Bringing Nunchaku 4-bit Diffusion Inference to Diffusers

Large diffusion transformers can create stunning images (or even videos, audio snippets, and now text), but loading a modern text-to-image model in BF16 precision often requires 20-30 GB of VRAM, which puts these models out of reach of most consumer GPUs. Quantization is a powerful solution to this problem, and Diffusers already integrates several quantization backends such as bitsandbytes, GGUF, torchao, and Quanto, which we covered in Exploring Quantization Backends in Diffusers.

Most of these backends are weight-only. This means that they store the weights in low precision and dequantize them back to high precision at compute time. This reduces memory usage significantly, but it usually does not make inference faster, and can even add a small latency overhead.

SVDQuant, the quantization method behind the popular Nunchaku inference engine, takes a different approach. It runs the main transformer layers with 4-bit weights and activations (W4A4), reducing memory while also speeding up the denoising loop. The details are covered below, but until now, using these checkpoints required a separate inference library.

With current Diffusers, loading a Nunchaku checkpoint is as simple as calling from_pretrained(), with no local CUDA compilation required thanks to the kernels package. In addition, the companion diffuse-compressor toolkit lets you quantize new architectures yourself and publish them as regular Diffusers repositories.

Table of Contents

• Getting started with Nunchaku Lite
• Background: SVDQuant and Nunchaku
• Introducing Nunchaku Lite
• Native loading in Diffusers
• Getting more speed and lower memory
• Benchmarks
• Quantizing your own model
• Ready-to-use checkpoints
• Conclusion
• Acknowledgements

Getting started with Nunchaku Lite

First, install the requirements. You need a recent version of Diffusers and the Hugging Face kernels package:

pip install -U diffusers transformers accelerate kernels bitsandbytes

Then load a pre-quantized pipeline like any other Diffusers model:

import torch
from diffusers import ErnieImagePipeline

pipe = ErnieImagePipeline.from_pretrained(
    "lite-infer/ERNIE-Image-Turbo-nunchaku-lite-nvfp4_r32-bnb4-text-encoder",
    torch_dtype=torch.bfloat16,
).to("cuda")

image = pipe(
    prompt="A cinematic portrait of a red fox in a misty forest at sunrise, "
           "detailed fur, volumetric light",
    height=1024,
    width=1024,
    num_inference_steps=8,
    guidance_scale=1.0,
    generator=torch.Generator("cuda").manual_seed(42),
).images[0]
image.save("output.png")

No custom pipeline class or separate inference engine is needed, and there is nothing to compile locally. The NVFP4 kernels are downloaded from the Hub through the Nunchaku Lite kernels page the first time they are used. This checkpoint pairs a Nunchaku NVFP4 transformer with a bitsandbytes NF4 text encoder, and generates a 1024x1024 image in about 1.7 seconds on an RTX 5090 with a peak memory usage of about 12 GB, compared with about 24 GB for the BF16 pipeline. You can find more details about the Nunchaku Lite checkpoint format in the official Diffusers documentation.

NVFP4 checkpoints require an NVIDIA Blackwell GPU (RTX 50 series, RTX PRO 6000, B200). For earlier generations, use the INT4 variants. See the hardware support table below for details.

Background: SVDQuant and Nunchaku

SVDQuant is the quantization method behind Nunchaku, its reference CUDA inference engine. Standard 4-bit quantization is difficult for diffusion transformers because both weights and activations contain large outliers. SVDQuant handles this by moving activation outliers into the weights, representing the hardest part of each weight matrix with a small 16-bit low-rank branch, and quantizing the remaining residual to 4 bits. Nunchaku makes this fast with fused kernels for the 4-bit path and the low-rank branch.

Introducing Nunchaku Lite

The original Nunchaku engine gets much of its speed from model-specific fused execution paths, such as fused QKV projections and fused GELU/MLP kernels. Those optimizations are tied to each architecture's module layout and checkpoint format, so supporting a new model family usually requires model-specific integration work.

Nunchaku Lite is the new integration path in Diffusers. With it, Diffusers can load Nunchaku-style checkpoints without a custom pipeline or a separate inference engine. Under the hood, Nunchaku Lite patches the relevant nn.Linear modules of a stock Diffusers model with runtime SVDQ/AWQ linear layers before the checkpoint is loaded. The CUDA kernels come from the Hub through the kernels package. Two kernel families are used:

• svdq_w4a4: 4-bit weights and activations with the SVDQuant low-rank correction. This layer is used for the transformer's attention and MLP projections, where nearly all of the compute is spent, and is available in INT4 and NVFP4 variants.
• awq_w4a16: 4-bit weights with 16-bit activations, used for adaptive normalization and modulation projections such as FLUX adanorm_single / adanorm_zero or Qwen-Image modulation layers. These layers are memory-bound and precision-sensitive, making AWQ a good fit to preserve precision while still saving memory and space.

The trade-off is that, without architecture-specific fused kernels and modules, Nunchaku Lite cannot match the speedup of the original Nunchaku engine. However, the bare-bones implementation still delivers around 30% speedup while retaining the same level of VRAM reduction.

Native loading in Diffusers

If you have used bitsandbytes or torchao in Diffusers, the mechanics will feel familiar. A Nunchaku Lite model repository is an ordinary Diffusers repository. The only special part is a quantization_config block inside the transformer's config.json:

"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",
            "layers.0.self_attention.to_k",
            "..."
        ]
    },
    "awq_w4a16": {
        "precision": "int4",
        "group_size": 64,
        "targets": [
            "adaLN_modulation.1",
            "..."
        ]
    }
}

This config tells Diffusers which modules were quantized, which scheme they use, and which Nunchaku Lite runtime layer to instantiate (SVDQW4A4Linear or AWQW4A16Linear).

Because the quantized model keeps the exact module structure of the dense one, everything downstream (schedulers, LoRA loading hooks, offloading, torch.compile) sees a normal Diffusers model.

Hardware support

Nunchaku Lite uses different kernel variants depending on the GPU generation and checkpoint precision:

Volta and Hopper GPUs are currently not supported by the 4-bit kernels. The quantizer validates the GPU's CUDA capability at load time and raises a clear error instead of producing incorrect outputs.

Getting more speed and lower memory

Nunchaku Lite can be combined with other Diffusers memory and speed optimizations.

torch.compile. Compiling the transformer improves the end-to-end speedup from 1.35x to 1.8x:

pipe.transformer.compile(fullgraph=True)



pipe.transformer.compile_repeated_blocks(fullgraph=True)

Quantized text encoders. The transformer is not the only component with a large memory footprint. Text encoders such as T5 or Qwen3 can occupy several gigabytes on their own. Further quantizing the text encoder with bitsandbytes NF4 reduces peak VRAM by about 22% in our benchmark.

Offloading. Diffusers offloading helpers such as enable_model_cpu_offload() and enable_sequential_cpu_offload() work as usual if you need to fit the pipeline onto a smaller GPU.

Benchmarks

All numbers below were measured on an NVIDIA RTX PRO 6000 (Blackwell) at 1024x1024 using rootonchair/ERNIE-Image-Turbo-nunchaku-lite-int4-bnb4-text-encoder.

End-to-end latency and memory

As shown above, Nunchaku reduces peak VRAM by up to 50% while still improving latency by roughly 30%. The remaining overhead comes largely from extra kernel launches, which torch.compile can mitigate, bringing the full pipeline down to 1.68 s, or 1.8x faster than the BF16 baseline.

Image quality

Quantizing your own model

Nunchaku Lite support in Diffusers is architecture-agnostic, and the diffuse-compressor toolkit provides an end-to-end SVDQuant workflow for Diffusers models: calibrate, quantize, package, and publish.

Below, we walk through quantizing FLUX.2 Klein 4B as an example. It covers the main steps: inspect the model, calibrate and quantize the transformer, package the result as a Diffusers pipeline, then verify and push it to the Hub. The full tutorial covers every flag in detail.

1. Inspect what will be quantized

The generic scanner walks the model and decides what to target: compatible linears inside the repeated transformer-block stack become SVDQ W4A4 targets, recognized modulation linears become AWQ W4A16 targets, and everything else stays dense.

python examples/text_to_image/quantize_hf.py black-forest-labs/FLUX.2-klein-4B \
  --precision int4 --rank 32 --inspect-config

Always read this report before quantizing. For FLUX.2 Klein 4B, the expected result is 100 SVDQ targets, 3 AWQ targets, and 6 dense outer linears, with no missing patterns or duplicate names.

2. Run quantization

The following command runs SVDQuant on the transformer and writes the quantized checkpoint to outputs/checkpoints/svdq-int4_r32-flux-2-klein-4b.safetensors:

python examples/text_to_image/quantize_hf.py black-forest-labs/FLUX.2-klein-4B \
  --precision int4 \
  --output outputs/checkpoints/svdq-int4_r32-flux-2-klein-4b.safetensors

Replace --precision int4 with nvfp4 to build Blackwell-native weights.

3. Package a Diffusers pipeline

The converter combines the quantized transformer with the base pipeline's other components, writes the compact nunchaku_lite configuration into transformer/config.json, and can optionally convert text encoders to NF4:

python examples/convert_nunchaku_lite_diffusers.py \
  --checkpoint outputs/checkpoints/svdq-int4_r32-flux-2-klein-4b.safetensors \
  --model-id black-forest-labs/FLUX.2-klein-4B \
  --bnb4-text-encoder text_encoder \
  --compute-dtype bfloat16 \
  --output-dir outputs/diffusers/FLUX.2-klein-4B-nunchaku-lite-int4-bnb4-text-encoder

4. Load, verify, and push to the Hub

import torch
from diffusers import DiffusionPipeline

pipe = DiffusionPipeline.from_pretrained(
    "outputs/diffusers/FLUX.2-klein-4B-nunchaku-lite-int4-bnb4-text-encoder",
    device_map="cuda",
)
image = pipe(
    "A glass robot in a greenhouse, cinematic lighting",
    num_inference_steps=4, guidance_scale=1.0,
    generator=torch.Generator("cuda").manual