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William Lin 552d7dfadc [perf] PAN2 sampling: Wan2.2 VAE decode kernel, split decode, fused DiT kernels

- fastvideo-kernel/wan22_vae_decode: the Wan2.2 (TI2V-5B) VAE decoder on sm_100a / sm_103a at 704x1248, built on
  first use against fastvideo-kernel's CUTLASS submodule. It frees intermediates early and carries 4 latent frames
  per pass: a 5 s video peaks at 20.3 GiB, down from 94.6 GiB in the kernel's first version, with the same output.
- WanVAEConfig.decode_kernel routes AutoencoderKLWan.decode through it where it applies (bf16 with fp32
  accumulation, about 7x faster than the fp32 decode on a B300, PSNR 61-63 dB, SSIM 0.998 against it);
  decode_kernel_parallel splits the decode over the sequence-parallel group (2, 4 or 8 GPUs) and broadcasts the
  video, with the ranks agreeing on whether every one of them could build the kernel.
- PAN2's DiT runs fused Triton kernels for Q/K RMSNorm + RoPE and LayerNorm + modulation (pan2_fusions,
  set_fused_kernels).
- PAN2PipelineConfig turns all three on (vae_decode_kernel, vae_decode_kernel_parallel, dit_fused_kernels).
- VideoGenerator copies a decoded RGB video's frames to the host once instead of once per frame.
- The PAN2 presets sample at guidance 2.5, the reference launcher's default since 2026-10-23.

2026-11-02 21:42:30 +00:00

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PAN 2 Text/Image to Video

PAN 2 is a 29B mixture-of-transformers video DiT with separate video and text streams, a Qwen3.5 text encoder and the Wan2.2 VAE. One pipeline serves text-to-video and image-to-video: pass image_path to condition on a first frame and leave it out for text only.

Model

Hub repo	Pipeline	Workloads	Example
`IFM/PAN2`	`PAN2ModularPipeline`	T2V, I2V	`basic_pan2.py`

PAN2 checkpoints use the Modular Diffusers layout (modular_model_index.json). Any repo id or local directory whose name contains pan2 (or whose manifest declares PAN2ModularPipeline) resolves to this pipeline. The defaults come from the pan2_t2v and pan2_i2v presets:

Setting	Default
Resolution	704x1248 (height and width must be multiples of 32)
Frames	313, 13 s at 24 fps (`num_frames - 1` must be a multiple of 4)
Steps	50, uniform sigmas from 1 to 0 with the checkpoint's flow shift (9 for `IFM/PAN2-TEST`, the reference sampler's value)
Guidance	2.5, classifier-free guidance, the reference launcher's default (3 before 2026-10-23). Image-to-video keeps the first latent frame out of guidance (`i2v_skip_first_latent_cfg`, on by default, as in the reference sampler)
Negative prompt	None unless the request carries one. The reference sampler always sends the release's structured negative caption (`negative_prompt.json` of the PAN2 repository) with its three `time_range_sec` values set to the clip length, `0 to 20` for 481 frames at 24 fps

PAN 2 is trained on the structured captions its prompt rewriter produces, serialized as one-line JSON. Short raw prompts work, with weaker results.

Usage

python examples/inference/basic/basic_pan2.py

from fastvideo import VideoGenerator

generator = VideoGenerator.from_pretrained(
    "IFM/PAN2",
    num_gpus=1,
    dit_cpu_offload=False,
    dit_layerwise_offload=False,
    text_encoder_cpu_offload=True,
)
generator.generate_video(
    "A red fox trots through fresh snow in a pine forest at dawn.",
    image_path=None,  # or the path of a first frame for image-to-video
    output_path="video_samples_pan2",
)

For image-to-video, the first frame is scaled to cover the requested resolution and center cropped, then encoded by the VAE into the first latent frame of the conditioning input.

Parallelism

With num_gpus > 1, FastVideo runs Ulysses sequence parallelism: the video tokens are sharded across ranks and the text tokens are replicated. The number of GPUs must divide the 32 attention heads. Sequence-parallel padding of the video tokens is trimmed before attention, so no attention mask is needed.

Padding is always excluded, without an opt-in setting. At 704x1248 with 313 frames, the 67,782 video tokens need two padding tokens at SP=4. They are removed before joining the text tokens in every attention block. This provides the same exclusion as vLLM-Omni's unconditional padding mask in revision e0c4c731. The trimmed sequence can use either FlashAttention or Torch SDPA.

PAN2_NUM_GPUS=4 python examples/inference/basic/basic_pan2.py

Performance switches

PAN2 sampling turns these on by default (PAN2PipelineConfig); pass any of them as False to VideoGenerator.from_pretrained to turn it off:

Switch	What it does
`vae_decode_kernel`	Decodes with fastvideo-kernel's Wan2.2 decoder on B200 / GB200 / B300 (704x1248, batch 1): bf16 with fp32 accumulation, 7x faster than the fp32 decode on a B300 at PSNR 61-63 dB and SSIM 0.998 against it, at a peak of about 20 GiB for a 5 s video. It builds on first use (minutes) into `WAN22_VAE_DECODE_BUILD_DIR`, against fastvideo-kernel's CUTLASS submodule (`git submodule update --init fastvideo-kernel/include/cutlass`, or set `CUTLASS_DIR`). Other GPUs and sizes keep the fp32 decode.
`vae_decode_kernel_parallel`	With 2, 4 or 8 GPUs, splits that decode over them instead of decoding the whole video on each: a 20 s video takes 3.0 s on one B300, 1.55 s on two, 0.79 s on four and 0.47 s on eight.
`dit_fused_kernels`	Runs the DiT's fused Triton kernels for Q/K RMSNorm + RoPE and LayerNorm + modulation. Their rounding differs slightly from the unfused path.

On GPUs with room for every component (B200, B300), keep the text encoder resident as well: text_encoder_cpu_offload=False.

Memory and offloading

The example keeps the DiT on the GPUs and offloads the text encoder between requests. Sequence parallelism shards video tokens; each rank still holds the DiT weights. Increasing num_gpus alone does not shard those weights. For devices that cannot hold the DiT, see CPU offloading for dit_cpu_offload and dit_layerwise_offload. Hardware recipes and timings reported by vLLM-Omni use its own offload implementation; they have not been validated for FastVideo.

The full IFM/PAN2 checkpoint is not public yet. Local parity tests use the public random-weight wuqing157/tiny-pan2-modular-pipe checkpoint; it verifies the integration and produces noise rather than useful video.

Differences from the vLLM-Omni integration

The optional prompt and video guardrails (pan2-guardrail) are not included.
The Cache-DiT quality policy and CFG-parallel execution are vLLM-Omni serving features and are not ported.
The text encoder is a FastVideo-native Qwen3.5 port. Its Gated DeltaNet layers use a PyTorch chunked delta rule instead of the fla kernels.

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