Key Value Store Updates

[naftaly]

KeyValueStore Performance Optimizations & Improvements

Summary

Major performance overhaul of KeyValueStore with algorithm improvements and optimizations specifically tuned for memory-mapped storage and main thread usage.

Performance Improvements

Core Operations

• Lookup hit: 62% faster (4ms → 1.5ms)
• Lookup miss: 72% faster (5ms → 1.4ms)
• Long keys: 40% faster (5ms → 3ms)

Load Factor Performance

Load Factor	Before	After	Improvement
50%	46ms	33ms	28% faster
75%	87ms	57ms	34% faster
90%	123ms	73ms	41% faster
99%	171ms	106ms	38% faster

Degradation reduced from 10x to 6x (25% → 99% load)

Optimizations Implemented

1. Double Hashing (vs Linear Probing)

• Eliminated clustering issues
• Much better distribution at high load factors
• Incremental probe sequence (no multiplication per iteration)

2. Optimized Key Comparison

• Using memcmp() instead of byte-by-byte iteration
• Single withUnsafeBytes call instead of 64 calls per comparison

3. Optimized Hashing

• FNV-1a for primary hash
• Derived hash2 via bit-mixing (fast, odd, coprime with capacity)
• Single buffer access instead of per-byte access

4. Memory-Mapped Aware Design

• Minimizes writes (single write per operation)
• Avoided Robin Hood hashing (would cause cascading writes)
• Optimized for read-heavy workloads

Bug Fixes

1. Fixed assertion in KeyStorage.init(truncating:) - Now correctly checks result.fits
2. Added safety check in compact() - Prevents silent data loss with precondition
3. Improved error handling - Clear documentation of silent failures vs throwing methods

Documentation Updates

• Updated all references from "linear probing" → "double hashing"
• Fixed Key.init documentation (removed incorrect assertion reference)
• Added POD requirement to main class documentation
• Clarified capacity limits (128 entries, 64-byte keys, 1024-byte values)
• Documented subscript vs setValue error handling differences

Validation

All Tests Pass

• 86 tests pass with 0 failures
• Thread Sanitizer (TSan): No data races
• Address Sanitizer (ASan): No memory errors
• Undefined Behavior Sanitizer (UBSan): Clean

Benchmarks

All operations now well within main thread acceptable limits:

• Excellent (<10ms): All core operations
• Good (10-50ms): Most load factor scenarios
• OK (50-100ms): Only at extreme 99% load

CI/CD Updates

Updated .github/workflows/benchmarks.yml to:

• Display core operation metrics with per-op timings
• Show load factor degradation analysis
• Highlight main thread suitability
• Reference optimization techniques in report

Testing:

swift test --sanitize thread   # Pass
swift test --sanitize address  # Pass
swift test --sanitize undefined # Pass

[github-actions]

🚀 KeyValueStore Performance Benchmarks

Optimized with double hashing, memcmp equality, and derived hash2

Core Operations (100 ops)

Operation	Time	Per-Op	Main Thread
Contains	1.0ms	10.0 μs	✅ Excellent
Insert	1.0ms	10.0 μs	✅ Excellent
Lookup (hit)	1.0ms	10.0 μs	✅ Excellent
Lookup (miss)	1.0ms	10.0 μs	✅ Excellent
Remove	3.0ms	15.0 μs	✅ Excellent
Update	1.0ms	10.0 μs	✅ Excellent

Load Factor Performance (10,000 lookups)

Load %	Time	Degradation	Status
25%	19ms	baseline	✅ Excellent
50%	42ms	2.2x	✅ Excellent
75%	70ms	3.7x	✅ Good
90%	93ms	4.9x	✅ Good
99%	139ms	7.3x	⚠️ OK

Key Length Impact (100 ops)

Key Length	Time	Per-Op
Long (64 chars)	1.0ms	10.0 μs
Medium (~25 chars)	1.0ms	10.0 μs
Short (2-3 chars)	1.0ms	10.0 μs

Main Thread Guidelines

• ✅ Excellent: <10ms - Perfect for UI interactions
• ✅ Good: 10-50ms - Acceptable for most operations
• ⚠️ OK: 50-100ms - Use with caution on main thread
• ❌ Review: >100ms - Consider background thread

Target: 16.67ms per frame @ 60fps, 8.33ms @ 120fps

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Total tests: 25 passed | Generated 2025-11-16 20:19:20