The TC_W4A16 branch of coli_cuda_expert_group handled every expert in a
per-expert loop: rows >= threshold got the Tensor Core path, everything
below fell back to 4 naive launches per expert. At decode every expert
has 1 row, so the whole group rode the fallback — ~981 quant_matmul
micro-launches per token (#431's measured flood) — while the grouped
3-launch path (grouped_hidden_w4_dual + silu + grouped_down_w4) sat one
else-if below, unreachable whenever the PREFILL tuning flag was set.
Gate the branch on 'at least one expert reaches the TC row threshold':
all-small groups (decode) now fall through to the grouped kernels.
Measured on 6x RTX 5090 (full residency, 39 forwards under nsys):
expert-side quant_matmul instances drop 981 -> 337 per forward, total
launches ~1,490 -> ~850 per token (-43%). Wall-clock is parity at the
A/B operating point — the win is structural (PR-C graph node count,
launch-tax share at champion speed).
Behavioural fix folded in: before this change, toggling TC_W4A16 — a
prefill-only optimization — changed DECODE output text (kernel-family
divergence, #100 class). After it, decode always uses the grouped
family: TC_W4A16=1 and =0 now produce byte-identical decode text
(verified, 96-token greedy A/B), and the flag affects only the prefill
it was built for.
ZacharyZcR
34f6e50091