Under SP>1 the H3 video VAE decoded all temporal chunks serially on the
output rank while the other ranks idled (#1703's gate), and every rank
encoded the full reference video redundantly. Chunk decodes and clip
encodes have no cross-chunk data dependency - only the joining (overlap
blend, trim, denormalize, moment concat) is sequential - so both are
round-robined across the sequence-parallel ranks:
- fastvideo/models/vaes/minimax_h3_parallel.py: decode_to_pixels_parallel
gathers each round's decoded segments (body+halo tail, one contiguous
slice per chunk) to the SP group's first rank via NCCL gather (or
all_gather, FASTVIDEO_VAE_PARALLEL_DECODE_STRATEGY), which replays the
serial blend/trim/denormalize/copy semantics with the same VAE methods -
bitwise-equal to serial decode_to_pixels by construction. Placeholder
rounds keep collective participation uniform; the leader decodes chunk 0
first and broadcasts dtype/shape metadata so placeholders never guess the
autocast dtype. encode_pixels_parallel all-gathers per-clip moments
(latent-sized) so every rank keeps the identical full posterior,
preserving the all-ranks-hold-latents contract.
- decoding stage: output gate moves from world rank 0 to the SP group's
first rank (identical in the single-group e2e case; correct for the
trainer validation callback, which consumes each group leader's batch);
with vae_parallel_decode every rank enters the decode body so no
rank-dependent branch guards the collectives.
- latent preparation: opt-in clip-parallel reference encode on the same
seam (vae_parallel_encode).
- knobs: FastVideoArgs.vae_parallel_decode/encode (+ --vae-parallel-decode,
--vae-parallel-encode, FASTVIDEO_VAE_PARALLEL_DECODE/ENCODE env
parse-once adapters), default OFF.
- _copy_chunk_pixels factored out of _decode_to_pixels so serial and
parallel share one output-copy path (behavior unchanged).
Tests: threaded fake-group CPU suite drives the real SPMD functions
end-to-end (world sizes 2-5, both strategies, pad/blend/trim geometries,
token_drop=0, batched slicing, placeholder rounds) bit-exact vs the serial
APIs; GPU regression (torchrun world>1 gated) asserts bitwise parity under
fp16 autocast with real NCCL plus repeat-determinism x3.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
