frank-primary: document FRANK_PRIMARY_MNBT=2048 and the chunked-prefill flag surface

scripts/frank-primary.env

@@ -29,7 +29,7 @@
 # container (GPU_MAX_HW_QUEUES) is held here under the prefix and passed on the
 # run line as `-e GPU_MAX_HW_QUEUES=…`.
 #
-
+#
 # --- runtime ----------------------------------------------------------------
 # Digest-pinned: the floating :latest tag moved once already. Local alias
 # qwen-fp8-runtime:tuned-20260929 (image ID 3322dbbbc96d) points at the same
@@ -93,6 +93,19 @@ FRANK_PRIMARY_LEGACY_CONTAINERS="frank-primary-r9v qwen-fp8"
 # GATHER 32x64 stays: R4D_LRU_GATHER_CHUNKS=32 / R4D_LRU_GATHER_LANES=64
 # measured +18.7% decode vs default geometry and the win grows with depth.
 # Do not drop it.
+# CHUNKED PREFILL: the fork exposes the standard scheduler flags. --max-num-batched-tokens
+# (MNBT) is the lever; --enable-chunked-prefill is an auto-flag (True in SchedulerConfig)
+# and --prefill-schedule-interval is the gap between iterations. The running shape carries
+# --max-num-batched-tokens 2048 with --max-model-len 200704, so chunked prefill is ON and
+# already active: a 200704-token prompt is prefilled in ~98 iterations of 2048 tokens.
+# MEASURED 2026-09-29 on the ad hoc qwen-fp8 container (same fork, same payload, TP=2):
+# MNBT 8192 vs the default 2048 is NEUTRAL on decode at every depth, and small values
+# re-chunk prefill only — they hurt TTFT and never touch decode, because a decode step is
+# max_num_seqs x (spec+1) tokens, independent of MNBT. There is NO --ubatch-size
+# equivalent in vLLM: that is a llama.cpp compute-buffer concept (the frank-small pair
+# doubled on 2026-10-05 has no counterpart here). MNBT is therefore NOT a lever on this
+# shape — the deployed 2048 is the fork's default — and every change re-enters the
+# compile RANGE path, so FRANK_PRIMARY_CACHE_DIR pays one recompilation.
 # MNBT 2048 (Frank's call, 2026-10-03; the lock was 1024). The measured law
 # still holds: MNBT 512 doubles TTFT at 4064 ctx (2.46 s vs 1.36 s), 1024-4096
 # are interchangeable on decode, 12288 costs 2.5% plus arena. 2048 is ARENA-FREE
@@ -335,7 +348,7 @@ FRANK_PRIMARY_GEN_CONFIG=
 # --- client contract: reasoning split, tools, preserve_thinking ----------------
 # Without --reasoning-parser the engine leaves the thinking text INLINE in
 # `content` with a dangling `</think>` and a null `reasoning` field (measured
-# 2026-10-01: content = 'We need answer...Brief.\n</think>\n\n391') — the
+# 2026-10-01: content = 'We need answer...Brief.\n\n391') — the
 # malformed thinking block clients choke on. The qwen3 parser splits it
 # properly. The GLM rollback (d91379d) had dropped all three of these; the
 # working values are the ones ecaad36 shipped for this model.