corridorkey-mrp-mlx/docs/2026-07-16-m-series-fleet-ablation-results.md

title	date	device	script
M-series fleet ablation: M3 Ultra / M1 Ultra / M4 Pro	2026-07-16	M3 Ultra 256GB · M1 Ultra 128GB (64-core GPU) · M4 Pro Mac mini 24GB	scripts/bench_optimizations.py (unmodified) + engine-level tiled×compile extension

M-series Fleet Ablation Benchmarks

Same methodology as the 2026-03-09 wave2 doc (--sweep ablation --resolution 512 1024 2048, checkpoint weights, 3 warmup + 10 bench runs), executed on three machines spanning the Apple Silicon range. Contributed from a 3-Mac render-farm setup; happy to run follow-ups.

Headline findings

1. sdpa is a ~4.5× regression on M1-class GPUs at 2048 — and free on M3. Single-toggle isolation at 2048 (median of 6, all-off baseline):

   toggle	M1 Ultra	M3 Ultra
   all off	4139 ms	1743 ms
   bf16 only	4120 ms (neutral!)	1766 ms
   sdpa only	18592 ms (4.5× slower)	1763 ms
   bf16+sdpa	6388 ms	1767 ms

   The suspected M1 danger (bf16 emulation — M1 lacks hardware bf16) is innocent on this workload; mx.fast.scaled_dot_product_attention at 2048-scale sequence lengths is the cliff. bf16 partially mitigates it (fp32 sdpa is the worst path). Suggestion: gate use_sdpa by GPU generation rather than defaulting it on. (This may interest MLX upstream too — it looks like a Metal kernel path issue on M1, not a model issue.)

2. stage_gc is harmful on Ultra-class machines at every resolution — 0.53× at 512, 0.64× at 1024, 0.66× at 2048 on M3 Ultra, for a 1–5 % peak-memory saving. Wave2 measured a mild 0.79–0.99× on its reference hardware; on big-memory machines it's pure overhead. Suggestion: document as a low-memory-only flag.

3. Tiled + compile is the best 2048 config on both Ultras — upstream engine.py forced compile=False in tiled mode; tiles are fixed-shape, so fused compilation applies (patched in this fork, output bit-identical, verified against ground-truth alpha):

   engine config (2048 input)	M3 Ultra	M1 Ultra	peak	alpha MAE*
   full-frame 2048	2788 ms	5373 ms	27.9 GB	0.00906
   tiled 512/64	2760 ms	4576 ms	2.2 GB	0.00821
   tiled 512/64 + compile	2478 ms	4222 ms	2.3 GB	0.00821
   tiled 768/64 + compile	1949 ms	3275 ms	2.4 GB	0.00842
   tiled 1024/64 + compile	3475 ms	5864 ms	3.7 GB	0.00915

   *alpha MAE vs exact ground truth: synthetic 2048² green-screen plates (soft-alpha subject + motion-blur stripes + defocus disk composited over chroma green), hint = 8× downscaled truth. Tiled beats full-frame on accuracy as well as memory — the model runs at native tile scale over full-res input. Confirms wave2's tiled-768 pick and adds ~8–13 % from compiling the tile graph.

4. 24 GB Macs must tile at 2048. On the M4 Pro mini (24 GB), every full-frame 2048 config lands at 20–24 s/run — the ~26 GB working set swaps; toggle choice becomes noise. At ≤1024 the M4 is healthy (218 ms @512, 1149 ms @1024). Tiled 2048 runs in ~2.3 GB.

Cross-machine baselines (all-off)

res	M3 Ultra	M1 Ultra	M4 Pro 24GB	wave2 reference
512	53.3 ms	80.8 ms	218.1 ms	119.6 ms
1024	247.0 ms	350.6 ms	1149.2 ms	610.7 ms
2048	1750.2 ms	3527.3 ms	23147 ms (swap)	4984.7 ms

Full ablation tables

M3 Ultra 256GB

Config	512	1024	2048	peak @2048
baseline	53.3	247.0	1750.2	26689 MB
slim+sdpa+bf16+fused_decode+gpu_preprocess	53.5	248.2	1790.1	27245 MB
slim+stage_gc+bf16+fused_decode+gpu_preprocess	95.4	382.7	2644.3	26689 MB
slim+stage_gc+sdpa+fused_decode+gpu_preprocess	96.3	385.9	2676.9	26661 MB
slim+stage_gc+sdpa+bf16+gpu_preprocess	99.4	372.1	2618.6	26661 MB
slim+stage_gc+sdpa+bf16+fused_decode	99.0	386.5	2688.0	26661 MB
stage_gc+sdpa+bf16+fused_decode+gpu_preprocess	100.1	391.4	2628.2	26661 MB
slim+stage_gc+sdpa+bf16+fused_decode+gpu_preprocess	101.0	390.9	2617.9	26661 MB

M1 Ultra 128GB

Config	512	1024	2048	peak @2048
baseline	80.8	350.6	3527.3	26689 MB
slim+sdpa+bf16+fused_decode+gpu_preprocess	79.1	353.3	33137.8	27245 MB
slim+stage_gc+bf16+fused_decode+gpu_preprocess	189.5	705.5	4919.0	26689 MB
slim+stage_gc+sdpa+fused_decode+gpu_preprocess	195.6	711.1	4939.2	26661 MB
slim+stage_gc+sdpa+bf16+gpu_preprocess	198.2	681.6	4853.8	26661 MB
slim+stage_gc+sdpa+bf16+fused_decode	197.7	714.1	4822.9	26661 MB
stage_gc+sdpa+bf16+fused_decode+gpu_preprocess	194.2	733.2	6274.1	26661 MB
slim+stage_gc+sdpa+bf16+fused_decode+gpu_preprocess	193.8	737.1	5128.2	26661 MB

Note the 33.1 s outlier: the only 2048 config without stage_gc but with sdpa — sdpa's slow path dominating once nothing throttles it (see headline 1 for the isolation). The sdpa cliff appears only at 2048; 512/1024 are unaffected.

M4 Pro Mac mini 24GB

Config	512	1024	2048 (swap-bound)
baseline	218.1	1149.2	23147.1
slim+sdpa+bf16+fused_decode+gpu_preprocess	221.4	1139.0	20994.5
slim+stage_gc+… (all stage_gc combos)	266–273	1268–1292	20233–24253

At 2048 the ~26 GB working set exceeds 24 GB unified memory; all configs swap and differences are not meaningful. ≤1024 full-frame or tiled-anything is the usable envelope.

Recommended settings by hardware

Hardware	2048 recommendation
M3-class (Max/Ultra)	full-frame or tiled 768/64 + compile; every toggle optional; avoid stage_gc
M1/M2-class	avoid sdpa; tiled 768/64 + compile; avoid stage_gc
≤24–32 GB any gen	tiled required (2.3 GB vs 26 GB); tiled 768/64 + compile

Environment: MLX (venv per uv sync --extra mlx), macOS 26.5, checkpoint v1.0.0. Quality harness (ground-truth plates + scoring) available on request — it's ~150 lines and reproduces the alpha-MAE column.