E4-v1 / v2 / pressured-v1 all failed to fire admission rejections in
this workload because the default 0.6 mem-fraction-static gives
288K-token kv_pool per decoder, more than enough to absorb the
50-session trace even at concurrency=32.
This commit adds:
--decode-mem-fraction-static (overrides per-decode SGLang arg)
--prefill-mem-fraction-static (symmetric for completeness)
Plumbed via topology.{decode,prefill}_extra_server_args. The
pressured sweep now uses --decode-mem-fraction-static 0.4 which
shrinks decoder kv_pool to ~192K tokens — should force enough
admission rejections to actually exercise the D→P snapshot path.
E4-v1 produced 272 admission rejects (good) but zero /_snapshot HTTP
calls (bad, entrance gate bug fixed in e729d62). E4-v2 went the other
way: 0 rejects through 53% of trace, sync function never even called.
E4-pressured locks in the *fix-verified* code path by lowering
--kvcache-migration-reject-threshold from 3 to 1. After ONE
rejection the policy forces session migration, which lands in
_invoke_kvcache_seeded_router → _attempt_d_to_p_sync.
With the e729d62 fix in place, the d-to-p-sync.jsonl structural log
should now capture every prepare/dump/finalize decision so we can
forensic verify the D→P fast path is actually delivering KV bytes
to P's radix tree.
