# 4-bit quantization

[QLoRA](https://hf.co/papers/2305.14314) is a finetuning method that quantizes a model to 4-bits and adds a set of low-rank adaptation (LoRA) weights to the model and tuning them through the quantized weights. This method also introduces a new data type, 4-bit NormalFloat (`LinearNF4`) in addition to the standard Float4 data type (`LinearFP4`). `LinearNF4` is a quantization data type for normally distributed data and can improve performance.

## Linear4bit[[bitsandbytes.nn.Linear4bit]]

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<docstring><name>class bitsandbytes.nn.Linear4bit</name><anchor>bitsandbytes.nn.Linear4bit</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L413</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_type", "val": " = 'fp4'"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters></docstring>

This class is the base module for the 4-bit quantization algorithm presented in [QLoRA](https://arxiv.org/abs/2305.14314).
QLoRA 4-bit linear layers uses blockwise k-bit quantization under the hood, with the possibility of selecting various
compute datatypes such as FP4 and NF4.

In order to quantize a linear layer one should first load the original fp16 / bf16 weights into
the Linear4bit module, then call `quantized_module.to("cuda")` to quantize the fp16 / bf16 weights.

<ExampleCodeBlock anchor="bitsandbytes.nn.Linear4bit.example">

Example:

```python
import torch
import torch.nn as nn

import bitsandbytes as bnb
from bnb.nn import Linear4bit

fp16_model = nn.Sequential(
    nn.Linear(64, 64),
    nn.Linear(64, 64)
)

quantized_model = nn.Sequential(
    Linear4bit(64, 64),
    Linear4bit(64, 64)
)

quantized_model.load_state_dict(fp16_model.state_dict())
quantized_model = quantized_model.to(0) # Quantization happens here
```

</ExampleCodeBlock>



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<docstring><name>__init__</name><anchor>bitsandbytes.nn.Linear4bit.__init__</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L446</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_type", "val": " = 'fp4'"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters><paramsdesc>- **input_features** (`str`) --
  Number of input features of the linear layer.
- **output_features** (`str`) --
  Number of output features of the linear layer.
- **bias** (`bool`, defaults to `True`) --
  Whether the linear class uses the bias term as well.</paramsdesc><paramgroups>0</paramgroups></docstring>

Initialize Linear4bit class.




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## LinearFP4[[bitsandbytes.nn.LinearFP4]]

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<docstring><name>class bitsandbytes.nn.LinearFP4</name><anchor>bitsandbytes.nn.LinearFP4</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L535</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters></docstring>

Implements the FP4 data type.



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<docstring><name>__init__</name><anchor>bitsandbytes.nn.LinearFP4.__init__</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L540</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters><paramsdesc>- **input_features** (`str`) --
  Number of input features of the linear layer.
- **output_features** (`str`) --
  Number of output features of the linear layer.
- **bias** (`bool`, defaults to `True`) --
  Whether the linear class uses the bias term as well.</paramsdesc><paramgroups>0</paramgroups></docstring>




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## LinearNF4[[bitsandbytes.nn.LinearNF4]]

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<docstring><name>class bitsandbytes.nn.LinearNF4</name><anchor>bitsandbytes.nn.LinearNF4</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L571</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters></docstring>
Implements the NF4 data type.

Constructs a quantization data type where each bin has equal area under a standard normal distribution N(0, 1) that
is normalized into the range [-1, 1].

For more information read the paper: QLoRA: Efficient Finetuning of Quantized LLMs (https://arxiv.org/abs/2305.14314)

Implementation of the NF4 data type in bitsandbytes can be found in the `create_normal_map` function in
the `functional.py` file: https://github.com/TimDettmers/bitsandbytes/blob/main/bitsandbytes/functional.py#L236.



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<docstring><name>__init__</name><anchor>bitsandbytes.nn.LinearNF4.__init__</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L583</source><parameters>[{"name": "input_features", "val": ""}, {"name": "output_features", "val": ""}, {"name": "bias", "val": " = True"}, {"name": "compute_dtype", "val": " = None"}, {"name": "compress_statistics", "val": " = True"}, {"name": "quant_storage", "val": " = torch.uint8"}, {"name": "device", "val": " = None"}]</parameters><paramsdesc>- **input_features** (`str`) --
  Number of input features of the linear layer.
- **output_features** (`str`) --
  Number of output features of the linear layer.
- **bias** (`bool`, defaults to `True`) --
  Whether the linear class uses the bias term as well.</paramsdesc><paramgroups>0</paramgroups></docstring>




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## Params4bit[[bitsandbytes.nn.Params4bit]]

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<docstring><name>class bitsandbytes.nn.Params4bit</name><anchor>bitsandbytes.nn.Params4bit</anchor><source>https://github.com/bitsandbytes-foundation/bitsandbytes/blob/main/bitsandbytes/nn/modules.py#L207</source><parameters>[{"name": "data", "val": ": typing.Optional[torch.Tensor] = None"}, {"name": "requires_grad", "val": " = False"}, {"name": "quant_state", "val": ": typing.Optional[bitsandbytes.functional.QuantState] = None"}, {"name": "blocksize", "val": ": typing.Optional[int] = None"}, {"name": "compress_statistics", "val": ": bool = True"}, {"name": "quant_type", "val": ": str = 'fp4'"}, {"name": "quant_storage", "val": ": dtype = torch.uint8"}, {"name": "module", "val": ": typing.Optional[ForwardRef('Linear4bit')] = None"}, {"name": "bnb_quantized", "val": ": bool = False"}]</parameters></docstring>



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<docstring><name>__init__</name><anchor>bitsandbytes.nn.Params4bit.__init__</anchor><parameters>[{"name": "*args", "val": ""}, {"name": "**kwargs", "val": ""}]</parameters></docstring>
Initialize self.  See help(type(self)) for accurate signature.

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