Quantize an LLM to INT4

You can improve vLLM performance using 4‑bit quantization. In this section, you’ll quantize the deepseek-ai/DeepSeek-V2-Lite model to 4-bit integer (INT4) weights.

Install quantization tools

Install the quantization dependencies used by vLLM and the llmcompressor toolkit:

    

        
        
pip install --no-deps compressed-tensors
pip install llmcompressor

    

• compressed-tensors provides the underlying tensor storage and compression utilities used for quantized model formats.
• llmcompressor includes quantization, pruning, and weight clustering utilities compatible with Hugging Face Transformers and vLLM runtime formats.

If you recently rebuilt vLLM, reinstall your locally built wheel to ensure compatibility with the quantization extensions:

    

        
        
pip install --no-deps dist/*.whl

    

Authenticate with Hugging Face (if required):

If the model you plan to quantize is gated on Hugging Face (e.g., DeepSeek or proprietary models), log in to authenticate your credentials before downloading model weights:

    

        
        
huggingface-cli login

    

Apply the INT4 quantization recipe

Using a file editor of your choice, save the following code into a file named quantize_vllm_models.py:

    

        
        
import argparse
import os
import torch
from transformers import AutoModelForCausalLM, AutoTokenizer
 
from compressed_tensors.quantization import QuantizationScheme
from compressed_tensors.quantization.quant_args import (
    QuantizationArgs,
    QuantizationStrategy,
    QuantizationType,
)
 
from llmcompressor import oneshot
from llmcompressor.modifiers.quantization.quantization import QuantizationModifier
 
 
def main():
    parser = argparse.ArgumentParser(description="Quantize a model using INT4 with minmax or mse and dynamic activation quantization.")
    parser.add_argument("model_id", type=str, help="Model identifier or path")
    parser.add_argument("--method", type=str, choices=["minmax", "mse"], default="mse", help="Quantization method")
    parser.add_argument("--scheme", type=str, choices=["channelwise", "groupwise"], required=True, help="Quantization scheme for weights")
    parser.add_argument("--groupsize", type=int, default=32, help="Group size for groupwise quantization")
    args = parser.parse_args()
 
    # Extract base model name for output dir
    base_model_name = os.path.basename(args.model_id.rstrip("/"))
    act_tag = "a8dyn"
    suffix = f"{args.method}-{args.scheme}"
    if args.scheme == "groupwise":
        suffix += f"-g{args.groupsize}"
    output_dir = f"{base_model_name}-w4{act_tag}-{suffix}"
 
    print(f"Loading model '{args.model_id}'...")
    model = AutoModelForCausalLM.from_pretrained(
        args.model_id, trust_remote_code=True
    )
    model = model.to(torch.float32)
    tokenizer = AutoTokenizer.from_pretrained(args.model_id)
 
    # Weight quantization args
    strategy = QuantizationStrategy.CHANNEL if args.scheme == "channelwise" else QuantizationStrategy.GROUP
    weights_args = QuantizationArgs(
        num_bits=4,
        type=QuantizationType.INT,
        strategy=strategy,
        symmetric=True,
        dynamic=False,
        group_size=args.groupsize if args.scheme == "groupwise" else None,
        observer=args.method,
    )
 
    # Activation quantization
    input_acts = QuantizationArgs(
        num_bits=8,
        type=QuantizationType.INT,
        strategy=QuantizationStrategy.TOKEN,
        symmetric=False,
        dynamic=True,
        observer=None,
    )
    output_acts = None
 
    # Create quantization scheme and recipe
    scheme = QuantizationScheme(
        targets=["Linear"],
        weights=weights_args,
        input_activations=input_acts,
        output_activations=output_acts,
    )
    recipe = QuantizationModifier(
        config_groups={"group_0": scheme},
        ignore=["lm_head"],
    )
 
    # Run quantization
    oneshot(
        model=model,
        recipe=recipe,
        tokenizer=tokenizer,
        output_dir=output_dir,
        trust_remote_code_model=True,
    )
 
    print(f"Quantized model saved to: {output_dir}")
 
 
if __name__ == "__main__":
    main()

    

This script creates an INT4 quantized copy of the vLLM model and saves it to a new directory.

Quantize DeepSeek‑V2‑Lite model

Quantizing your model to INT4 format significantly reduces memory usage and improves inference speed on Arm CPUs. In this section, you’ll apply the quantization script to the DeepSeek‑V2‑Lite model, tuning key parameters for optimal performance and accuracy. This process prepares your model for efficient deployment with vLLM on Arm-based servers.

Tune quantization parameters

Quantization parameters control how the model’s floating-point weights and activations are converted to lower-precision integer formats. The right settings help you balance accuracy, memory usage, and performance on Arm CPUs.

• Use minmax for faster quantization, or mse for higher accuracy (but slower)
• Choose channelwise for most models; it’s a reliable default
• Try groupwise for potentially better accuracy; --groupsize 32 is a common choice

Pick the combination that fits your accuracy and speed needs.

Execute the following command to quantize the DeepSeek-V2-Lite model:

    

        
        
# DeepSeek example
python3 quantize_vllm_models.py deepseek-ai/DeepSeek-V2-Lite \
  --scheme channelwise --method mse

    

This will generate an INT4 quantized model directory such as:

    

        
        
DeepSeek-V2-Lite-w4a8dyn-mse-channelwise

    

You will load this quantized model directory with vLLM in the next step.