docs(ArcGate): record #167's moved dispatch gate as an open question needing a hardware A/B

[heydryft]

Issues are disabled on this repo, so this is the tracking record for the one merged change in the 2026-08-20 queue sweep that is not inert.

docs/engineering/OPEN_QUESTIONS.md is the right home: it is explicitly the register of things "deliberately not done, with the reasoning that led to each decision" and "the honest list of what has not been measured. Nothing on that list may be claimed, quoted, or reasoned from."

What it records

#167 (master b2841f5eb) moved the MoE dispatch gate from token count to m-tile occupancy. b=1 decode and full prefill are unchanged, but 9 ≤ n ≤ 682 now prefers gather-GEMV over grouped GEMM — batched decode at B≥9 and every prefill chunk under 683 tokens, which is where chunked prefill operates.

It merged on a green-CI gate, and CI cannot see performance. The path it now prefers is a no-dedup GEMV with 3.15× redundant reads, so it may be a regression exactly where it fires. Exposure is performance, not correctness — both paths compute the same thing.

The entry names the A/B that settles it (B = 16/32/64; prefill chunks 128/256/512), states plainly that it needs hardware, and records the coupling with the open #99: raising GROUPED_TILE_M to 64 on SM90+ moves this threshold ~4×, so #99 must make the gate read grouped_tile_m_for_cc rather than the constant — otherwise the gate and the kernel disagree about tile size, which is a wrong-answer bug rather than a perf bug.

It also carries the caveat covering the whole sweep: every performance number attached to the seven merged PRs was measured on that branch's own base, not on current master. None is a current fact until re-measured — including the 15.0 → 32.6 tok/s ladder figures on the still-open #185–#189.

Blast radius

Documentation only. One file, one new subsection under §2 "Ready to run, not yet run". No code, no kernel, no format, no dependency movement.

🤖 Generated with Claude Code

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