<!--Copyright 2024 The HuggingFace Team. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.

⚠️ Note that this file is in Markdown but contains specific syntax for our doc-builder (similar to MDX) that may not be
rendered properly in your Markdown viewer.

-->

# Bitsandbytes

The [bitsandbytes](https://github.com/bitsandbytes-foundation/bitsandbytes) library provides quantization tools for LLMs through a lightweight Python wrapper around hardware accelerator functions. It enables working with large models using limited computational resources by reducing their memory footprint.

At its core, bitsandbytes provides:

- **Quantized Linear Layers**: `Linear8bitLt` and `Linear4bit` layers that replace standard PyTorch linear layers with memory-efficient quantized alternatives
- **Optimized Optimizers**: 8-bit versions of common optimizers through its `optim` module, enabling training of large models with reduced memory requirements
- **Matrix Multiplication**: Optimized matrix multiplication operations that leverage the quantized format

bitsandbytes offers two main quantization features:

1. **LLM.int8()** - An 8-bit quantization method that makes inference more accessible without significant performance degradation. Unlike naive quantization, [LLM.int8()](https://hf.co/papers/2208.07339) dynamically preserves higher precision for critical computations, preventing information loss in sensitive parts of the model.

2. **QLoRA** - A 4-bit quantization technique that compresses models even further while maintaining trainability by inserting a small set of trainable low-rank adaptation (LoRA) weights.

> **Note:** For a user-friendly quantization experience, you can use the `bitsandbytes` [community space](https://huggingface.co/spaces/bnb-community/bnb-my-repo).

Run the command below to install bitsandbytes.

```bash
pip install --upgrade transformers accelerate bitsandbytes
```

To compile from source, follow the instructions in the [bitsandbytes installation guide](https://huggingface.co/docs/bitsandbytes/main/en/installation).

## Hardware Compatibility

bitsandbytes is supported on NVIDIA GPUs for CUDA versions 11.8 - 13.0, Intel XPU, Intel Gaudi (HPU), and CPU. There is an ongoing effort to support additional platforms. If you're interested in providing feedback or testing, check out the [bitsandbytes repository](https://github.com/bitsandbytes-foundation/bitsandbytes) for more information.

### NVIDIA GPUs (CUDA)

This backend is supported on Linux x86-64, Linux aarch64, and Windows platforms.

| Feature | Minimum Hardware Requirement |
|---------|-------------------------------|
| 8-bit optimizers | NVIDIA Pascal (GTX 10X0 series, P100) or newer GPUs * |
| LLM.int8() | NVIDIA Turing (RTX 20X0 series, T4) or newer GPUs |
| NF4/FP4 quantization | NVIDIA Pascal (GTX 10X0 series, P100) or newer GPUs * |

### Intel GPUs (XPU)

This backend is supported on Linux x86-64 and Windows x86-64 platforms.

### Intel Gaudi (HPU)

This backend is supported on Linux x86-64 for Gaudi2 and Gaudi3.

### CPU

This backend is supported on Linux x86-64, Linux aarch64, and Windows x86-64 platforms.

## Quantization Examples

Quantize a model by passing a [`BitsAndBytesConfig`] to [`~PreTrainedModel.from_pretrained`]. This works for any model in any modality, as long as it supports [Accelerate](https://huggingface.co/docs/accelerate/index) and contains [torch.nn.Linear](https://pytorch.org/docs/stable/generated/torch.nn.Linear.html) layers.

<hfoptions id="bnb">
<hfoption id="8-bit">
<div class="bnb-container" style="border: 1px solid #ddd; border-radius: 8px; padding: 20px; margin: 20px 0">
Quantizing a model in 8-bit halves the memory-usage, and for large models, set `device_map="auto"` to efficiently distribute the weights across all available GPUs.

```py
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)

model_8bit = AutoModelForCausalLM.from_pretrained(
    "bigscience/bloom-1b7", 
    device_map="auto",
    quantization_config=quantization_config
)
```

By default, all other modules such as [torch.nn.LayerNorm](https://pytorch.org/docs/stable/generated/torch.nn.LayerNorm.html) are set to the default torch dtype. You can change the data type of these modules with the `dtype` parameter. Setting `dtype="auto"` loads the model in the data type defined in a model's `config.json` file.

```py
import torch
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)

model_8bit = AutoModelForCausalLM.from_pretrained(
    "facebook/opt-350m", 
    device_map="auto",
    quantization_config=quantization_config, 
    dtype="auto"
)
model_8bit.model.decoder.layers[-1].final_layer_norm.weight.dtype
```

Once a model is quantized to 8-bit, you can't push the quantized weights to the Hub unless you're using the latest version of Transformers and bitsandbytes. If you have the latest versions, then you can push the 8-bit model to the Hub with [`~PreTrainedModel.push_to_hub`]. The quantization config.json file is pushed first, followed by the quantized model weights.

```py
from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)

model = AutoModelForCausalLM.from_pretrained(
    "bigscience/bloom-560m", 
    device_map="auto",
    quantization_config=quantization_config
)

model.push_to_hub("bloom-560m-8bit")
```

</div>
</hfoption>
<hfoption id="4-bit">
<div class="bnb-container" style="border: 1px solid #ddd; border-radius: 8px; padding: 20px; margin: 20px 0">
Quantizing a model in 4-bit reduces your memory-usage by 4x, and for large models, set `device_map="auto"` to efficiently distribute the weights across all available GPUs.

```py
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_4bit=True)

model_4bit = AutoModelForCausalLM.from_pretrained(
    "bigscience/bloom-1b7",
    device_map="auto",
    quantization_config=quantization_config
)
```

By default, all other modules such as [torch.nn.LayerNorm](https://pytorch.org/docs/stable/generated/torch.nn.LayerNorm.html) are converted to `torch.float16`. You can change the data type of these modules with the `dtype` parameter. Setting `dtype="auto"` loads the model in the data type defined in a model's `config.json` file.

```py
import torch
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_4bit=True)

model_4bit = AutoModelForCausalLM.from_pretrained(
    "facebook/opt-350m",
    device_map="auto",
    quantization_config=quantization_config, 
    dtype="auto"
)
model_4bit.model.decoder.layers[-1].final_layer_norm.weight.dtype
```

Make sure you have the latest bitsandbytes version so you can serialize 4-bit models and push them to the Hub with [`~PreTrainedModel.push_to_hub`]. Use [`~PreTrainedModel.save_pretrained`] to save the 4-bit model locally.  

```py
from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_4bit=True)

model = AutoModelForCausalLM.from_pretrained(
    "bigscience/bloom-560m", 
    device_map="auto",
    quantization_config=quantization_config
)

model.push_to_hub("bloom-560m-4bit")
```

</div>
</hfoption>
</hfoptions>

> [!WARNING]
> 8 and 4-bit training is only supported for training *extra* parameters.

Check your memory footprint with `get_memory_footprint`.

```py
print(model.get_memory_footprint())
```

Load quantized models with [`~PreTrainedModel.from_pretrained`] without a `quantization_config`.

```py
from transformers import AutoModelForCausalLM, AutoTokenizer

model = AutoModelForCausalLM.from_pretrained("{your_username}/bloom-560m-8bit", device_map="auto")
```

## LLM.int8

This section explores some of the specific features of 8-bit quantization, such as offloading, outlier thresholds, skipping module conversion, and finetuning.

### Offloading

8-bit models can offload weights between the CPU and GPU to fit very large models into memory. The weights dispatched to the CPU are stored in **float32** and aren't converted to 8-bit. For example, enable offloading for [bigscience/bloom-1b7](https://huggingface.co/bigscience/bloom-1b7) through [`BitsAndBytesConfig`].

```py
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(llm_int8_enable_fp32_cpu_offload=True)
```

Design a custom device map to fit everything on your GPU except for the `lm_head`, which is dispatched to the CPU.

```py
device_map = {
    "transformer.word_embeddings": 0,
    "transformer.word_embeddings_layernorm": 0,
    "lm_head": "cpu",
    "transformer.h": 0,
    "transformer.ln_f": 0,
}
```

Now load your model with the custom `device_map` and `quantization_config`.

```py
model_8bit = AutoModelForCausalLM.from_pretrained(
    "bigscience/bloom-1b7",
    dtype="auto",
    device_map=device_map,
    quantization_config=quantization_config,
)
```

### Outlier threshold

An "outlier" is a hidden state value greater than a certain threshold, and these values are computed in fp16. While the values are usually normally distributed ([-3.5, 3.5]), this distribution can be very different for large models ([-60, 6] or [6, 60]). 8-bit quantization works well for values ~5, but beyond that, there is a significant performance penalty. A good default threshold value is 6, but a lower threshold may be needed for more unstable models (small models or finetuning).

To find the best threshold for your model, experiment with the `llm_int8_threshold` parameter in [`BitsAndBytesConfig`]. For example, setting the threshold to `0.0` significantly speeds up inference at the potential cost of some accuracy loss.

```py
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

model_id = "bigscience/bloom-1b7"

quantization_config = BitsAndBytesConfig(
    llm_int8_threshold=0.0,
    llm_int8_enable_fp32_cpu_offload=True
)

model_8bit = AutoModelForCausalLM.from_pretrained(
    model_id,
    dtype="auto",
    device_map=device_map,
    quantization_config=quantization_config,
)
```

### Skip module conversion

For some models, like [Jukebox](model_doc/jukebox), you don't need to quantize every module to 8-bit because it can actually cause instability. With Jukebox, there are several `lm_head` modules that should be skipped using the `llm_int8_skip_modules` parameter in [`BitsAndBytesConfig`].

```py
from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

model_id = "bigscience/bloom-1b7"

quantization_config = BitsAndBytesConfig(
    llm_int8_skip_modules=["lm_head"],
)

model_8bit = AutoModelForCausalLM.from_pretrained(
    model_id,
    dtype="auto",
    device_map="auto",
    quantization_config=quantization_config,
)
```

### Finetuning

The [PEFT](https://github.com/huggingface/peft) library supports fine-tuning large models like [flan-t5-large](https://huggingface.co/google/flan-t5-large) and [facebook/opt-6.7b](https://huggingface.co/facebook/opt-6.7b) with 8-bit quantization. You don't need to pass the `device_map` parameter for training because it automatically loads your model on a GPU. However, you can still customize the device map with the `device_map` parameter (`device_map="auto"` should only be used for inference).

## QLoRA

This section explores some of the specific features of 4-bit quantization, such as changing the compute data type, the Normal Float 4 (NF4) data type, and nested quantization.

### Compute data type

Change the data type from float32 (the default value) to bf16 in [`BitsAndBytesConfig`] to speedup computation.

```py
import torch
from transformers import BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_4bit=True, bnb_4bit_compute_dtype=torch.bfloat16)
```

### Normal Float 4 (NF4)

NF4 is a 4-bit data type from the [QLoRA](https://hf.co/papers/2305.14314) paper, adapted for weights initialized from a normal distribution. You should use NF4 for training 4-bit base models.

```py
from transformers import BitsAndBytesConfig

nf4_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
)

model_nf4 = AutoModelForCausalLM.from_pretrained(model_id, dtype="auto", quantization_config=nf4_config)
```

For inference, the `bnb_4bit_quant_type` does not have a huge impact on performance. However, to remain consistent with the model weights, you should use the `bnb_4bit_compute_dtype` and `dtype` values.

### Nested quantization

Nested quantization can save additional memory at no additional performance cost. This feature performs a second quantization of the already quantized weights to save an additional 0.4 bits/parameter. For example, with nested quantization, you can finetune a [Llama-13b](https://huggingface.co/meta-llama/Llama-2-13b) model on a 16GB NVIDIA T4 GPU with a sequence length of 1024, a batch size of 1, and enable gradient accumulation with 4 steps.

```py
from transformers import BitsAndBytesConfig

double_quant_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_use_double_quant=True,
)

model_double_quant = AutoModelForCausalLM.from_pretrained("meta-llama/Llama-2-13b-chat-hf", dtype="auto", quantization_config=double_quant_config)
```

## Dequantizing bitsandbytes models

Once quantized, you can [`~PreTrainedModel.dequantize`] a model to the original precision but this may result in some quality loss. Make sure you have enough GPU memory to fit the dequantized model.

```python
from transformers import AutoModelForCausalLM, BitsAndBytesConfig, AutoTokenizer

model = AutoModelForCausalLM.from_pretrained("facebook/opt-125m", BitsAndBytesConfig(load_in_4bit=True))
model.dequantize()
```

## Resources

Learn more about the details of 8-bit quantization in [A Gentle Introduction to 8-bit Matrix Multiplication for transformers at scale using Hugging Face Transformers, Accelerate and bitsandbytes](https://huggingface.co/blog/hf-bitsandbytes-integration).

Try 4-bit quantization in this [notebook](https://colab.research.google.com/drive/1ge2F1QSK8Q7h0hn3YKuBCOAS0bK8E0wf) and learn more about its details in [Making LLMs even more accessible with bitsandbytes, 4-bit quantization and QLoRA](https://huggingface.co/blog/4bit-transformers-bitsandbytes).
