vllm-omni/docs/user_guide/diffusion/quantization/overview.md

Quantization

vLLM-Omni provides a unified quantization framework that supports both diffusion models (DiT, text encoders, VAEs) and multi-stage omni models (e.g., Qwen3-Omni thinker). Quantization reduces memory usage and accelerates inference.

Supported Methods

Diffusion Models

Method	Guide	Description	Tested Models	Min GPU
FP8	FP8	FP8 W8A8, dynamic or static	Z-Image, Qwen-Image, Flux, Bagel	SM 89 (Ada)
Int8	Int8	Int8 W8A8	Z-Image, Qwen-Image	SM 89 (Ada) / Ascend NPU
GGUF	GGUF	GGUF format, dequant+GEMM for N-D tensors	Z-Image, Flux	SM 60

Multi-stage Omni Models (Pre-quantized Checkpoints)

Method	Description	Tested Models	Min GPU
ModelOpt FP8	Pre-quantized FP8 via NVIDIA ModelOpt	Qwen3-Omni (thinker)	SM 89 (Ada/Hopper)
ModelOpt NVFP4	Pre-quantized NVFP4 via NVIDIA ModelOpt	Qwen3-Omni (experimental, quality issues)	SM 100 (Blackwell)

!!! note AWQ, GPTQ, and BitsAndBytes are supported by vLLM's upstream quantization registry but have not been tested with vLLM-Omni pipelines. They may work via build_quant_config() but are not validated.

Pre-quantized LLM Checkpoints (Multi-stage Models)

For multi-stage models like Qwen3-Omni, the unified quantization framework auto-detects pre-quantized checkpoints via quantization_config in the HF config. Quantization is automatically scoped to the thinker's language_model — audio encoder, vision encoder, talker, and code2wav remain in BF16.

Format	quant_algo	Hardware	Example	Status
ModelOpt FP8	FP8	Ada/Hopper (SM 89+)	asdazd/Qwen3-Omni-30B-A3B-Instruct_modelopt_FP8	Tested — 47% memory reduction, comparable throughput
ModelOpt NVFP4	NVFP4	Blackwell (SM 100+)	shunyang90/Qwen3-Omni-30B-A3B-Instruct-NVFP4	Experimental — loads and runs but output quality is unacceptable

Tested FP8 results (Qwen3-Omni, 1×H200):

Config	Model Memory (GiB)	Mem Reduction	Decode (tok/s)
BF16 baseline	59.26	—	41.6
FP8 (ModelOpt)	31.41	47%	39.9

FP8 enables the full 3-stage pipeline (thinker + talker + code2wav) on a single 64GB GPU, which is impossible with BF16 (thinker alone requires 59.26 GiB).

Quantization Scope

Diffusion Models

When --quantization fp8 is enabled for diffusion models:

Component	What Gets Quantized	Mechanism
DiT (transformer)	nn.Linear layers	vLLM W8A8 FP8 compute (Ada/Hopper) or weight-only (older GPUs)
Text encoder	nn.Linear layers	FP8 weight storage, BF16 compute
VAE	nn.Conv2d, nn.Conv3d layers	FP8 weight storage, BF16 compute

Multi-stage Omni Models

For pre-quantized omni model checkpoints, quantization is scoped per component:

Component	Quantized?	Notes
Thinker (language_model)	Yes	FP8 or NVFP4 via ModelOpt
Audio encoder	No	Stays BF16
Vision encoder	No	Stays BF16
Talker	No	Stays BF16
Code2Wav	No	Stays BF16

!!! note Not all models support all components. See the FP8 supported models table for per-model details.

Device Compatibility for FP8

Device	Example Hardware	FP8	NVFP4
Blackwell GPU (SM 100+)	RTX 5090, B100, B200	Yes	Yes (native FP4 HW)
Ada/Hopper GPU (SM 89+)	RTX 4090, H100, H200	Yes (W8A8 native)	No
Turing/Ampere GPU (SM 75-86)	RTX 3090, A100	Yes (weight-only Marlin)	No
Ascend NPU	Atlas 800T A2 (910B)	Not yet supported	No

Kernel selection is automatic on CUDA GPUs.

Device Compatibility for Int8

Device Type	Generation	Example	Int8 Mode
NVIDIA GPU	Ada/Hopper (SM 89+)	RTX 4090, H100, H200	Full W8A8 with native hardware
Ascend NPU	Atlas A2/Atlas A3	Atlas 800T A2/Atlas 900 A3	Full W8A8 with native hardware

Python API

The build_quant_config() factory accepts multiple input formats:

from vllm_omni.quantization import build_quant_config

# String
config = build_quant_config("fp8")

# Dict with parameters
config = build_quant_config({"method": "fp8", "activation_scheme": "static"})

# Per-component dict
config = build_quant_config({
    "transformer": {"method": "fp8"},
    "vae": None,
})

# None / "none"
config = build_quant_config(None)  # returns None

Architecture

build_quant_config(spec)
    ├── str  → _build_single(method) → vLLM registry / _OVERRIDES
    ├── dict with "method" → _build_single(method, **kwargs)
    ├── per-component dict → ComponentQuantizationConfig
    │       routes get_quant_method() by longest-prefix match
    ├── QuantizationConfig → passthrough
    └── None / "none" → None

Migration Guide

Before (v0.14.x)

# Old diffusion-specific API (removed)
from vllm_omni.diffusion.quantization import get_diffusion_quant_config
config = get_diffusion_quant_config("fp8", activation_scheme="static")

After (v0.16.0+)

# Unified API — delegates to vLLM's registry
from vllm_omni.quantization import build_quant_config
config = build_quant_config({"method": "fp8", "activation_scheme": "static"})

# Per-component (new)
config = build_quant_config({
    "transformer": {"method": "fp8"},
    "vae": None,
})