transformers/docs/source/en/quantization/bitsandbytes.md

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# Bitsandbytes

The [bitsandbytes](https://github.com/bitsandbytes-foundation/bitsandbytes) library provides quantization tools for LLMs through a lightweight Python wrapper around CUDA 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 currently only supported on CUDA GPUs for CUDA versions 11.0 - 12.8. However, there's an ongoing multi-backend effort under development, which is currently in alpha. If you're interested in providing feedback or testing, check out the [bitsandbytes repository](https://github.com/bitsandbytes-foundation/bitsandbytes) for more information.

### CUDA

| Feature | Minimum Hardware Requirement |
|---------|-------------------------------|
| 8-bit optimizers | NVIDIA Maxwell (GTX 900 series, TITAN X, M40) or newer GPUs * |
| LLM.int8() | NVIDIA Turing (RTX 20 series, T4) or newer GPUs |
| NF4/FP4 quantization | NVIDIA Maxwell (GTX 900 series, TITAN X, M40) or newer GPUs * |

### Multi-backend

| Backend | Supported Versions | Python versions | Architecture Support | Status |
|---------|-------------------|----------------|---------------------|---------|
| AMD ROCm | 6.1+ | 3.10+ | minimum CDNA - gfx90a, RDNA - gfx1100 | Alpha |
| Apple Silicon (MPS) | WIP | 3.10+ | M1/M2 chips | Planned |
| Intel CPU | v2.4.0+ (ipex) | 3.10+ | Intel CPU | Alpha |
| Intel GPU | v2.4.0+ (ipex) | 3.10+ | Intel GPU | Experimental |
| Ascend NPU | 2.1.0+ (torch_npu) | 3.10+ | Ascend NPU | Experimental |

> **Note:** Bitsandbytes is moving away from the multi-backend approach towards using [Pytorch Custom Operators](https://pytorch.org/tutorials/advanced/custom_ops_landing_page.html), as the main mechanism for supporting new hardware, and dispatching to the correct backend.

## 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 `torch_dtype` parameter. Setting `torch_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, 
    torch_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 `torch_dtype` parameter.. Setting `torch_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, 
    torch_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",
    torch_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,
    torch_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,
    torch_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, torch_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 `torch_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", torch_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 it's details in [Making LLMs even more accessible with bitsandbytes, 4-bit quantization and QLoRA](https://huggingface.co/blog/4bit-transformers-bitsandbytes).