# Performance Evolution: July 29 → July 31 → August 4, 2025

## Timeline Summary
- **July 29**: Initial benchmarks before arena implementation
- **July 31**: First arena implementation completed
- **August 4**: After recursive evaluator removal and Expression API improvements

## Performance Comparison Table

### Time Per Iteration (7 expressions)
| Approach | July 29 | July 31 | August 4 | Total Improvement |
|----------|----------|----------|-----------|-------------------|
| Individual evaluation | 340.9 µs | 262.9 µs | 329.0 µs | +3.5% (slightly slower) |
| BatchBuilder/Arena | 142.7 µs | 86.8 µs | 118.3 µs | +17.1% faster |

### Time Per Expression
| Approach | July 29 | July 31 | August 4 | Total Improvement |
|----------|----------|----------|-----------|-------------------|
| Individual | 48.7 µs | 37.6 µs | 47.0 µs | +3.5% faster |
| BatchBuilder/Arena | 20.4 µs | 12.4 µs | 16.9 µs | +17.2% faster |

### Maximum Throughput (expressions/second)
| Approach | July 29 | July 31 | August 4 | Total Improvement |
|----------|----------|----------|-----------|-------------------|
| Individual | 20,533 | 26,628 | 21,275 | +3.6% |
| BatchBuilder/Arena | 49,070 | 80,682 | 59,164 | +20.6% |

## Analysis

### What Happened Between July 31 → August 4?

1. **Major Architectural Changes**:
   - Removed recursive evaluator (`eval_ast_inner`)
   - Migrated everything to iterative evaluator
   - Added Expression API as primary interface
   - Fixed expression function evaluation to use arena allocation

2. **Performance Regression Explanation**:
   - The regression from July 31 to August 4 appears to be due to:
     - Additional overhead from Expression API abstraction
     - More complex expression function handling through iterative path
     - Possible additional validation/safety checks

3. **Still Better Than Original**:
   - BatchBuilder/Arena is still 17.1% faster than original (July 29)
   - Individual evaluation returned close to original performance
   - Memory benefits (zero allocations) are preserved

## Memory Comparison (Unchanged Benefits)

### Before Arena (July 29)
- Memory allocations: 1.05M/second
- Memory traffic: 28.0 MB/s

### After Arena (July 31 & August 4)
- Memory allocations: 0/second
- Memory traffic: 0 MB/s
- **100% elimination of runtime allocations maintained**

## Key Takeaways

1. **Arena Benefits Preserved**: Despite some performance regression, the zero-allocation benefit remains

2. **Trade-offs Made**:
   - Simpler, more maintainable code (no recursive evaluator)
   - Better expression function support
   - Slightly lower performance than peak (July 31)

3. **Still Meeting Requirements**:
   - At 1000Hz: 11.8% CPU usage (was 14.3% originally)
   - 88.2% CPU available for other tasks
   - Well within real-time constraints

4. **Future Optimization Opportunities**:
   - The Expression API could be optimized
   - Some abstraction overhead could be reduced
   - Profile-guided optimization might help

## Conclusion

While there was a performance regression between July 31 and August 4, the implementation is still significantly better than the original:
- **17.1% faster** for batch evaluation
- **Zero runtime allocations**
- **Cleaner architecture** without recursive evaluator
- **Better feature support** (expression functions work correctly)

The trade-off of slightly lower performance for better maintainability and correctness appears reasonable, especially given that the performance still exceeds original requirements.