 routing sweep (5 policies)        ~1.5 h  GPU
  Phase C partial refresh              ~30 min CPU
  ── HARD STOP, hand results back ──

Window 2 on dash0 (B4 + B5, ~6 h GPU, only after review):
  B4 SRR sweep (5 policies × bisect)   ~4 h    GPU
  B5 failure attribution               ~2 h    GPU
  Phase C final refresh                ~1 h    CPU
```

## Decisions (locked 2026-05-25)

1. **Target model**: Qwen3-Coder-30B-A3B. Compute
   `kv_bytes_per_token` from this model's config at manifest time.
2. **GPU topology**: TP1 × 8 vLLM instances (DP8). All proxies and
   sweeps assume 8 worker endpoints.
3. **Trace for B3/B4**: `traces/w600_r0.0015_st30.jsonl` (~850
   requests). No resampling.
4. **E2E SLO**: per-class. Split requests by `requested_output_tokens`
   decile, set separate E2E thresholds per class. No normalized-E2E
   headline.
5. **vLLM scheduler patch**: accepted. Step-level JSONL log goes
   through a patch under `patches/`. Polling falls back to per-engine
   `/metrics` for sanity only.
6. **GPU phasing**: hard stop after B2 and B3. Hand results back for
   review before committing to B4 SRR sweep or B5 attribution.

## What stays with the interns

- Re-running `summarize_runs.py` after each GPU batch (mechanical).
- Reviewing the auto-generated `current_results.md` for typos.
- Maintaining `main_claim_allowed_runs.md` if new traces are added.
- Anything reading the audit package — not extending it.

## Out of scope for this plan

- New routing policy design (Unified-v2 / PUSH variants).
- Production-grade KV transfer engineering.
- Any change to the production paper figures in
  `analysis/pd_sep_paper_section/`.
- vLLM upstream contributions.

These are downstream of characterization; once B2/B3/B5 attribution is
in, we decide separately.
