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int8 Weight and Activation Quantization

llm-compressor supports quantizing weights and activations to int for memory savings and inference acceleration with vLLM

int8 compuation is supported on Nvidia GPUs with compute capability > 7.5 (Turing, Ampere, Ada Lovelace, Hopper).

Installation

To get started, install:

pip install llmcompressor

Quickstart

The example includes an end-to-end script for applying the quantization algorithm.

python3 llama3_example.py

The resulting model Meta-Llama-3-8B-Instruct-W8A8-Dynamic-Per-Token is ready to be loaded into vLLM.

Code Walkthough

Now, we will step though the code in the example. There are four steps:

  1. Load model
  2. Prepare calibration data
  3. Apply quantization
  4. Evaluate accuracy in vLLM

1) Load Model

Load the model using AutoModelForCausalLM for handling quantized saving and loading.

from transformers import AutoTokenizer, AutoModelForCausalLM

MODEL_ID = "meta-llama/Meta-Llama-3-8B-Instruct"
model = AutoModelForCausalLM.from_pretrained(
    MODEL_ID, device_map="auto", torch_dtype="auto",
)
tokenizer = AutoTokenizer.from_pretrained(MODEL_ID)

2) Prepare Calibration Data

Prepare the calibration data. When quantizing activations of a model to int8, we need some sample data to estimate the activation scales. As a result, it is very useful to use calibration data that closely matches the type of data used in deployment. If you have fine-tuned a model, using a sample of your training data is a good idea.

In our case, we are quantizing an Instruction tuned generic model, so we will use the ultrachat dataset. Some best practices include:

  • 512 samples is a good place to start (increase if accuracy drops)
  • 2048 sequence length is a good place to start
  • Use the chat template or instrucion template that the model is trained with
from datasets import load_dataset

NUM_CALIBRATION_SAMPLES=512
MAX_SEQUENCE_LENGTH=2048

# Load dataset.
ds = load_dataset("HuggingFaceH4/ultrachat_200k", split="train_sft")
ds = ds.shuffle(seed=42).select(range(NUM_CALIBRATION_SAMPLES))

# Preprocess the data into the format the model is trained with.
def preprocess(example):
    return {"text": tokenizer.apply_chat_template(example["messages"], tokenize=False,)}
ds = ds.map(preprocess)

# Tokenize the data (be careful with bos tokens - we need add_special_tokens=False since the chat_template already added it).
def tokenize(sample):
    return tokenizer(sample["text"], padding=False, max_length=MAX_SEQUENCE_LENGTH, truncation=True, add_special_tokens=False)
ds = ds.map(tokenize, remove_columns=ds.column_names)

3) Apply Quantization

With the dataset ready, we will now apply quantization.

We first select the quantization algorithm. For W8A8, we want to:

  • Run SmoothQuant to make the activations easier to quantize
  • Quantize the weights to 8 bits with channelwise scales using GPTQ
  • Quantize the activations with dynamic per token strategy

See the Recipes documentation for more information on recipes

from llmcompressor.transformers import oneshot
from llmcompressor.modifiers.quantization import GPTQModifier
from llmcompressor.modifiers.smoothquant import SmoothQuantModifier

# Configure the quantization algorithms to run.
recipe = [
    SmoothQuantModifier(smoothing_strength=0.8),
    GPTQModifier(targets="Linear", scheme="W8A8", ignore=["lm_head"]),
]

# Apply quantization.
oneshot(
    model=model,
    dataset=ds,
    recipe=recipe,
    max_seq_length=MAX_SEQUENCE_LENGTH,
    num_calibration_samples=NUM_CALIBRATION_SAMPLES,
)

# Save to disk compressed.
SAVE_DIR = MODEL_ID.split("/")[1] + "-W8A8-Dynamic-Per-Token"
model.save_pretrained(SAVE_DIR, save_compressed=True)
tokenizer.save_pretrained(SAVE_DIR)

We have successfully created an w8a8 model with weights and activations quantized to 8-bit integers!

4) Evaluate Accuracy

With the model created, we can now load and run in vLLM (after installing).

from vllm import LLM
model = LLM("./Meta-Llama-3-8B-Instruct-W8A8-Dynamic-Per-Token")

We can evaluate accuracy with lm_eval (pip install lm_eval==v0.4.3):

Note: quantized models can be sensitive to the presence of the bos token. lm_eval does not add a bos token by default, so make sure to include the add_bos_token=True argument when running your evaluations.

Run the following to test accuracy on GSM-8K:

lm_eval --model vllm \
  --model_args pretrained="./Meta-Llama-3-8B-Instruct-W8A8-Dynamic-Per-Token",add_bos_token=true \
  --tasks gsm8k \
  --num_fewshot 5 \
  --limit 250 \
  --batch_size 'auto'

We can see the resulting scores look good!

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.752|±  |0.0274|
|     |       |strict-match    |     5|exact_match|↑  |0.756|±  |0.0272|

Questions or Feature Request?

Please open up an issue on vllm-project/llm-compressor