#
Quantization

Quantization techniques focus on representing data with less information while also trying to not lose too much accuracy. This often means converting a data type to represent the same information with fewer bits. For example, if your model weights are stored as 32-bit floating points and they’re quantized to 16-bit floating points, this halves the model size which makes it easier to store and reduces memory-usage. Lower precision can also speedup inference because it takes less time to perform calculations with fewer bits.

Transformers supports several quantization schemes to help you run inference with large language models (LLMs) and finetune adapters on quantized models. This guide will show you how to use Activation-aware Weight Quantization (AWQ), AutoGPTQ, and bitsandbytes.

##
AWQ

Try AWQ quantization with this notebook!

Activation-aware Weight Quantization (AWQ) doesn’t quantize all the weights in a model, and instead, it preserves a small percentage of weights that are important for LLM performance. This significantly reduces quantization loss such that you can run models in 4-bit precision without experiencing any performance degradation.

There are several libraries for quantizing models with the AWQ algorithm, such as llm-awq, autoawq or optimum-intel. Transformers supports loading models quantized with the llm-awq and autoawq libraries. This guide will show you how to load models quantized with autoawq, but the processs is similar for llm-awq quantized models.

Make sure you have autoawq installed:

    pip install autoawq

AWQ-quantized models can be identified by checking the `quantization_config` attribute in the model’s config.json file:

    {
      "_name_or_path": "/workspace/process/huggingfaceh4_zephyr-7b-alpha/source",
      "architectures": [
        "MistralForCausalLM"
      ],
      ...
      ...
      ...
      "quantization_config": {
        "quant_method": "awq",
        "zero_point": true,
        "group_size": 128,
        "bits": 4,
        "version": "gemm"
      }
    }

A quantized model is loaded with the from_pretrained() method. If you loaded your model on the CPU, make sure to move it to a GPU device first. Use the `device_map` parameter to specify where to place the model:

    from transformers import AutoModelForCausalLM, AutoTokenizer

    model_id = "TheBloke/zephyr-7B-alpha-AWQ"
    model = AutoModelForCausalLM.from_pretrained(model_id, device_map="cuda:0")

Loading an AWQ-quantized model automatically sets other weights to fp16 by default for performance reasons. If you want to load these other weights in a different format, use the `torch_dtype` parameter:

    from transformers import AutoModelForCausalLM, AutoTokenizer

    model_id = "TheBloke/zephyr-7B-alpha-AWQ"
    model = AutoModelForCausalLM.from_pretrained(model_id, torch_dtype=torch.float32)

AWQ quantization can also be combined with FlashAttention-2 to further accelerate inference:

    from transformers import AutoModelForCausalLM, AutoTokenizer

    model = AutoModelForCausalLM.from_pretrained("TheBloke/zephyr-7B-alpha-AWQ", use_flash_attention_2=True, device_map="cuda:0")

##
AutoGPTQ

Try GPTQ quantization with PEFT in this notebook and learn more about it’s details in this blog post!

The AutoGPTQ library implements the GPTQ algorithm, a post-training quantization technique where each row of the weight matrix is quantized independently to find a version of the weights that minimizes the error. These weights are quantized to int4, but they’re restored to fp16 on the fly during inference. This can save your memory-usage by 4x because the int4 weights are dequantized in a fused kernel rather than a GPU’s global memory, and you can also expect a speedup in inference because using a lower bitwidth takes less time to communicate.

Before you begin, make sure the following libraries are installed:

    pip install auto-gptq
    pip install git+https://github.com/huggingface/optimum.git
    pip install git+https://github.com/huggingface/transformers.git
    pip install --upgrade accelerate

To quantize a model (currently only supported for text models), you need to create a GPTQConfig class and set the number of bits to quantize to, a dataset to calibrate the weights for quantization, and a tokenizer to prepare the dataset.

    from transformers import AutoModelForCausalLM, AutoTokenizer, GPTQConfig

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

You could also pass your own dataset as a list of strings, but it is highly recommended to use the same dataset from the GPTQ paper.

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

Load a model to quantize and pass the `gptq_config` to the from_pretrained() method. Set `device_map="auto"` to automatically offload the model to a CPU to help fit the model in memory, and allow the model modules to be moved between the CPU and GPU for quantization.

    quantized_model = AutoModelForCausalLM.from_pretrained(model_id, device_map="auto", quantization_config=gptq_config)

If you’re running out of memory because a dataset is too large, disk offloading is not supported. If this is the case, try passing the `max_memory` parameter to allocate the amount of memory to use on your device (GPU and CPU):

    quantized_model = AutoModelForCausalLM.from_pretrained(model_id, device_map="auto", max_memory={0: "30GiB", 1: "46GiB", "cpu": "30GiB"}, quantization_config=gptq_config)

Depending on your hardware, it can take some time to quantize a model from scratch. It can take ~5 minutes to quantize the faceboook/opt-350m model on a free-tier Google Colab GPU, but it’ll take ~4 hours to quantize a 175B parameter model on a NVIDIA A100. Before you quantize a model, it is a good idea to check the Hub if a GPTQ-quantized version of the model already exists.

Once your model is quantized, you can push the model and tokenizer to the Hub where it can be easily shared and accessed. Use the push_to_hub() method to save the GPTQConfig:

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

You could also save your quantized model locally with the save_pretrained() method. If the model was quantized with the `device_map` parameter, make sure to move the entire model to a GPU or CPU before saving it. For example, to save the model on a CPU:

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

    # if quantized with device_map set
    quantized_model.to("cpu")
    quantized_model.save_pretrained("opt-125m-gptq")

Reload a quantized model with the from_pretrained() method, and set `device_map="auto"` to automatically distribute the model on all available GPUs to load the model faster without using more memory than needed.

    from transformers import AutoModelForCausalLM

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

###
ExLlama

ExLlama is a Python/C++/CUDA implementation of the Llama model that is designed for faster inference with 4-bit GPTQ weights (check out these benchmarks). The ExLlama kernel is activated by default when you create a GPTQConfig object. To boost inference speed even further, use the ExLlamaV2 kernels by configuring the `exllama_config` parameter:

    import torch
    from transformers import AutoModelForCausalLM, GPTQConfig

    gptq_config = GPTQConfig(bits=4, exllama_config={"version":2})
    model = AutoModelForCausalLM.from_pretrained("{your_username}/opt-125m-gptq", device_map="auto", quantization_config=gptq_config)

Only 4-bit models are supported, and we recommend deactivating the ExLlama kernels if you’re finetuning a quantized model with PEFT.

The ExLlama kernels are only supported when the entire model is on the GPU. If you’re doing inference on a CPU with AutoGPTQ (version > 0.4.2), then you’ll need to disable the ExLlama kernel. This overwrites the attributes related to the ExLlama kernels in the quantization config of the config.json file.

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

##
bitsandbytes

bitsandbytes is the easiest option for quantizing a model to 8 and 4-bit. 8-bit quantization multiplies outliers in fp16 with non-outliers in int8, converts the non-outlier values back to fp16, and then adds them together to return the weights in fp16. This reduces the degradative effect outlier values have on a model’s performance. 4-bit quantization compresses a model even further, and it is commonly used with QLoRA to finetune quantized LLMs.

To use bitsandbytes, make sure you have the following libraries installed:

8-bit
4-bit

    pip install transformers accelerate bitsandbytes>0.37.0

Now you can quantize a model with the `load_in_8bit` or `load_in_4bit` parameters in the from_pretrained() method. This works for any model in any modality, as long as it supports loading with Accelerate and contains `torch.nn.Linear` layers.

8-bit
4-bit

Quantizing a model in 8-bit halves the memory-usage, and for large models, set `device_map="auto"` to efficiently use the GPUs available:

    from transformers import AutoModelForCausalLM

    model_8bit = AutoModelForCausalLM.from_pretrained("bigscience/bloom-1b7", device_map="auto", load_in_8bit=True)

By default, all the other modules such as `torch.nn.LayerNorm` are converted to `torch.float16`. You can change the data type of these modules with the `torch_dtype` parameter if you want:

    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

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 the push_to_hub() method. The quantization config.json file is pushed first, followed by the quantized model weights.

    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")

Training with 8-bit and 4-bit weights are only supported for training extra parameters.

You can check your memory footprint with the `get_memory_footprint` method:

    print(model.get_memory_footprint())

Quantized models can be loaded from the from_pretrained() method without needing to specify the `load_in_8bit` or `load_in_4bit` parameters:

    from transformers import AutoModelForCausalLM, AutoTokenizer

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

###
8-bit

Learn more about the details of 8-bit quantization in this blog post!

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

####
Offloading

8-bit models can offload weights between the CPU and GPU to support fitting very large models into memory. The weights dispatched to the CPU are actually stored in float32, and aren’t converted to 8-bit. For example, to enable offloading for the bigscience/bloom-1b7 model, start by creating a BitsAndBytesConfig:

    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 you’ll dispatch to the CPU:

    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`:

    model_8bit = AutoModelForCausalLM.from_pretrained(
        "bigscience/bloom-1b7",
        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, we recommend experimenting with the `llm_int8_threshold` parameter in BitsAndBytesConfig:

    from transformers import AutoModelForCausalLM, 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,
    )

####
Skip module conversion

For some models, like Jukebox, you don’t need to quantize every module to 8-bit which 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:

    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="auto",
        quantization_config=quantization_config,
    )

####
Finetuning

With the PEFT library, you can finetune large models like flan-t5-large and facebook/opt-6.7b with 8-bit quantization. You don’t need to pass the `device_map` parameter for training because it’ll automatically load your model on a GPU. However, you can still customize the device map with the `device_map` parameter if you want to (`device_map="auto"` should only be used for inference).

###
4-bit

Try 4-bit quantization in this notebook and learn more about it’s details in this blog post.

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

####
Compute data type

To speedup computation, you can change the data type from float32 (the default value) to bf16 using the `bnb_4bit_compute_dtype` parameter in BitsAndBytesConfig:

    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 paper, adapted for weights initialized from a normal distribution. You should use NF4 for training 4-bit base models. This can be configured with the `bnb_4bit_quant_type` parameter in the BitsAndBytesConfig:

    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)

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 is a technique that can save additional memory at no additional performance cost. This feature performs a second quantization of the already quantized weights to save an addition 0.4 bits/parameter. For example, with nested quantization, you can finetune a Llama-13b model on a 16GB NVIDIA T4 GPU with a sequence length of 1024, a batch size of 1, and enabling gradient accumulation with 4 steps.

    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", quantization_config=double_quant_config)

##
Optimum

The Optimum library supports quantization for Intel, Furiosa, ONNX Runtime, GPTQ, and lower-level PyTorch quantization functions. Consider using Optimum for quantization if you’re using specific and optimized hardware like Intel CPUs, Furiosa NPUs or a model accelerator like ONNX Runtime.

##
Benchmarks

To compare the speed, throughput, and latency of each quantization scheme, check the following benchmarks obtained from the optimum-benchmark library. The benchmark was run on a NVIDIA A1000 for the TheBloke/Mistral-7B-v0.1-AWQ and TheBloke/Mistral-7B-v0.1-GPTQ models. These were also tested against the bitsandbytes quantization methods as well as a native fp16 model.

forward peak memory/batch size
generate peak memory/batch size
generate throughput/batch size
forward latency/batch size

The benchmarks indicate AWQ quantization is the fastest for inference, text generation, and has the lowest peak memory for text generation. However, AWQ has the largest forward latency per batch size. For a more detailed discussion about the pros and cons of each quantization method, read the Overview of natively supported quantization schemes in 🤗 Transformers blog post.