KvAwarePolicy now accepts an optional cold_d_bonus int. When > 0,
fresh requests (sticky=0, i.e. no prior D for this session) receive
the bonus added to lex-score position 0 (overlap+sticky_bonus) for
any D worker that has never been assigned a session yet
(decode_assignment_counts == 0). This breaks the pathology
documented in docs/E1_E2_RESULTS_ZH.md §5d where workloads with
shared cross-session prefix (e.g. Inferact's "permissions
instructions" boilerplate) cause every D that has hosted any session
to dominate the overlap term against any cold D, leaving the cold D
permanently unused.
Sticky behavior is preserved: turn 1+ requests of an existing
session continue to stick to their original D because the bonus is
gated on `not sticky`.
Plumbed through ReplayConfig.kvcache_cold_d_bonus (default 0,
keeping current behavior unchanged), BenchmarkConfig, and CLI flag
--kvcache-cold-d-bonus on both `replay` and `benchmark-live`
subcommands. Set above max expected boilerplate overlap (Inferact's
~50 24-token blocks → 1000 is safe).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Replay-side changes paired with the SGLang admission hint:
- DecodeResidencyState gains pause_until_s; admission probe parses
recommended_pause_ms and updates the per-D pause window.
- _wait_for_decode_pause is invoked at request entry points
(_invoke_router, _invoke_session_direct) so requests stall before
hitting a saturated D instead of timing out via mooncake.
- New CLI flags: --enable-backpressure (default off, baseline preserved),
--backpressure-max-pause-s (cap on per-request sleep, default 2s).
Structural instrumentation written under <run_dir>/structural/:
- admission-events.jsonl: every admission probe (RTT, queue_depth,
pause_ms, available_tokens, evicted_count)
- backpressure-events.jsonl: every actual pause sleep
- session-d-binding.jsonl: per-request policy decision
Used to validate the structural claims documented separately.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hostile audit of the original report flagged three load-bearing errors:
1. held_tokens semantic was inverted. session_held_tokens() at
session_aware_cache.py:278-282 sums (kv_allocated_len - cache_protected_len)
per slot, i.e. slot-private (NOT in radix tree). So "other = cap - held -
avail" actually CONTAINS the radix-tree protected prefix cache (likely the
single biggest component for shared agentic prefixes), not just running
batch + in-flight as the original report claimed.
2. Admission-race causal hypothesis for the 415 EXP2+profile errors is
contradicted by the data: 414/415 errors have kv_transfer_blocks > 0 — they
passed admission and died downstream ("generate stream ended before
producing any token", raised by the client when a 200 response had an empty
stream).
3. Polling deconfound was too quickly dismissed. Mode counts shift ~1:1
(session-cap-fb -356 / kvcache-centric +406), and /server_info is not a
passive read — it dispatches into the scheduler main loop and iterates
every session slot.
Plus: per-D error% confounded by sticky session affinity (only 18 unique
sessions cause 415 errors, decode-3 had 0 errors only because no high-error
session landed there); decile 10 "recovery" was an equal-time binning
artifact (24.5% under equal-count); v5 vs v5+profile time gap was 21h not
6h; p50/p90 latency comparison is N=1.
Rewritten report (docs/V5_PROFILE_INVESTIGATION_ZH.md) marks each correction
with ⚠️ and demotes admission-race to one of four hypotheses (H1-H4).
Action items split into P0 (verify, must do first) and P1 (instrument):
P0 — scripts/sweep_tp1_v5_baseline_rerun_exp2.sh runs 3x v5 baseline EXP2
(no polling, identical config to the original v5 run) to test whether the
9-error baseline result is reproducible. If 3 runs give ~9 errors and
profile gives 415, polling is the leading suspect. Currently running
in background.
P1 — scheduler.py:_compute_pool_breakdown_for_diagnostics adds a read-only
"pool_breakdown" dict to /server_info covering: radix_evictable_tokens,
radix_protected_tokens, slot_private_held_tokens, session_slot_count,
running_batch_{reqs,kv_tokens}, transfer_queue_{reqs,tokens},
prealloc_queue_{reqs,tokens}, retracted_queue_{reqs,tokens}. With these,
"unaccounted = cap - sum(known)" exposes true leakage. replay.py captures
all fields into the per-tick row; analyzer prints the decomposition and
gracefully handles old timeseries (prints "P1 instrument absent").
Mock-tested end-to-end. SGLang patch is read-only and does not affect
admission/scheduling. Old v5+profile data still analyzes correctly.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
v5 dropped errors but pushed session-cap fallback to 46-51%. Before adding
v6 mitigations we need to attribute that capacity loss to one of:
(a) active sessions — real footprint
(b) idle-evictable sessions — LRU not aggressive enough
(c) prefill backup blocks / in-flight / fragmentation — release timing
Without this it's all guessing. Plumb a 1Hz poller into replay that hits
each P/D worker's /server_info, captures session_cache + memory_usage, and
writes a per-worker time-series JSONL to <run_dir>/d-pool-timeseries.jsonl.
Off by default (--pool-poll-interval-s 0); v5+profile sweep enables it at
1.0s. Per-tick HTTP cost is ~8 parallel /server_info calls — negligible
relative to the 50min run.
Analyzer (scripts/analysis/analyze_pool_timeseries.py) decomposes each D's
capacity into active_held / idle_evictable / other (= cap-held-avail, the
backup-blocks bucket) / free, and reports session residency churn across
workers as a starvation/thrashing signal.
Mock-tested poller end-to-end (cancellation clean, file flushed, sessions
captured); analyzer validated against synthetic timeseries.
Next: run scripts/sweep_tp1_v5_optD_profile.sh on hardware (~90min), then
analyze results to pick a v6 direction.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
v4 (cap=16) saw 35% session-cap fallback because the local soft_cap
min(16, usable / target) evaluates to 1-2 for large agentic inputs.
The cap was hit not because D was full but because replay's heuristic
underestimated capacity.
This change makes worker admission_mode authoritative for ALL paths:
SGLang side:
- io_struct.py: DirectAppendAdmissionReqInput gains a `mode` field
("direct_append" | "seed", default "direct_append" preserves prior
behavior).
- scheduler.py:admit_direct_append: when mode == "seed", skip the
resident-on-D requirement and run the same capacity check + LRU
eviction (maybe_trim_decode_session_cache) that direct_append uses.
This lets D atomically decide if a new session can be admitted based
on actual token_to_kv_pool_allocator state.
Replay side (replay.py):
- _query_decode_direct_admission gains a `mode` parameter.
- _reserve_decode_session_capacity: in worker admission_mode, the
seed/reseed branch now queries D with mode="seed" and trusts the
result, instead of estimating capacity from the residency snapshot.
- _should_admit_new_decode_session: in worker mode, skip the local
soft_cap pre-check and let D decide. Same-D session fast-path is
preserved.
Effects:
- Local hardcoded cap of 16 is bypassed under worker mode; D's real
KV pool size is the only constraint.
- LRU eviction runs in D's process atomically with admission, so
starvation (the v3 bimodal "lucky vs starved sessions" pattern)
should resolve.
scripts/sweep_tp1_v5_optD.sh added to run the same 1P7D / 2P6D
configs as v4 with the new admission path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Document the iterative debugging from v1 (broken KVC) through v4
(routing fixed + session cap raised), with code-level analysis of
the two main bugs encountered:
1. v2 root cause (mis-diagnosed previously as `allow_local_prefill`):
`--policy default` for KVC mechanism caused replay's round-robin
policy and the PD router's round-robin to diverge, sending requests
with `session_params` to a D worker that did not have the session
open. Resulted in 56-61% truncation with finish_reason
"session id X does not exist".
Fix: use `--policy kv-aware` (sweep_tp1_v3_kvaware.sh) so replay
emits `x-smg-target-worker` and PD router uses consistent_hashing.
2. v3 new bottleneck: `pd-router-fallback-large-append-session-cap`
dominated 52-65% of requests. Root cause was hardcoded
`min(4, ...)` in `_decode_session_soft_cap`. With 7 D workers x 4
sessions = 28 slots for 52 trace sessions, ~24 sessions starved
permanently (bimodal direct-to-D rate of 0% or 99%).
Fix: raise the cap to 16 (replay.py).
Also includes the v3 finding that direct-to-d-session path P50=0.495s
and TTFT P50=0.043s already beats the 8-way DP baseline (0.65s/0.093s)
- the KVC core mechanism works when fallback paths are avoided.
Files:
- docs/KVC_DEBUG_JOURNEY_V1_TO_V4.md: full journey + code location index
- docs/SWEBENCH_EXPERIMENT_{PROGRESS,RESULTS}.md: prior session notes
- scripts/sweep_tp1_v{2,3,4}*.sh: experiment driver scripts
- src/agentic_pd_hybrid/replay.py: cap 4 -> 16, audit fields
- src/agentic_pd_hybrid/pd_router.py: strip session_params from prefill
- src/agentic_pd_hybrid/metrics.py: truncated_request_count
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
