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fp8 Weight and Activation Quantization

llmcompressor supports quantizing weights and activations to fp8 for memory savings and inference acceleration with vllm

fp8 compuation is supported on Nvidia GPUs with compute capability > 8.9 (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-FP8-Dynamic is ready to be loaded into vLLM.

Code Walkthough

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

  1. Load model
  2. Apply quantization
  3. Evaluate accuracy in vLLM

1) Load Model

Load the model using AutoModelForCausalLM

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) Apply Quantization

For fp8 quantization, we can recover accuracy with simple PTQ quantization.

We recommend targeting all Linear layers using the FP8_DYNAMIC scheme, which uses:

  • Static, per-channel quantization on the weights
  • Dynamic, per-token quantization on the activations

Since simple PTQ does not require data for weight quantization and the activations are quantized dynamically, we do not need any calibration data for this quantization flow.

from llmcompressor.transformers import oneshot
from llmcompressor.modifiers.quantization import QuantizationModifier

# Configure the simple PTQ quantization
recipe = QuantizationModifier(
  targets="Linear", scheme="FP8_DYNAMIC", ignore=["lm_head"])

# Apply the quantization algorithm.
oneshot(model=model, recipe=recipe)

# Save the model.
SAVE_DIR = MODEL_ID.split("/")[1] + "-FP8-Dynamic"
model.save_pretrained(SAVE_DIR)
tokenizer.save_pretrained(SAVE_DIR)

We have successfully created an fp8 model!

3) Evaluate Accuracy

Install vllm and lm-evaluation-harness:

pip install vllm lm_eval==0.4.3

Load and run the model in vllm:

from vllm import LLM
model = LLM("./Meta-Llama-3-8B-Instruct-FP8-Dynamic")
model.generate("Hello my name is")

Evaluate accuracy with lm_eval (for example on 250 samples of gsm8k):

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.

MODEL=$PWD/Meta-Llama-3-8B-Instruct-FP8-Dynamic 
lm_eval \
  --model vllm \
  --model_args pretrained=$MODEL,add_bos_token=True \
  --tasks gsm8k  --num_fewshot 5 --batch_size auto --limit 250

We can see the resulting scores look good:

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.768|±  |0.0268|
|     |       |strict-match    |     5|exact_match|↑  |0.768|±  |0.0268|

Questions or Feature Request?

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