Quantize 🤗 Transformers models

AutoGPTQ Integration

🤗 Transformers has integrated optimum API to perform GPTQ quantization on language models. You can load and quantize your model in 8, 4, 3 or even 2 bits without a big drop of performance and faster inference speed! This is supported by most GPU hardwares.

To learn more about the the quantization model, check out:

• the GPTQ paper
• the optimum guide on GPTQ quantization
• the AutoGPTQ library used as the backend

Requirements

You need to have the following requirements installed to run the code below:

• Install latest AutoGPTQ library pip install auto-gptq

• Install latest optimum from source pip install git+https://github.com/huggingface/optimum.git

• Install latest transformers from source pip install git+https://github.com/huggingface/transformers.git

• Install latest accelerate library pip install --upgrade accelerate

Note that GPTQ integration supports for now only text models and you may encounter unexpected behaviour for vision, speech or multi-modal models.

Load and quantize a model

GPTQ is a quantization method that requires weights calibration before using the quantized models. If you want to quantize transformers model from scratch, it might take some time before producing the quantized model (~5 min on a Google colab for facebook/opt-350m model).

Hence, there are two different scenarios where you want to use GPTQ-quantized models. The first use case would be to load models that has been already quantized by other users that are available on the Hub, the second use case would be to quantize your model from scratch and save it or push it on the Hub so that other users can also use it.

GPTQ Configuration

In order to load and quantize a model, you need to create a GPTQConfig. You need to pass the number of bits, a dataset in order to calibrate the quantization and the tokenizer of the model in order prepare the dataset.

model_id = "facebook/opt-125m"
tokenizer = AutoTokenizer.from_pretrained(model_id)
gptq_config = GPTQConfig(bits=4, dataset = "c4", tokenizer=tokenizer)

Note that you can pass your own dataset as a list of string. However, it is highly recommended to use the dataset from the GPTQ paper.

dataset = ["auto-gptq is an easy-to-use model quantization library with user-friendly apis, based on GPTQ algorithm."]
quantization = GPTQConfig(bits=4, dataset = dataset, tokenizer=tokenizer)

Quantization

You can quantize a model by using from_pretrained and setting the quantization_config.

from transformers import AutoModelForCausalLM
model = AutoModelForCausalLM.from_pretrained(model_id, quantization_config=gptq_config)

Note that you will need a GPU to quantize a model. We will put the model in the cpu and move the modules back and forth to the gpu in order to quantize them.

If you want to maximize your gpus usage while using cpu offload, you can set device_map = "auto".

from transformers import AutoModelForCausalLM
model = AutoModelForCausalLM.from_pretrained(model_id, device_map="auto", quantization_config=gptq_config)

Note that disk offload is not supported. Furthermore, if you are out of memory because of the dataset, you may have to pass max_memory in from_pretained. Checkout this guide to learn more about device_map and max_memory.

Push quantized model to 🤗 Hub

You can push the quantized model like any 🤗 model to Hub with push_to_hub. The quantization config will be saved and pushed along the model.

quantized_model.push_to_hub("opt-125m-gptq")
tokenizer.push_to_hub("opt-125m-gptq")

If you want to save your quantized model on your local machine, you can also do it with save_pretrained:

quantized_model.save_pretrained("opt-125m-gptq")
tokenizer.save_pretrained("opt-125m-gptq")

Note that if you have quantized your model with a device_map, make sure to move the entire model to one of your gpus or the cpu before saving it.

quantized_model.to("cpu")
quantized_model.save_pretrained("opt-125m-gptq")

Load a quantized model from the 🤗 Hub

You can load a quantized model from the Hub by using from_pretrained. Make sure that the pushed weights are quantized, by checking that the attribute quantization_config is present in the model configuration object.

from transformers import AutoModelForCausalLM
model = AutoModelForCausalLM.from_pretrained("{your_username}/opt-125m-gptq")

If you want to load a model faster and without allocating more memory than needed, the device_map argument also works with quantized model. Make sure that you have accelerate library installed.

from transformers import AutoModelForCausalLM
model = AutoModelForCausalLM.from_pretrained("{your_username}/opt-125m-gptq", device_map="auto")

Exllama kernels for faster inference

For 4-bit model, you can use the exllama kernels in order to a faster inference speed. It is activated by default. You can change that behavior by passing disable_exllama in GPTQConfig. This will overwrite the quantization config stored in the config. Note that you will only be able to overwrite the attributes related to the kernels. Furthermore, you need to have the entire model on gpus if you want to use exllama kernels.

import torch
gptq_config = GPTQConfig(bits=4, disable_exllama=False)
model = AutoModelForCausalLM.from_pretrained("{your_username}/opt-125m-gptq", device_map="auto", quantization_config = gptq_config)

Note that only 4-bit models are supported for now. Furthermore, it is recommended to deactivate the exllama kernels if you are finetuning a quantized model with peft.

Fine-tune a quantized model

With the official support of adapters in the Hugging Face ecosystem, you can fine-tune models that have been quantized with GPTQ. Please have a look at peft library for more details.

Example demo

Check out the Google Colab notebook to learn how to quantize your model with GPTQ and how finetune the quantized model with peft.

GPTQConfig

bitsandbytes Integration

🤗 Transformers is closely integrated with most used modules on bitsandbytes. You can load your model in 8-bit precision with few lines of code. This is supported by most of the GPU hardwares since the 0.37.0 release of bitsandbytes.

Learn more about the quantization method in the LLM.int8() paper, or the blogpost about the collaboration.

Since its 0.39.0 release, you can load any model that supports device_map using 4-bit quantization, leveraging FP4 data type.

If you want to quantize your own pytorch model, check out this documentation from 🤗 Accelerate library.

Here are the things you can do using bitsandbytes integration

General usage

You can quantize a model by using the load_in_8bit or load_in_4bit argument when calling the from_pretrained() method as long as your model supports loading with 🤗 Accelerate and contains torch.nn.Linear layers. This should work for any modality as well.

from transformers import AutoModelForCausalLM

model_8bit = AutoModelForCausalLM.from_pretrained("facebook/opt-350m", load_in_8bit=True)
model_4bit = AutoModelForCausalLM.from_pretrained("facebook/opt-350m", load_in_4bit=True)

By default all other modules (e.g. torch.nn.LayerNorm) will be converted in torch.float16, but if you want to change their dtype you can overwrite the torch_dtype argument:

>>> import torch
>>> from transformers import AutoModelForCausalLM

>>> model_8bit = AutoModelForCausalLM.from_pretrained("facebook/opt-350m", load_in_8bit=True, torch_dtype=torch.float32)
>>> model_8bit.model.decoder.layers[-1].final_layer_norm.weight.dtype
torch.float32

FP4 quantization

Requirements

Make sure that you have installed the requirements below before running any of the code snippets below.

• Latest bitsandbytes library pip install bitsandbytes>=0.39.0

• Install latest accelerate pip install --upgrade accelerate

• Install latest transformers pip install --upgrade transformers

Tips and best practices

• Advanced usage: Refer to this Google Colab notebook for advanced usage of 4-bit quantization with all the possible options.

• Faster inference with batch_size=1 : Since the 0.40.0 release of bitsandbytes, for batch_size=1 you can benefit from fast inference. Check out these release notes and make sure to have a version that is greater than 0.40.0 to benefit from this feature out of the box.

• Training: According to QLoRA paper, for training 4-bit base models (e.g. using LoRA adapters) one should use bnb_4bit_quant_type='nf4'.

• Inference: For inference, bnb_4bit_quant_type does not have a huge impact on the performance. However for consistency with the model’s weights, make sure you use the same bnb_4bit_compute_dtype and torch_dtype arguments.

Load a large model in 4bit

By using load_in_4bit=True when calling the .from_pretrained method, you can divide your memory use by 4 (roughly).

# pip install transformers accelerate bitsandbytes
from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "bigscience/bloom-1b7"

tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(model_id, device_map="auto", load_in_4bit=True)

Load a large model in 8bit

You can load a model by roughly halving the memory requirements by using load_in_8bit=True argument when calling .from_pretrained method

# pip install transformers accelerate bitsandbytes
from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "bigscience/bloom-1b7"

tokenizer = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(model_id, device_map="auto", load_in_8bit=True)

Then, use your model as you would usually use a PreTrainedModel.

You can check the memory footprint of your model with get_memory_footprint method.

print(model.get_memory_footprint())

With this integration we were able to load large models on smaller devices and run them without any issue.

Advanced use cases

Here we will cover some advanced use cases you can perform with FP4 quantization

Change the compute dtype

The compute dtype is used to change the dtype that will be used during computation. For example, hidden states could be in float32 but computation can be set to bf16 for speedups. By default, the compute dtype is set to float32.

import torch
from transformers import BitsAndBytesConfig

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

Using NF4 (Normal Float 4) data type

You can also use the NF4 data type, which is a new 4bit datatype adapted for weights that have been initialized using a normal distribution. For that run:

from transformers import BitsAndBytesConfig

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

model_nf4 = AutoModelForCausalLM.from_pretrained(model_id, quantization_config=nf4_config)

Use nested quantization for more memory efficient inference

We also advise users to use the nested quantization technique. This saves more memory at no additional performance - from our empirical observations, this enables fine-tuning llama-13b model on an NVIDIA-T4 16GB with a sequence length of 1024, batch size of 1 and gradient accumulation steps of 4.

from transformers import BitsAndBytesConfig

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

model_double_quant = AutoModelForCausalLM.from_pretrained(model_id, quantization_config=double_quant_config)

Push quantized models on the 🤗 Hub

You can push a quantized model on the Hub by naively using push_to_hub method. This will first push the quantization configuration file, then push the quantized model weights. Make sure to use bitsandbytes>0.37.2 (at this time of writing, we tested it on bitsandbytes==0.38.0.post1) to be able to use this feature.

from transformers import AutoModelForCausalLM, AutoTokenizer

model = AutoModelForCausalLM.from_pretrained("bigscience/bloom-560m", device_map="auto", load_in_8bit=True)
tokenizer = AutoTokenizer.from_pretrained("bigscience/bloom-560m")

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

Load a quantized model from the 🤗 Hub

You can load a quantized model from the Hub by using from_pretrained method. Make sure that the pushed weights are quantized, by checking that the attribute quantization_config is present in the model configuration object.

from transformers import AutoModelForCausalLM, AutoTokenizer

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

Note that in this case, you don’t need to specify the arguments load_in_8bit=True, but you need to make sure that bitsandbytes and accelerate are installed. Note also that device_map is optional but setting device_map = 'auto' is prefered for inference as it will dispatch efficiently the model on the available ressources.

Advanced use cases

This section is intended to advanced users, that want to explore what it is possible to do beyond loading and running 8-bit models.

Offload between cpu and gpu

One of the advanced use case of this is being able to load a model and dispatch the weights between CPU and GPU. Note that the weights that will be dispatched on CPU will not be converted in 8-bit, thus kept in float32. This feature is intended for users that want to fit a very large model and dispatch the model between GPU and CPU.

First, load a BitsAndBytesConfig from transformers and set the attribute llm_int8_enable_fp32_cpu_offload to True:

from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(llm_int8_enable_fp32_cpu_offload=True)

Let’s say you want to load bigscience/bloom-1b7 model, and you have just enough GPU RAM to fit the entire model except the lm_head. Therefore write a custom device_map as follows:

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

And load your model as follows:

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

And that’s it! Enjoy your model!

Play with llm_int8_threshold

You can play with the llm_int8_threshold argument to change the threshold of the outliers. An “outlier” is a hidden state value that is greater than a certain threshold. This corresponds to the outlier threshold for outlier detection as described in LLM.int8() paper. Any hidden states value that is above this threshold will be considered an outlier and the operation on those values will be done in fp16. Values are usually normally distributed, that is, most values are in the range [-3.5, 3.5], but there are some exceptional systematic outliers that are very differently distributed for large models. These outliers are often in the interval [-60, -6] or [6, 60]. Int8 quantization works well for values of magnitude ~5, but beyond that, there is a significant performance penalty. A good default threshold is 6, but a lower threshold might be needed for more unstable models (small models, fine-tuning). This argument can impact the inference speed of the model. We suggest to play with this parameter to find which one is the best for your use case.

from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

model_id = "bigscience/bloom-1b7"

quantization_config = BitsAndBytesConfig(
    llm_int8_threshold=10,
)

model_8bit = AutoModelForCausalLM.from_pretrained(
    model_id,
    device_map=device_map,
    quantization_config=quantization_config,
)
tokenizer = AutoTokenizer.from_pretrained(model_id)

Skip the conversion of some modules

Some models has several modules that needs to be not converted in 8-bit to ensure stability. For example Jukebox model has several lm_head modules that should be skipped. Play with llm_int8_skip_modules

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,
    device_map=device_map,
    quantization_config=quantization_config,
)
tokenizer = AutoTokenizer.from_pretrained(model_id)

Fine-tune a model that has been loaded in 8-bit

With the official support of adapters in the Hugging Face ecosystem, you can fine-tune models that have been loaded in 8-bit. This enables fine-tuning large models such as flan-t5-large or facebook/opt-6.7b in a single google Colab. Please have a look at peft library for more details.

Note that you don’t need to pass device_map when loading the model for training. It will automatically load your model on your GPU. You can also set the device map to a specific device if needed (e.g. cuda:0, 0, torch.device('cuda:0')). Please note that device_map=auto should be used for inference only.

BitsAndBytesConfig

Quantization with 🤗 optimum

Please have a look at Optimum documentation to learn more about quantization methods that are supported by optimum and see if these are applicable for your use case.