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# Nunchaku Lite

Nunchaku Lite is a quantization backend for loading prequantized checkpoints in Diffusers. Create compatible checkpoints with [diffuse-compressor](https://github.com/rootonchair/diffuse-compressor). It quantizes and exports a transformer, then packages it as a Diffusers pipeline.

Nunchaku Lite builds on the original [Nunchaku](https://github.com/nunchaku-ai/nunchaku) inference engine,
[DeepCompressor](https://github.com/nunchaku-ai/deepcompressor) quantization library, and
[SVDQuant paper](https://arxiv.org/abs/2411.05007).

## Install the CUDA kernels

The kernels package supplies the optimized CUDA kernels, which load automatically. Install it first.

```bash
pip install -U kernels
```

Nunchaku Lite loads its kernels from the [`rootonchair/nunchaku-lite-kernels`](https://huggingface.co/rootonchair/nunchaku-lite-kernels) repository, whose publisher is not a trusted kernel publisher on the Hub. Loading it downloads and executes code from the Hub, so Diffusers requires you to explicitly opt in by setting `DIFFUSERS_TRUST_REMOTE_KERNELS=true`. See [Trusting remote kernels](../optimization/attention_backends#trusting-remote-kernels) for details.

## Load a quantized pipeline

Load the prequantized pipeline with [`~DiffusionPipeline.from_pretrained`], which reads the quantization
config from `config.json`.

```python
import torch
from diffusers import DiffusionPipeline

model_id = "rootonchair/ERNIE-Image-Turbo-nunchaku-lite-nvfp4"

pipe = DiffusionPipeline.from_pretrained(
    model_id, dtype=torch.bfloat16,
).to("cuda")

prompt = "A modern red armchair in a quiet studio, soft window light, realistic product photography"
image = pipe(
    prompt=prompt,
    height=1024,
    width=1024,
    num_inference_steps=8,
    guidance_scale=1.0,
).images[0]
image.save("ernie-image-turbo-nunchaku-lite.png")
```

> [!NOTE]
> The exported state dict must match the target Diffusers model architecture exactly. For example, a checkpoint
> quantized with fused QKV projections won't load into a model config that expects separate Q, K, and V projection
> modules.

## Supported quantization types

Nunchaku Lite supports the following quantized linear layer formats.

> [!TIP]
> Use `nvfp4` on Blackwell GPUs. Running `int4` checkpoints on Blackwell can be slower than `nvfp4`.

The CUDA kernels currently support the following NVIDIA GPU architectures:

- `sm_75` (Turing, for example RTX 2080)
- `sm_80` (Ampere, for example A100)
- `sm_86` (Ampere, for example RTX 3090 and RTX A6000)
- `sm_89` (Ada, for example RTX 4090)
- `sm_120` (Blackwell, for example RTX 5090)

> [!NOTE]
> Hopper GPUs, such as `sm_90` H100 and H200, are not currently supported.

`nvfp4` checkpoints require a Blackwell or newer NVIDIA GPU. On Blackwell GPUs, use PyTorch >= 2.7 with CUDA >= 12.8.
`int4` checkpoints require a Turing or newer NVIDIA GPU.

| Method | Precision | Group size | Notes |
|---|---:|---:|---|
| `svdq_w4a4` | `nvfp4` | 16 | Uses NVFP4 runtime kernels with SVDQ low-rank correction. |
| `svdq_w4a4` | `int4` | 64 | Uses INT4 W4A4 kernels with SVDQ low-rank correction. |
| `awq_w4a16` | `int4` | 64 | Uses INT4 weight-only AWQ-style kernels. |

## NunchakuLiteQuantizationConfig

The `config.json` file must include a [`NunchakuLiteQuantizationConfig`]. It defines the runtime
`compute_dtype` and the target modules for each Nunchaku Lite quantization method.

- `compute_dtype`: runtime dtype for floating-point buffers in quantized modules, typically `torch.bfloat16`.
- `svdq_w4a4`: SVDQ W4A4 target config with `precision`, `group_size`, `rank`, and `targets`.
- `awq_w4a16`: AWQ W4A16 target config with `precision`, `group_size`, and `targets`.

Each entry in `targets` must point to a linear layer. Diffusers swaps each `svdq_w4a4` target for an SVDQ W4A4 layer and each `awq_w4a16` target for an AWQ W4A16 layer. The example below shows the
expected shape with shortened target lists.

List each module you want to quantize under `svdq_w4a4` or `awq_w4a16`. A module can only use one method, so don't list the same target under both.

```json
{
  "_class_name": "ErnieImageTransformer2DModel",
  "quantization_config": {
    "quant_method": "nunchaku_lite",
    "compute_dtype": "bfloat16",
    "svdq_w4a4": {
      "precision": "nvfp4",
      "group_size": 16,
      "rank": 32,
      "targets": ["layers.0.self_attention.to_q"]
    },
    "awq_w4a16": {
      "precision": "int4",
      "group_size": 64,
      "targets": ["final_linear"]
    }
  }
}
```

## torch.compile

Nunchaku Lite kernels and quantized linear layers are compatible with [`torch.compile`](../optimization/fp16#torchcompile).
Compile the quantized transformer after loading the pipeline for faster inference.

```python
pipe.transformer = torch.compile(pipe.transformer, mode="default", fullgraph=True)
```

The compiled Nunchaku Lite NVFP4 pipeline runs 1.8x faster than the original BF16 pipeline (2.271s → 1.675s on an RTX PRO 6000).

## Resources

- [diffuse-compressor](https://github.com/rootonchair/diffuse-compressor)
- [Nunchaku installation requirements](https://nunchaku.tech/docs/nunchaku/installation/installation.html)
