pub const ROUTER_BATCH_MIN_T: usize = 8;Expand description
FAST-ROUTER batch twin (lane/fast-router, 2026-08-02). The concat-prime exactness fix (router_prefill_exact_on) routes prefill through router_gemv — m-invariant, but a per-(expert,token) GEMV program with zero operand reuse, so q35 board-2048 prefill paid -10% on the 5090. router_gemv_f32_w8_batch register-tiles (8x8 expert-x-token) the same per-row FP chains (BIT-IDENTICAL per row — kernel-check sweeps m=1..2048 on real router weights), so the t crossover below is pure perf, not a numeric config. Swept on-box (research/fast-router-20260802/crossover-router*.jsonl): plain wins t<=4, batch +7-9% at t=8, 1.9x at t=16 rising to 3.45x at t=2048 — MIN_T=8. Decode t=1 and spec verify t<8 keep the plain w8 form. MEMRA_ROUTER_BATCH=0 forces plain at every t (rollback seam, perf-only: bits are equal by the kernel-check gate). Killed arms (same sweep, JSONL is the record): the 8x16 tile lost to 8x8 at every t (128-accumulator register pressure beats the halved w-traffic), and the same-shape sigmoid_dot_rows twin (out_f=1) measured 0.62-0.89x at every prefill t (launch-latency-bound, ~7us/layer at m=2048) — both bit-identity-PASSED before dying.