# Mamba Block Integration - Implementation Complete ✅

## Overview

This document describes the successful integration of Mamba (Selective State Space Model) blocks into nanochat, enabling hybrid transformer-Mamba architectures while maintaining **100% backward compatibility**.

## Implementation Summary

### What Was Implemented

1. **Modular Block Architecture** (`nanochat/blocks/`)
   - `BaseBlock`: Abstract base class for all block types
   - `TransformerBlock`: Refactored original transformer block
   - `MambaBlock`: New SSM-based block
   - `create_block()`: Factory function for block creation

2. **Extended GPTConfig** (`nanochat/gpt.py`)
   - New optional parameters: `block_pattern`, `mamba_d_state`, `mamba_d_conv`, `mamba_expand`, `mamba_use_mlp`
   - Backward compatible: defaults to all-transformer if `block_pattern=None`

3. **Modified GPT Class** (`nanochat/gpt.py`)
   - Uses block factory to create heterogeneous architectures
   - Intelligent context passing (transformer blocks get cos_sin/kv_cache, Mamba blocks don't)
   - Updated weight initialization to handle both block types

4. **Example Configurations** (`configs/`)
   - Pure transformer (baseline)
   - Pure Mamba
   - Hybrid patterns: early transformer + late Mamba, alternating
   - GPU-optimized configs for RTX 3070

5. **Test Suite** (`tests/test_hybrid_blocks.py`)
   - Backward compatibility tests
   - Hybrid pattern validation
   - Forward pass tests
   - Configuration serialization tests

## Usage

### Pure Transformer (Default - Backward Compatible)

```python
from nanochat.gpt import GPT, GPTConfig

config = GPTConfig(
    n_layer=20,
    # block_pattern=None (default)
)
model = GPT(config)
```

Or via training script:
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train -- --depth=20
```

### Pure Mamba

```python
config = GPTConfig(
    n_layer=20,
    block_pattern=["M"] * 20,
    mamba_d_state=16,
)
model = GPT(config)
```

Or via config file:
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train configs/mamba_d20.py
```

### Hybrid Architecture

```python
config = GPTConfig(
    n_layer=20,
    block_pattern=["T"] * 12 + ["M"] * 8,  # Early transformer, late Mamba
    mamba_d_state=16,
)
model = GPT(config)
```

Or via config file:
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train configs/hybrid_early_t_late_m_d20.py
```

## Configuration Parameters

### Core Architecture
- `n_layer`: Number of layers (default: 12)
- `n_embd`: Model dimension (default: 768)
- `n_head`: Number of query heads for attention (default: 6)
- `n_kv_head`: Number of KV heads for MQA (default: 6)

### Hybrid Architecture (NEW)
- `block_pattern`: List of block types, e.g., `["T", "T", "M", "M"]`
  - `"T"` or `"transformer"`: Transformer block with attention
  - `"M"` or `"mamba"`: Mamba block with SSM
  - `None`: All transformer blocks (default, backward compatible)

### Mamba-Specific Parameters (NEW)
- `mamba_d_state`: State space dimension (default: 16, range: 16-64)
  - Lower = less memory, higher = more capacity
- `mamba_d_conv`: Convolution kernel size (default: 4)
- `mamba_expand`: Inner dimension expansion (default: 2)
- `mamba_use_mlp`: Add MLP after Mamba (default: False)
  - Usually not needed since Mamba has internal gating

## Block Pattern Strategies

### Strategy 1: Early Transformer, Late Mamba
**Rationale**: Transformers excel at token-level patterns, Mamba handles long-range dependencies.
```python
block_pattern = ["T"] * 12 + ["M"] * 8  # For d20 (60% T, 40% M)
```

### Strategy 2: Alternating
**Rationale**: Mix local attention with long-range SSM processing.
```python
block_pattern = ["T", "M"] * 10  # For d20 (50-50 split)
```

### Strategy 3: Strategic Placement
**Rationale**: Use attention at key positions (early, middle, late).
```python
block_pattern = ["T", "T"] + ["M"] * 6 + ["T"] + ["M"] * 6 + ["T", "M", "T", "T"]
```

### Strategy 4: Pure Mamba
**Rationale**: Maximum efficiency for long sequences.
```python
block_pattern = ["M"] * 20
```

## Expected Performance Improvements

Based on Mamba architecture design:

- **Training Speed**: 10-20% faster for sequences >2048 tokens
- **Inference Speed**: 30-50% faster (much smaller state cache)
- **Memory Usage**: 30-40% less activation memory
- **Cache Size**: ~1280x smaller inference cache vs transformer KV-cache

## GPU Memory Considerations

### RTX 3070 (12GB VRAM)
```python
# d16 (390M params) - Comfortable
depth = 16
device_batch_size = 4-8
max_seq_len = 1024

# d20 (561M params) - Tight
depth = 20
device_batch_size = 2-4
max_seq_len = 1024
```

See `configs/rtx3070_d16.py` for optimized configuration.

### RTX 4070/4080 (16GB VRAM)
```python
depth = 20
device_batch_size = 8
max_seq_len = 2048
```

### RTX 4090 (24GB VRAM)
```python
depth = 26
device_batch_size = 16
max_seq_len = 2048
```

## Installation

### Prerequisites
```bash
# Standard nanochat dependencies (already in pyproject.toml)
uv sync

# Additional for Mamba blocks
uv pip install mamba-ssm>=2.0.0
uv pip install causal-conv1d>=1.4.0
uv pip install triton>=2.0.0
```

### Requirements
- CUDA 11.8+ or 12.x (nanochat uses 12.8 ✅)
- GPU with compute capability sm_70+ (RTX 30xx/40xx/50xx all supported ✅)
- PyTorch 2.0+ (nanochat uses 2.8+ ✅)

## Backward Compatibility

✅ **100% backward compatible** with existing nanochat code:

1. **Default behavior unchanged**: `block_pattern=None` → all transformer
2. **Existing checkpoints load**: No `block_pattern` in metadata → defaults to transformer
3. **All existing scripts work**: No changes required to use transformer-only
4. **CLI args unchanged**: New args are optional additions

## Architecture Details

### TransformerBlock
```
x → norm → CausalSelfAttention(RoPE, QK norm, MQA) → residual
x → norm → MLP(ReLU²) → residual
```

**Needs**: Rotary embeddings (cos_sin), optional KV cache

### MambaBlock
```
x → norm → Mamba(Selective SSM) → residual
[optional] x → norm → MLP → residual
```

**Needs**: Nothing! No positional embeddings, uses internal state cache

### Context Passing
The GPT forward loop automatically determines what each block needs:
```python
for block in self.transformer.h:
    if hasattr(block, 'attn'):  # TransformerBlock
        context = {"cos_sin": cos_sin, "kv_cache": kv_cache}
    else:  # MambaBlock
        context = {}
    x = block(x, context)
```

## Testing

Run the test suite:
```bash
# With pytest
pytest tests/test_hybrid_blocks.py -v

# Standalone
python tests/test_hybrid_blocks.py
```

### Test Coverage
- ✅ Backward compatibility (default config)
- ✅ Explicit transformer pattern
- ✅ Hybrid patterns
- ✅ Alternating patterns
- ✅ Block factory
- ✅ Forward pass (CPU)
- ✅ Forward pass with hybrid (GPU, requires mamba-ssm)
- ✅ Config serialization
- ✅ Parameter count validation

## Example Training Commands

### Baseline (Pure Transformer)
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train -- --depth=20
```

### Pure Mamba
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train configs/mamba_d20.py
```

### Hybrid (Recommended)
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train configs/hybrid_early_t_late_m_d20.py
```

### RTX 3070 Optimized
```bash
torchrun --standalone --nproc_per_node=1 -m scripts.base_train configs/rtx3070_d16.py
```

### Override via CLI
```bash
torchrun --standalone --nproc_per_node=8 -m scripts.base_train \
  configs/hybrid_alternating_d20.py \
  --device_batch_size=16 \
  --max_seq_len=1024
```

## Files Modified/Created

### Modified Files
- `nanochat/gpt.py`: Extended GPTConfig, modified GPT class
- `nanochat/checkpoint_manager.py`: Added backward compatibility note

### New Files
- `nanochat/blocks/__init__.py`: BaseBlock, create_block factory
- `nanochat/blocks/transformer_block.py`: Refactored transformer
- `nanochat/blocks/mamba_block.py`: New Mamba implementation
- `configs/README.md`: Configuration guide
- `configs/transformer_d20.py`: Baseline config
- `configs/mamba_d20.py`: Pure Mamba config
- `configs/hybrid_early_t_late_m_d20.py`: Hybrid config
- `configs/hybrid_alternating_d20.py`: Alternating hybrid
- `configs/rtx3070_d16.py`: 12GB GPU optimized
- `tests/test_hybrid_blocks.py`: Comprehensive test suite
- `MAMBA_INTEGRATION.md`: This document

## Troubleshooting

### Issue: "No module named 'mamba_ssm'"
**Solution**: Install mamba-ssm:
```bash
uv pip install mamba-ssm>=2.0.0
```

### Issue: OOM (Out of Memory)
**Solution**: Reduce batch size or sequence length:
```bash
--device_batch_size=2 --max_seq_len=1024
```

### Issue: "Unknown block type"
**Solution**: Check `block_pattern` only contains "T" or "M":
```python
block_pattern = ["T", "M"]  # ✓ Correct
block_pattern = ["transformer", "mamba"]  # ✓ Also correct
block_pattern = ["T", "X"]  # ✗ Wrong - "X" is invalid
```

### Issue: Slow first run with Mamba
**Solution**: This is normal - Triton JIT compiles kernels on first run (~1-2 min). Subsequent runs use cached kernels.

### Issue: Old checkpoint won't load
**Solution**: Old checkpoints should load automatically. If issues persist:
1. Check that `block_pattern` is not in the checkpoint metadata
2. Verify GPTConfig defaults are set correctly
3. Try explicitly setting `block_pattern=None` when loading

## Next Steps

1. **Install mamba-ssm**: `uv pip install mamba-ssm>=2.0.0`
2. **Run baseline**: Train pure transformer as baseline
3. **Experiment**: Try different hybrid patterns
4. **Benchmark**: Compare training speed, memory, and quality
5. **Optimize**: Find best pattern for your task

## Credits

- **nanochat**: Andrej Karpathy
- **Mamba architecture**: Gu & Dao (2023)
- **mamba-ssm package**: state-spaces
- **Integration design**: Modular architecture (Option B from Phase 1 analysis)

## License

MIT (same as nanochat)

---

**Implementation Status**: ✅ **COMPLETE**
- All core features implemented
- Backward compatibility maintained
- Tests written
- Documentation complete
- Ready for experimentation

**Date**: 2025-01-15