# Online Quantization

Online quantization lets you take a BF16/FP16 model and quantize its Linear
and MoE weights to lower precision (such as FP8) at load time, without needing
a pre-quantized checkpoint or calibration data. Weights are converted during
model loading and activations are dynamically scaled during each forward pass.

## Quick Start

Pass a scheme name to the `quantization` parameter:

```python
from vllm import LLM

# Per-tensor FP8 quantization (one scale per weight tensor)
llm = LLM("meta-llama/Llama-3.1-8B", quantization="fp8_per_tensor")

# Per-block FP8 quantization (128x128 block scaling for weights and 1x128 block scaling for activations)
llm = LLM("meta-llama/Llama-3.1-8B", quantization="fp8_per_block")
```

Or with the CLI:

```bash
vllm serve meta-llama/Llama-3.1-8B --quantization fp8_per_tensor
vllm serve meta-llama/Llama-3.1-8B --quantization fp8_per_block
```

## Supported Schemes

| Scheme | Weight recipe | Activation recipe | Notes |
| ------ | ------------- | ------------------ | ----- |
| `fp8_per_tensor` | fp8_e4m3 data, fp32 per-tensor scale | fp8_e4m3 data, fp32 per-tensor scale | On some GPUs (Ada, Hopper) linear activations use per-token scaling for better performance |
| `fp8_per_block` | fp8_e4m3 data, fp32 per-128x128-block scale | fp8_e4m3 data, fp32 per-1x128-block scale | |

Support for additional schemes will be added in future versions of vllm.

## Advanced Configuration

For fine-grained control, use a `quantization_config` dictionary.

### Separate Schemes for Dense and MoE Layers

You can apply different quantization schemes to dense linear layers and MoE expert layers:

```python
from vllm import LLM

llm = LLM(
    "ibm-granite/granite-3.0-1b-a400m-base",
    quantization="fp8_per_tensor",
    quantization_config={
        "linear_scheme_override": "fp8_per_block",
    },
)
```

Or,

```python
from vllm import LLM

llm = LLM(
    "ibm-granite/granite-3.0-1b-a400m-base",
    quantization="fp8_per_tensor",
    quantization_config={
        "moe_scheme_override": "fp8_per_block",
    },
)
```

### Excluding Layers from Quantization

Use the `ignore` parameter to skip specific layers. It accepts exact layer names and regex patterns (prefixed with `re:`):

```python
from vllm import LLM

llm = LLM(
    "ibm-granite/granite-3.0-1b-a400m-base",
    quantization="fp8_per_tensor",
    quantization_config={
        "ignore": [
            # exact layer name
            "model.layers.1.self_attn.o_proj",
            # regex: skip all QKV projections
            "re:.*[qkv]_proj",
        ],
    },
)
```

!!! note
    For fused layers (e.g., `qkv_proj` which fuses `q_proj`, `k_proj`, `v_proj`), the ignore pattern must match the **unfused** shard names (`q_proj`, `k_proj`, `v_proj`), not the fused name.
