# Model Card: BitLinear

## Model Description

**BitLinear** is a PyTorch implementation of ultra-low-precision (1.58-bit) ternary linear layers that can serve as drop-in replacements for `nn.Linear` in neural networks, particularly Transformers. It achieves ~19x memory compression while maintaining high output similarity.

### Model Details

- **Developed by:** BitLinear Contributors
- **Model type:** Quantization / Compression
- **Language:** Python, C++, CUDA
- **License:** MIT
- **Repository:** https://github.com/yourusername/bitlinear

## Intended Use

### Primary Use Cases

- **Edge Deployment:** Deploying large models on memory-constrained devices
- **Production Inference:** Reducing memory footprint for serving large language models
- **Research:** Exploring ultra-low-precision neural networks
- **Cost Optimization:** Reducing cloud infrastructure costs through memory savings

### Out-of-Scope Use Cases

- Training from scratch (requires quantization-aware training)
- Applications requiring exact numerical precision
- Real-time applications where Python overhead is prohibitive (use C++/CUDA extensions)

## How to Use

### Basic Usage

```python
import torch
from bitlinear import BitLinear

# Create a BitLinear layer (same interface as nn.Linear)
layer = BitLinear(in_features=512, out_features=1024, bias=True)

# Forward pass
x = torch.randn(32, 128, 512)
output = layer(x)  # Same as nn.Linear
```

### Converting Existing Models

```python
import torch.nn as nn
from bitlinear import convert_linear_to_bitlinear

# Convert a pre-trained model
model = nn.TransformerEncoderLayer(d_model=512, nhead=8)
model_compressed = convert_linear_to_bitlinear(model, inplace=False)

# Use as normal
x = torch.randn(10, 32, 512)
output = model_compressed(x)
```

### Multi-Ternary for Better Accuracy

```python
from bitlinear import MultiTernaryLinear

# Use k=3 components for 75% error reduction
layer = MultiTernaryLinear(in_features=512, out_features=1024, k=3)
```

## Performance

### Memory Compression

- **Average Compression:** 19.23x (95% of theoretical 20x)
- **GPT-2 Small Example:** 324 MB → 16.8 MB (307 MB saved)

| Layer Size | nn.Linear | BitLinear (Packed) | Compression |
|------------|-----------|-------------------|-------------|
| 512×512    | 1.00 MB   | 0.05 MB          | 18.6x       |
| 1024×1024  | 4.00 MB   | 0.21 MB          | 19.3x       |
| 4096×4096  | 64.02 MB  | 3.23 MB          | 19.8x       |

### Accuracy

- **Cosine Similarity:** > 0.96 (96%+)
- **Relative Error:** ~0.28 (28%)
- **Multi-Ternary (k=3):** 75% error reduction vs k=1

## Limitations

### Known Limitations

1. **Accuracy Trade-off:** Ternary quantization introduces approximation error (~3-5% typical)
2. **Training:** Requires quantization-aware training (QAT) for optimal results
3. **Speed:** Python implementation may be slower than nn.Linear (use C++/CUDA for production)
4. **Activation Quantization:** Currently only weights are quantized (full BitNet includes activation quantization)

### Recommendations

- Fine-tune converted models for best accuracy
- Use k≥2 for MultiTernaryLinear when accuracy is critical
- Profile performance on your specific hardware
- Test accuracy on your specific task before deployment

## Training

### Quantization-Aware Training (QAT)

For best results, fine-tune models with BitLinear layers:

```python
# Convert pre-trained model
model_bit = convert_linear_to_bitlinear(pretrained_model)

# Fine-tune with standard training loop
optimizer = torch.optim.AdamW(model_bit.parameters(), lr=1e-4)
# ... train as normal ...
```

### From Scratch Training

Training from scratch with ternary weights requires:
- Careful initialization
- Straight-through estimators for gradients
- Potentially modified learning rates

See `read/IMPLEMENTATION_GUIDE.md` for details.

## Technical Specifications

### Architecture

- **Weight Quantization:** Ternary {-1, 0, +1}
- **Scaling:** Per-output-channel absmax scaling
- **Packing:** Base-3 encoding (5 values per byte)
- **Decomposition:** Greedy residual quantization for multi-ternary

### Implementation

- **Python:** Pure PyTorch baseline
- **C++:** Optimized CPU kernels with PyBind11
- **CUDA:** GPU kernels with warp-level reductions and shared memory tiling

### Requirements

- Python ≥ 3.8
- PyTorch ≥ 2.0.0
- NumPy ≥ 1.20.0
- C++ compiler (for C++ extensions)
- CUDA toolkit (optional, for GPU support)

## Evaluation

### Benchmarks

Comprehensive benchmarks available in `BENCHMARKS.md`:
- Memory compression analysis
- Forward pass timing
- Accuracy metrics
- Real-world transformer examples

### Validation

All implementations validated against:
- Unit tests (pytest suite)
- Numerical correctness tests
- Integration tests with Transformers
- Cross-implementation consistency (Python vs C++)

## Citation

If you use BitLinear in your research, please cite:

```bibtex
@article{jmlr_ternary_2024,
  title={Ternary Representations of Neural Networks},
  journal={Journal of Machine Learning Research},
  volume={26},
  year={2024},
  url={https://jmlr.org/papers/volume26/24-2050/24-2050.pdf}
}

@article{bitnet2023,
  title={BitNet: Scaling 1-bit Transformers for Large Language Models},
  author={Wang, Hongyu and Ma, Shuming and Dong, Li and Huang, Shaohan and Wang, Huaijie and Ma, Lingxiao and Yang, Fan and Wang, Ruiping and Wu, Yi and Wei, Furu},
  journal={arXiv preprint arXiv:2310.11453},
  year={2023}
}
```

## Model Card Contact

For questions or issues, please open an issue on GitHub or contact the maintainers.

## Glossary

- **Ternary Quantization:** Representing weights with only three values {-1, 0, +1}
- **Absmax Scaling:** Scaling factor computed as max(abs(weights))
- **Base-3 Packing:** Encoding ternary values in base-3 for memory efficiency
- **Multi-Ternary:** Sum of k ternary components for improved approximation
- **QAT:** Quantization-Aware Training - training with quantization in the loop

## More Information

- **Documentation:** See `README.md` and `read/` directory
- **Examples:** See `examples/` directory
- **Benchmarks:** See `BENCHMARKS.md`
- **Implementation Guide:** See `read/IMPLEMENTATION_GUIDE.md`