# WeaselDB Threading Performance Analysis Report

## Summary

WeaselDB's /ok health check endpoint achieves 1M requests/second with 740ns of configurable CPU work per request through the 4-stage commit pipeline, while maintaining 0% CPU usage when idle. The configurable CPU work serves both as a health check (validating the full pipeline) and as a benchmarking tool for measuring per-request processing capacity.

## Performance Metrics

### Throughput
- **1.0M requests/second** /ok health check endpoint (4-stage commit pipeline)
- 8 I/O threads with 8 epoll instances
- Load tester used 12 network threads
- **0% CPU usage when idle** (optimized futex wake implementation)

### Threading Architecture
- **Four-stage commit pipeline**: Sequence → Resolve → Persist → Release
- Lock-free coordination using atomic ring buffer
- **Optimized futex wake**: Only wake on final pipeline stage
- Configurable CPU work performed serially in resolve stage

### Performance Characteristics

**Health Check Pipeline (/ok endpoint)**:
- **Throughput**: 1.0M requests/second
- **Configurable CPU work**: 740ns (4000 iterations, validated with nanobench)
- **Theoretical maximum CPU time**: 1000ns (1,000,000,000ns ÷ 1,000,000 req/s)
- **CPU work efficiency**: 74% (740ns ÷ 1000ns)
- **Pipeline stages**: Sequence (noop) → Resolve (CPU work) → Persist (response) → Release (cleanup)
- **CPU usage when idle**: 0%

### Key Optimizations

**Futex Wake Reduction**:
- **Previous approach**: Futex wake at every pipeline stage (10% CPU overhead)
- **Optimized approach**: Futex wake only at final stage to wake producers. Stages now do their futex wait on the beginning of the pipeline instead of the previous stage.
- **Result**: 23% increase in serial CPU budget (396ns → 488ns)
- **Benefits**: Higher throughput per CPU cycle + idle efficiency

**CPU-Friendly Spin Loop**:
- **Added**: `_mm_pause()` intrinsics in polling loop to reduce power consumption and improve hyperthreading efficiency
- **Maintained**: 100,000 spin iterations necessary to prevent thread descheduling
- **Result**: Same throughput with more efficient spinning

**Resolve Batch Size Optimization**:
- **Changed**: Resolve max batch size from unlimited to 1
- **Mechanism**: Single-item processing checks for work more frequently, keeping the thread in fast coordination paths instead of expensive spin/wait cycles

### Request Flow

**Health Check Pipeline** (/ok endpoint):
```
I/O Threads (8) → HttpHandler::on_batch_complete() → Commit Pipeline
    ↑                                                        ↓
    |                                                 Stage 0: Sequence (noop)
    |                                                        ↓
    |                                                 Stage 1: Resolve (740ns CPU work)
    |                                                 (spend_cpu_cycles(4000))
    |                                                        ↓
    |                                                 Stage 2: Persist (generate response)
    |                                                 (send "OK" response)
    |                                                        ↓
    |                                                 Stage 3: Release (connection return)
    |                                                 (optimized futex wake)
    |                                                        ↓
    └─────────────────────── Server::release_back_to_server()
```

## Test Configuration

- Server: test_benchmark_config.toml with 8 io_threads, 8 epoll_instances
- Configuration: `ok_resolve_iterations = 4000` (740ns CPU work)
- Load tester: targeting /ok endpoint
- Benchmark validation: ./bench_cpu_work 4000
- Build: ninja
- Command: ./weaseldb --config test_benchmark_config.toml
