The KV pool at the fp8 shape backed 386,904 tokens — 1.93 sessions of the
served 200,000 window, so a second full-height conversation preempted to the
host/disk prefix tiers. Moving the cache to bf16 (the engine's only
unquantized rung; `--kv-cache-dtype` takes bf16|fp8 and has no per-tensor
K/V split) doubles what a token costs: 7.3125 -> 14.625 KiB per rank
(kv_attn 26.00 -> 52.00 KiB per 4-token page + kv_qsa_bkey 3.25 -> 6.50).
Pool 3543 -> 6912 MiB (437k tokens, >= 2 x 200,000), and the weights slab
pays for all of it: 26071 -> 22208 MiB, the pair pinned to the measured
card ceiling 29,120 MiB (card 31,856 - 1,710 consumed outside both budgets
- 1,024 free). The resident set that leaves VRAM spills into the pinned host
pool, so 40960 -> 44928 MiB, and the prefix host tier doubles with the entry
size (58.50 -> 117 KiB): 4096 -> 8192 MiB. Pinned ledger ~147 GiB, ttm floor
140000 -> 150000 (cap 200 GiB).
--max-num-seqs 3 -> 2: two slots x the window is the capacity contract, and
the "seqs 2 cannot boot" law is the retired tcclaviger fork's torch.compile
query_start_loc constraint, not radiance's (radiance booted seqs 2 in the
fp8-native era; the 2026-10-06 sweep had 2 and 3 tied inside +-3%). Bonus:
the fused all-reduce is now declared over 2 x 3 = 6 rows, under the 25-row
bound at which the wht6 wire is bit-exact.
Cost, paid twice on this engine: idle KV VRAM is also the expert cache's
spillover (lends 2.69 GiB + 740 MiB of arena), so the pool steals weight
residency from the slab AND the lendable cache. Priors: 2.14 GiB of stolen
weights cost 2.5% decode (MAX_MODEL_LEN 262144), the EVC ladder's other
direction cost 8.9%. Prediction -3..-8% vs the fp8 control (136 t/s at 44.5k,
acceptance 0.7246); the deploy boot ledger and live probes are the referee.
Seams: MAX_SEQS/HOST_POOL/KV_DTYPE/PREFIX_HOST literals follow the shape, and
two new invariants guard it — the KV pool must back seqs x max-model-len at
the declared dtype (an engine hard limit: an over-long request is accepted
then refused as too large to ever fit; the formula reproduces the fp8 boot's
386,904 to within 0.1%), and the VRAM pair must stay inside the ceiling.
Frank
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