# round 31 track b experiment plan

owner: q2 decode combine helpers and dn_body in isolated /tmp/sol_r31_moe/strata. no interfaces, summation ordering, format layout, other kernels or live server changes.

hypotheses: half-mantissa and float-mantissa decode remove integer conversion overhead; packed loads/pointer stepping reduce address overhead. each is compared against immutable base kernel snapshot, with expert_parity --ab exact memcmp over NE=1..4,8/random and paired dn timings; gemv uses its built-in ab check. baseline self-ab first. accept only reproducible >=2% improvement without material regressions (>2%) across required cases; repeat apparent winners three times. all gpu invocations serial, reps=4/layers=1 initially to stay <10s (20s timeout for cold compile), no full-model work. reject failed parity, stop after bounded variants if gains not established. live server noise is uncontrolled; report rather than overclaim.

## result: ship half-mantissa combine only

patch: `/tmp/sol_r31_moe/sol_r31_moe.patch` (git apply --check against vendor/strata: exit 0). changes only `q2_combine<T_Q2_0R>` in moe.metal and `gemv_q2_raw_combine` in gemv.metal. no test code or dn loop changes survive. neither tracked vendor source nor :8080 touched.

machine: apple m5. baseline vendor head: `296b59d2f73c8f04a805df29c2fa97655adace78`, **dirty working tree**, so commit alone is not the measured source. exact baseline kernels preserved at `/tmp/sol_r31_moe/base/`; exact candidate at `/tmp/sol_r31_moe/half/` and `/tmp/sol_r31_moe/strata/metal/kernels/`. source hashes in `/tmp/sol_r31_moe/source_hashes.json`. prebuilt parity binaries from vendor/strata/build-metal; no evaluator edits. default safe math.

### correctness

all expert invocations exited 0 and printed `0 fail(s) ALL PASS`; the harness compares full plan, h, ye, y via memcmp for same-expert ne=1,2,3,4,8 and random routing at ntok=1,2,3,4,8. candidate was checked repeatedly against the immutable baseline. host exhaustive check of all 65,536 packed ushort words, all eight code positions, also exactly reproduces code-1 via half-bit decode. summation/fma order is unchanged.

gemv: bitwise parity at 640→2560, ntok=1,2,4 for **both Q2_0 and Q2_0R (142)**. Q2_0R is essential: Q2_0 uses the prepared-input path and does not exercise the changed helper. the raw helper is used only for ntok=1; raw ntok=2,4 retain the decoded path. logs: `/tmp/sol_r31_moe/gem_q2_nt{1,2,4}.log`, `/tmp/sol_r31_moe/gem_half_confirm*_nt{1,2,4}.log`.

### paired expert timings

command (same for baseline and all candidates):

```
STRATA_METAL_KERNELS=/tmp/sol_r31_moe/strata/metal/kernels \
  vendor/strata/build-metal/expert_parity --gu IQ2_S --dn Q2_0 \
  --ab /tmp/sol_r31_moe/base --bench --layers 1 --reps 4
```

baseline self-ab ratios 0.9868–1.0116 (baseline.log). first clean half run 0.8299–0.9218 (half.log). three confirmation runs, seven alternating pairs per case:

| case | confirm 1 | confirm 2 | confirm 3 | median ratio |
|---|---:|---:|---:|---:|
| ne=1 | 0.9145 | 0.9007 | 0.9086 | 0.9086 |
| ne=2 | 0.8600 | 0.8506 | 0.8539 | 0.8539 |
| ne=3 | 0.8417 | 0.8250 | 0.8261 | 0.8261 |
| ne=4 | 0.8221 | 0.8271 | 0.8258 | 0.8258 |
| ntok=1 | 0.9059 | 0.9036 | 0.9155 | 0.9059 |
| ntok=2 | 0.9246 | 0.9154 | 0.9173 | 0.9173 |
| ntok=4 | 0.9277 | 0.9194 | 0.9236 | 0.9236 |
| ntok=8 | 0.8900 | 0.8865 | 0.8936 | 0.8900 |

this is **7.6–17.4% lower dn latency** by median case ratio, not an end-to-end throughput claim. example confirmation 3: ne=1 45.4→40.8 us; ne=4 98.4→81.2 us; mixed ntok=8 266.7→238.2 us. unchanged gu is a useful control (~1.00). logs half_confirm1/2/3.log. all gpu invocations serial in this session; other tracks and live server may overlap, so parent will do exclusive confirmation. reps/layers were intentionally kept small to honor <10s invocations.

### gemv timings

```
STRATA_METAL_KERNELS=/tmp/sol_r31_moe/strata/metal/kernels \
  vendor/strata/build-metal/gemv_parity --ab /tmp/sol_r31_moe/base Q2_0R 640 2560 1 gemv
```

repeat with ntok 2/4 and with Q2_0. raw ntok=1 median paired ratios on confirmation repeats 2/3: **0.8754 / 0.8930** (12.5% / 10.7% lower). raw ntok=2/4 ratios ~0.995–0.999, as expected for untouched paths; Q2_0 ratios 0.9961–0.9999. first raw runs showed frequency ramping, recorded rather than discarded, and were followed by stable repeats. no material regressions observed.

### rejected alternatives / evidence

- packed_ushort2 down loads: bitwise pass, ratio 0.9970–1.0173, no established gain (`packed.log`).
- pointer stepping + packed load: bitwise pass, ne=3 +2.6% regression, mixed cases -2.1..-4.2% (`pointer.log`). combined with winning half against half-only: ratio 0.9824–1.0124, no >=2% incremental improvement (`half_pointer_vs_half.log`). excluded.
- unroll_count(5): bitwise pass, ne=2 +7.9%, ne=3 +6.0%, mixed ntok=8 +8.0% (`unroll.log`). excluded.
- fp32 mantissa: bitwise pass but severe external gpu interference (unchanged gu and dn times ballooned) makes its timing **inconclusive**, not a valid performance rejection (`float.log`). do not ship because half has reproducible wins while fp32 does not have clean confirmation.

all standalone processes finished below 10s (slowest float run 7.84s); timeout=20 allowed cold compilation. no full model benchmark. experiments stopped after confirmed half winner; quiet confirmation belongs to parent.

### review

reviewed complete scratch git diff visually, searched existing half-mantissa idioms (matches moe_mm), checked field ordering x/y/z/w and exact fma sequence. the two small helpers stay local to their kernel translation units; moving them to units.metal would violate ownership and broaden the patch. no unrelated edits, added debug output, or hard-coded shape assumptions.