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>
