GPTQ (Generative Pre-trained Transformer Quantization)

Post-training quantization method that compresses LLMs to 4-bit with minimal accuracy loss using group-wise quantization.

When to use GPTQ

Use GPTQ when:

• Need to fit large models (70B+) on limited GPU memory
• Want 4× memory reduction with <2% accuracy loss
• Deploying on consumer GPUs (RTX 4090, 3090)
• Need faster inference (3-4× speedup vs FP16)

Use AWQ instead when:

• Need slightly better accuracy (<1% loss)
• Have newer GPUs (Ampere, Ada)
• Want Marlin kernel support (2× faster on some GPUs)

Use bitsandbytes instead when:

• Need simple integration with transformers
• Want 8-bit quantization (less compression, better quality)
• Don't need pre-quantized model files

Quick start

Installation

# Install AutoGPTQ
pip install auto-gptq

# With Triton (Linux only, faster)
pip install auto-gptq[triton]

# With CUDA extensions (faster)
pip install auto-gptq --no-build-isolation

# Full installation
pip install auto-gptq transformers accelerate

Load pre-quantized model

from transformers import AutoTokenizer
from auto_gptq import AutoGPTQForCausalLM

# Load quantized model from HuggingFace
model_name = "TheBloke/Llama-2-7B-Chat-GPTQ"

model = AutoGPTQForCausalLM.from_quantized(
    model_name,
    device="cuda:0",
    use_triton=False  # Set True on Linux for speed
)

tokenizer = AutoTokenizer.from_pretrained(model_name)

# Generate
prompt = "Explain quantum computing"
inputs = tokenizer(prompt, return_tensors="pt").to("cuda:0")
outputs = model.generate(**inputs, max_new_tokens=200)
print(tokenizer.decode(outputs[0]))

Quantize your own model

from transformers import AutoTokenizer
from auto_gptq import AutoGPTQForCausalLM, BaseQuantizeConfig
from datasets import load_dataset

# Load model
model_name = "meta-llama/Llama-2-7b-chat-hf"
tokenizer = AutoTokenizer.from_pretrained(model_name)

# Quantization config
quantize_config = BaseQuantizeConfig(
    bits=4,              # 4-bit quantization
    group_size=128,      # Group size (recommended: 128)
    desc_act=False,      # Activation order (False for CUDA kernel)
    damp_percent=0.01    # Dampening factor
)

# Load model for quantization
model = AutoGPTQForCausalLM.from_pretrained(
    model_name,
    quantize_config=quantize_config
)

# Prepare calibration data
dataset = load_dataset("c4", split="train", streaming=True)
calibration_data = [
    tokenizer(example["text"])["input_ids"][:512]
    for example in dataset.take(128)
]

# Quantize
model.quantize(calibration_data)

# Save quantized model
model.save_quantized("llama-2-7b-gptq")
tokenizer.save_pretrained("llama-2-7b-gptq")

# Push to HuggingFace
model.push_to_hub("username/llama-2-7b-gptq")

Group-wise quantization

How GPTQ works:

1. Group weights: Divide each weight matrix into groups (typically 128 elements)
2. Quantize per-group: Each group has its own scale/zero-point
3. Minimize error: Uses Hessian information to minimize quantization error
4. Result: 4-bit weights with near-FP16 accuracy

Group size trade-off:

Example:

Weight matrix: [1024, 4096] = 4.2M elements

Group size = 128:
- Groups: 4.2M / 128 = 32,768 groups
- Each group: own 4-bit scale + zero-point
- Result: Better granularity → better accuracy

Quantization configurations

Standard 4-bit (recommended)

from auto_gptq import BaseQuantizeConfig

config = BaseQuantizeConfig(
    bits=4,              # 4-bit quantization
    group_size=128,      # Standard group size
    desc_act=False,      # Faster CUDA kernel
    damp_percent=0.01    # Dampening factor
)

Performance:

• Memory: 4× reduction (70B model: 140GB → 35GB)
• Accuracy: ~1.5% perplexity increase
• Speed: 3-4× faster than FP16

High accuracy (3-bit with larger groups)

config = BaseQuantizeConfig(
    bits=3,              # 3-bit (more compression)
    group_size=128,      # Keep standard group size
    desc_act=True,       # Better accuracy (slower)
    damp_percent=0.01
)

Trade-off:

• Memory: 5× reduction
• Accuracy: ~3% perplexity increase
• Speed: 5× faster (but less accurate)

Maximum accuracy (4-bit with small groups)

config = BaseQuantizeConfig(
    bits=4,
    group_size=32,       # Smaller groups (better accuracy)
    desc_act=True,       # Activation reordering
    damp_percent=0.005   # Lower dampening
)

Trade-off:

• Memory: 3.5× reduction (slightly larger)
• Accuracy: ~0.8% perplexity increase (best)
• Speed: 2-3× faster (kernel overhead)

Kernel backends

ExLlamaV2 (default, fastest)

model = AutoGPTQForCausalLM.from_quantized(
    model_name,
    device="cuda:0",
    use_exllama=True,      # Use ExLlamaV2
    exllama_config={"version": 2}
)

Performance: 1.5-2× faster than Triton

Marlin (Ampere+ GPUs)

# Quantize with Marlin format
config = BaseQuantizeConfig(
    bits=4,
    group_size=128,
    desc_act=False  # Required for Marlin
)

model.quantize(calibration_data, use_marlin=True)

# Load with Marlin
model = AutoGPTQForCausalLM.from_quantized(
    model_name,
    device="cuda:0",
    use_marlin=True  # 2× faster on A100/H100
)

Requirements:

• NVIDIA Ampere or newer (A100, H100, RTX 40xx)
• Compute capability ≥ 8.0

Triton (Linux only)

model = AutoGPTQForCausalLM.from_quantized(
    model_name,
    device="cuda:0",
    use_triton=True  # Linux only
)

Performance: 1.2-1.5× faster than CUDA backend

Integration with transformers

Direct transformers usage

from transformers import AutoModelForCausalLM, AutoTokenizer

# Load quantized model (transformers auto-detects GPTQ)
model = AutoModelForCausalLM.from_pretrained(
    "TheBloke/Llama-2-13B-Chat-GPTQ",
    device_map="auto",
    trust_remote_code=False
)

tokenizer = AutoTokenizer.from_pretrained("TheBloke/Llama-2-13B-Chat-GPTQ")

# Use like any transformers model
inputs = tokenizer("Hello", return_tensors="pt").to("cuda")
outputs = model.generate(**inputs, max_new_tokens=100)

QLoRA fine-tuning (GPTQ + LoRA)

from transformers import AutoModelForCausalLM
from peft import prepare_model_for_kbit_training, LoraConfig, get_peft_model

# Load GPTQ model
model = AutoModelForCausalLM.from_pretrained(
    "TheBloke/Llama-2-7B-GPTQ",
    device_map="auto"
)

# Prepare for LoRA training
model = prepare_model_for_kbit_training(model)

# LoRA config
lora_config = LoraConfig(
    r=16,
    lora_alpha=32,
    target_modules=["q_proj", "v_proj"],
    lora_dropout=0.05,
    bias="none",
    task_type="CAUSAL_LM"
)

# Add LoRA adapters
model = get_peft_model(model, lora_config)

# Fine-tune (memory efficient!)
# 70B model trainable on single A100 80GB

Performance benchmarks

Memory reduction

Enables:

• 70B on single A100 80GB (vs 2× A100 needed for FP16)
• 405B on 3× A100 80GB (vs 11× A100 needed for FP16)
• 13B on RTX 4090 24GB (vs OOM with FP16)

Inference speed (Llama 2-7B, A100)

Accuracy (perplexity on WikiText-2)

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