# round 18 track b

## contract / preflight
owner: metal grouped-residual kernels and Window::gr_norm dispatch selection. preserve operation order, generic shape fallback, apply=false semantics and buffer bindings. no live server or full-model runs.
question: do fixed-shape pointer hoisting / unrolling and stream staging improve latency without changing a single output bit? compare baseline 4s, optimized 4s, per-stream grid and staged 4s; compare baseline/optimized gr_down_q8_8_4.
evaluator: deterministic fuse_parity inputs, memcmp R/xn and down/inject for ntok 1..8; ordered alternating gpu timings, 3 exploratory and 5 confirmation pairs, 64 dispatch repetitions each. improvement must exceed 3% median, with bitwise parity mandatory. each invocation timeout 4.9s, no concurrent gpu tests; compile jobs exempt from test invocation limit. live server may introduce noise; report it.
source snapshots: /tmp/sol_r18_grcomb/baseline and candidate scratch tree; no tracked vendor edits. stop if compilation cannot be repaired in two attempts or tests exceed the invocation budget.

## result / decision

**ship the hoisted/unrolled `gr_norm_comb` with grid `(4, ntok)`.** keep optimized `gr_norm_comb_4s` available, but do not dispatch it from `Window::gr_norm`. shared-bo staging was slower. do not ship the `gr_down_f` change: it passed bitwise parity but did not establish a repeatable improvement.

main patch: `/tmp/sol_r18_grcomb/sol_r18_grcomb.patch` (three files; no `gr.metal` edits). rejected/neutral down experiment: `/tmp/sol_r18_grcomb/neutral_down.patch`. neither patch has been applied to tracked vendor sources.

## execution / evidence

- machine: Apple M5 (the local 10-core GPU laptop). no server access, full-model benchmarks, or spawned agents.
- vendor input HEAD: `296b59d2f73c8f04a805df29c2fa97655adace78`; input working-tree snapshots are in `/tmp/sol_r18_grcomb/baseline/`, since HEAD alone does not describe this already-dirty development tree.
- scratch baseline commit: `faeb472`; all executed shader fixtures, evaluator source, binary, and input/candidate hashes: `/tmp/sol_r18_grcomb/sha256.json`.
- all narrow GPU invocations completed below 1.6 seconds. first existing full `fuse_parity` invocation was terminated at 4.9s during cold shader compilation. parent then explicitly permitted a 20s timeout for the full test's compile overhead. full suite subsequently passed twice: 5.45s and 13.73s (including compilation), exit 0; final log `/tmp/sol_r18_grcomb/final_full_parity.log`.
- standalone compile/link succeeded; scratch `window.cpp` compiled successfully with the existing engine compiler flags, output `/tmp/sol_r18_grcomb/window.o`.
- final `git -C vendor/strata apply --check /tmp/sol_r18_grcomb/sol_r18_grcomb.patch`: exit 0. scratch `git diff --check`: exit 0.
- `--gr-ab ... 0 norm` rejects invalid token counts with exit 2 before creating a GPU context.

### correctness

`memcmp == 0` for both R and xn across ntok 1..8, apply false/true, topk 1/10, for optimized 4s, per-stream grid and staged-bo variants versus the original 4s. 96 final A/B parity cases passed. original full suite also checks both production combine kernels versus `moe_combine_sh` + `gr_norm`, and passed all 32 cases (plus gemv/router tests). down Q8_0 lo and inject are bitwise equal for ntok 1..8. generic n != 2560 behavior is retained but was not newly exercised.

### timing contract adjustment

initial 80ms/30ms warm-up left obvious clock ramps. before confirmation, increased warm-up to 300ms baseline + 100ms candidate, with five alternating-order pairs of 64 dispatches each. preserved both exploratory and confirmation logs rather than replacing the original evidence. the live machine can change clocks or have other GPU work; absolute timings are not stable, so a direct optimized-4s-vs-grid confirmation was also run. this is a kernel microbenchmark, **not** an end-to-end speedup estimate.

### original 4s vs selected per-stream kernel

medians of five paired batches, microseconds/dispatch:

| ntok | original 4s | selected grid4 | reduction |
|---:|---:|---:|---:|
| 1 | 22.673 | 4.458 | 80.3% |
| 2 | 22.820 | 4.680 | 79.5% |
| 3 | 22.882 | 5.902 | 74.2% |
| 4 | 22.878 | 6.453 | 71.8% |
| 5 | 54.246 | 16.863 | 68.9% |
| 6 | 22.977 | 8.649 | 62.4% |
| 7 | 43.543 | 13.086 | 69.9% |
| 8 | 22.951 | 10.456 | 54.4% |

optimized 4s reduced baseline time by 46–49% in this confirmation; staged shared bo reduced it by 43–46%. the selected grid kernel reduced it by 54–80%.

### direct optimized 4s vs selected grid4

these runs happened at much slower clocks for most token counts, but both candidates were measured back-to-back in alternating order:

| ntok | optimized 4s | selected grid4 | reduction |
|---:|---:|---:|---:|
| 1 | 51.937 | 20.042 | 61.4% |
| 2 | 51.971 | 20.734 | 60.1% |
| 3 | 52.184 | 26.059 | 50.1% |
| 4 | 52.386 | 28.171 | 46.2% |
| 5 | 52.582 | 31.563 | 40.0% |
| 6 | 11.881 | 8.551 | 28.0% |
| 7 | 54.546 | 30.451 | 44.2% |
| 8 | 52.787 | 46.443 | 12.0% |

all eight direct comparisons favor grid4 (12–61%), exceeding the 3% decision threshold.

### down experiment (not shipped)

implemented exactly the requested xnp pointer hoist, UNIT/16 load unroll, and KS reduction unroll. ntok 1..8 all bitwise PASS. confirmation median reductions were 1.30%, 4.40%, -0.14%, 0.13%, -0.18%, -0.03%, 0.11%, 0.12%. ntok2 was clock-noisy, not a broad repeatable win. preserve this as a measured neutral result, not a performance claim.

## reproduce

artifacts live under `/tmp/sol_r18_grcomb/`; `norm.metal` contains original 4s plus all three candidates, `down.metal` original/candidate down, `direct.metal` uses optimized 4s as the reference. the focused evaluator is the new `--gr-ab <absolute shader path> <ntok> <norm|down>` mode in `metal/tests/fuse_parity.cpp`.

from the project root (replace `$PWD` with the checkout path if launched elsewhere):

```text
clang++ -std=c++20 -O2 -Ivendor/strata/metal/src /tmp/sol_r18_grcomb/strata/metal/tests/fuse_parity.cpp vendor/strata/build-metal/libmtl_rt.a -framework Metal -framework Foundation -o /tmp/sol_r18_grcomb/fuse_parity
STRATA_METAL_KERNELS=/tmp/sol_r18_grcomb/strata/metal/kernels /tmp/sol_r18_grcomb/fuse_parity --gr-ab /tmp/sol_r18_grcomb/norm.metal 1 norm
STRATA_METAL_KERNELS=/tmp/sol_r18_grcomb/strata/metal/kernels /tmp/sol_r18_grcomb/fuse_parity --gr-ab /tmp/sol_r18_grcomb/direct.metal 1 norm
STRATA_METAL_KERNELS=/tmp/sol_r18_grcomb/strata/metal/kernels /tmp/sol_r18_grcomb/fuse_parity --gr-ab /tmp/sol_r18_grcomb/down.metal 1 down
```

repeat ntok 1..8, sequentially. paired logs: `confirm_norm_*.log`, `confirm_down_*.log`, `direct_*.log`; additional topk1 regression logs `final_norm_*.log`, post-review endpoint checks `postreview_norm_1.log` / `postreview_norm_8.log`. machine-readable medians: `measurements.json`.

## review

reviewed every patch hunk with `view_diff`, helper/Window context with `view_code`, then re-reviewed formatting fixes. no arithmetic reassociation or reduction-order changes; all new pointer accesses stay inside the existing fixed-shape specialization and topk guard, generic fallback remains. no live server settings changed.