frank-large: halogen container cutover, rootless rlimit law, and the DFlash2 drafter shape

serverAppConfigs-docs/LLM/VLLM Overview.md

@@ -237,6 +237,46 @@ vLLM supports multiple parallelism strategies:
 - Expert Parallelism (EP) distributes expert networks in MoE models
 - Context Parallelism (CP) splits context across GPUs
 
+
+## Chunked Prefill — the flag surface, and what it actually does
+
+**YES, the runtime exposes chunked-prefill options, and the running shape
+already uses them.** The tcclaviger fork is a vLLM fork, so the standard
+scheduler flags are present in the shipped image (verified by reading the
+`vllm.engine.arg_utils` and `vllm.config.scheduler` source in the live
+container `frank-primary-qwen`, not from a stub):
+
+| Flag | Default | What it controls |
+|---|---|---|
+| `--enable-chunked-prefill` | `True` in `SchedulerConfig`; the CLI arg is an auto-flag (`"default": None`) | whether prefill may be split into chunks |
+| `--max-num-batched-tokens` (MNBT) | 2048 (`DEFAULT_MAX_NUM_BATCHED_TOKENS`) | tokens processed per prefill iteration |
+| `--prefill-schedule-interval` | 0 | gap between prefill iterations |
+
+The running shape's argv carries `--max-num-batched-tokens 2048` alongside
+`--max-model-len 200704`, so chunked prefill is **ON and already active**: a
+200704-token prompt is prefilled in ~98 iterations of 2048 tokens, not in one
+shot.
+
+**It is a TTFT lever, not a throughput lever.** MEASURED 2026-09-29 on the
+ad hoc qwen-fp8 container (the same fork, same payload, same TP=2):
+
+- MNBT 8192 vs the default 2048: **neutral** on decode at every depth.
+- Small values re-chunk prefill only. They hurt TTFT (each chunk is its own
+  iteration, so the first token waits on more iterations) and they never touch
+  decode: a decode step is `max_num_seqs x (spec+1)` tokens, independent of
+  MNBT.
+
+**There is no `--ubatch-size` equivalent in vLLM.** `--ubatch-size` is a
+llama.cpp compute-buffer concept (`--batch-size`/`--ubatch-size`); the tcclaviger
+fork has no ubatch knob at all. The llama.cpp compute-buffer pair that frank-small
+was doubled on (2026-10-05, +4.6 % prefill at 8,928 tokens for 616 MiB) has no
+counterpart here, and the two backends should not be reasoned about by analogy.
+
+**MNBT is not a lever on this shape either.** The deployed 2048 is the fork's
+default, so a change is a deliberate departure from the shipped value, not a
+tune. The one thing MNBT does cost: every change re-enters the compile RANGE
+path, so `FRANK_PRIMARY_CACHE_DIR` pays one recompilation.
+
 ## See Also
 
 - [[VLLM Index]]