# Story 10.20: Performance Benchmarks Documentation

**Status:** Complete
**Priority:** Medium
**Depends on:** None

---

## Context / Background

Enterprise users evaluating fapilog need performance data to:
- Justify adoption to architecture review boards
- Plan capacity for production workloads
- Tune configuration for their environment

A comprehensive benchmark script exists at `scripts/benchmarking.py` (467 lines) that measures throughput, latency, and memory against Python stdlib logging. However, results are not published in the documentation.

The performance tuning guide (`docs/user-guide/performance-tuning.md`) provides configuration advice but no actual numbers to contextualize recommendations.

**Goal:** Publish indicative benchmark results with professional presentation — honest about scope and limitations, with clear reproducibility instructions.

---

## Scope (In / Out)

### In Scope

- Run existing benchmark script and capture results
- Create `docs/user-guide/benchmarks.md` with methodology, results, and caveats
- Document environment used for benchmarks
- Provide reproduction instructions
- Link from existing performance-tuning.md
- Add to docs navigation

### Out of Scope

- CI-based automated benchmark runs (see Story 10.19)
- Statistical rigor (multiple runs, confidence intervals)
- Hardware-specific tuning guides
- Comparative benchmarks against other logging libraries (loguru, structlog)

---

## Acceptance Criteria

### AC1: Benchmark Results Page Exists

**Description:** A new documentation page presents benchmark results with clear methodology.

**Validation:**
- File exists at `docs/user-guide/benchmarks.md`
- Page renders correctly in docs build
- Page is accessible from user-guide navigation (after performance-tuning)

### AC2: Methodology Section

**Description:** The methodology section explains what is measured and how.

**Validation:**
```markdown
## Methodology

- Baseline: Python stdlib `logging` to file
- Test: fapilog with rotating file sink
- Metrics: Throughput (logs/sec), latency (μs), peak memory (bytes)
- Warmup: 1,000 calls before measurement
- Iterations: 20,000 (throughput/memory), 5,000 (latency)
```

### AC3: Environment Documentation

**Description:** The test environment is documented for reproducibility context.

**Validation:**
Page includes:
- Python version
- OS and version
- CPU description
- Memory available
- fapilog version

### AC4: Results Tables

**Description:** Results are presented in clear tables covering throughput, latency, and memory.

**Validation:**
Three tables present:
1. **Throughput:** stdlib logs/sec, fapilog logs/sec, speedup factor
2. **Latency:** avg/median/p95 in microseconds for both
3. **Memory:** peak bytes for both, reduction percentage

### AC5: Reproduction Instructions

**Description:** Users can reproduce benchmarks on their own hardware.

**Validation:**
```markdown
## Reproducing These Results

```bash
python scripts/benchmarking.py --iterations 20000 --latency-iterations 5000
```
```

### AC6: Limitations Section

**Description:** Honest caveats prevent over-interpretation of results.

**Validation:**
Page includes explicit limitations:
- Single development machine, not production hardware
- Measures front-end log call rate, not sink I/O completion
- Results vary with CPU, disk, Python version, and workload
- Not a substitute for load testing in your environment

### AC7: Cross-Linking

**Description:** Related documentation is linked for discoverability.

**Validation:**
- `docs/user-guide/performance-tuning.md` links to benchmarks page
- Benchmarks page links to performance tuning guide

---

## Implementation Notes

### File Changes

```
docs/user-guide/benchmarks.md (NEW)
docs/user-guide/performance-tuning.md (MODIFIED - add link)
docs/user-guide/index.md (MODIFIED - add to toctree after performance-tuning)
```

Note: Adding to `user-guide/` keeps related performance content together and simplifies navigation.

### Benchmark Execution

```bash
# Run benchmark and capture output
python scripts/benchmarking.py --iterations 20000 --latency-iterations 5000 > benchmark_output.txt

# Extract environment info
python --version
uname -a
sysctl -n machdep.cpu.brand_string  # macOS
# or: cat /proc/cpuinfo | grep "model name" | head -1  # Linux
```

### Tone Guidelines

- Factual, no superlatives
- "Indicative results" not "proof of performance"
- Acknowledge limitations upfront
- Let numbers speak for themselves

---

## Tasks

### Phase 1: Benchmark Execution

- [ ] Run `scripts/benchmarking.py` with default parameters
- [ ] Capture environment metadata (Python, OS, CPU, memory)
- [ ] Save raw JSON output for reference

### Phase 2: Documentation

- [ ] Write `docs/user-guide/benchmarks.md` following structure
- [ ] Format results into markdown tables
- [ ] Write methodology section
- [ ] Write limitations section
- [ ] Add reproduction instructions

### Phase 3: Integration

- [ ] Add benchmarks page to docs navigation
- [ ] Add link from `docs/user-guide/performance-tuning.md`
- [ ] Verify docs build succeeds
- [ ] Review rendered output

### Phase 4: Finalization

- [ ] Update CHANGELOG.md

---

## Tests

### Manual Validation

- Docs build completes without errors
- Page renders correctly with tables formatted
- All links resolve correctly
- Reproduction instructions work when followed

No automated tests required (documentation-only change).

---

## Definition of Done

### Code Complete

- [ ] All acceptance criteria implemented
- [ ] Documentation follows project style

### Quality Assurance

- [ ] Docs build passes
- [ ] Links verified working
- [ ] Tables render correctly
- [ ] Reproduction instructions tested

### Documentation

- [ ] CHANGELOG updated
- [ ] Navigation updated

---

## Risks / Rollback

### Risks

1. **Risk:** Benchmark results could be misinterpreted as guarantees
   - **Mitigation:** Prominent limitations section, "indicative" language throughout

2. **Risk:** Results become stale as code changes
   - **Mitigation:** Include fapilog version, link to Story 10.19 for future CI automation

### Rollback Plan

Documentation-only change. If issues occur:
1. Remove `docs/user-guide/benchmarks.md`
2. Revert toctree entry in `docs/user-guide/index.md`
3. Revert link in `docs/user-guide/performance-tuning.md`

---

## Related Stories

- **Related:** Story 10.19 - Performance regression detection (future CI automation)
- **Enhances:** Existing `docs/user-guide/performance-tuning.md`

---

## Code Review

**Date:** 2026-01-19
**Reviewer:** Claude
**Verdict:** OK to PR

### Summary

Documentation-only change adding performance benchmarks page with honest presentation of results, methodology, environment details, limitations, and reproduction instructions.

### AC Verification

| Criterion | Evidence |
|-----------|----------|
| AC1: Page exists | `docs/user-guide/benchmarks.md:1-133` - file created, builds, added to toctree |
| AC2: Methodology | `benchmarks.md:6-18` - table with baseline, metrics, warmup, iterations |
| AC3: Environment | `benchmarks.md:22-30` - Python, OS, CPU, memory, fapilog version |
| AC4: Results tables | `benchmarks.md:34-92` - throughput, latency, slow sink, burst, memory |
| AC5: Reproduction | `benchmarks.md:113-131` - command and options table |
| AC6: Limitations | `benchmarks.md:106-111` - all four caveats documented |
| AC7: Cross-linking | `performance-tuning.md:3` ↔ `benchmarks.md:3,130` |

### Quality Gates

- [x] Docs build passed
- [x] Links verified working
- [x] Tables render correctly
- [x] CHANGELOG updated

### Issues Addressed

- [P1] Missing CHANGELOG entry - Fixed in amended commit

---

## Change Log

| Date | Change | Author |
|------|--------|--------|
| 2026-01-19 | Initial draft | Claude |
| 2026-01-19 | Review: moved to user-guide/, status → Ready | Claude |
| 2026-01-19 | Implementation complete, status → Ready for Code Review | Claude |
| 2026-01-19 | Code review passed, status → Complete | Claude |
