On GB10 the light H3 VAE's INT8 ConvRot overlay decodes 832x480x124 in
42 s and gains nothing from tile batching. The dense weights take 24 s
with one tile per decoder call, 14 s with 12 tiles per call and 9.8 s
compiled, at higher fidelity (77.7 vs 63.4 dB PSNR against an FP32
decode). FASTVIDEO_H3_VAE_INT8_OVERLAY=0 now skips the overlay (default
on, so other recipes are unchanged), and the single-Spark recipes use it
with FASTVIDEO_H3_VAE_TILE_BATCH=12 and a compiled VAE decoder.
FlashInfer's CUTLASS mm_fp4 is 2.7-3x slower than its cuDNN backend for
the H3 FFN projections on GB10 with identical outputs, so the recipes
select FASTVIDEO_NVFP4_MM_BACKEND=cudnn. FlashInfer autotuning was not
adopted: it made repeated calls non-deterministic.
One Spark, 832x480x124, seed 1234, warm calls: FastH3 Trim 121-122 s to
68 s (denoise 78 -> 58 s, decode 43 -> 9.5 s); FastH3 V2 139-141 s to
79 s. Repeated calls are bit-identical, which the previous V2 recipe was
not.
FP4 attention (FASTVIDEO_H3_VSA_FP4=1, built for sm_121a since #1598) is
documented for GB10 as opt-in and stays off: it is 24-33% faster in
denoise on V2 but changes the output (LPIPS 0.39-0.55 against BF16
attention), and the checkpoints were not trained for it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
