---

name: audit-performance-thread-safety description: Audit codebase for performance bottlenecks and thread-safety issues

Performance and Thread Safety Audit Guide

Purpose: Systematically audit Minecraft mod codebase for performance bottlenecks, thread-safety issues, and dimension isolation problems.

When to use:

• Before major releases
• After implementing new structure generation systems
• After adding new boss spawning systems
• When investigating multiplayer crashes or lag
• When profiling shows unexpected CPU usage

Output: Generates comprehensive audit report with findings, risk assessment, and fix recommendations.

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Audit Process

Phase 1: Identify Critical Code Paths

What to look for:

1. Event handlers registered with TickEvent.SERVER_POST or TickEvent.SERVER_LEVEL_POST
2. Structure generation code in worldgen/ packages
3. Boss spawning systems in worldgen/spawning/
4. Dimension-specific handlers that process multiple dimensions
5. Mixin classes that modify structure placement

Commands:

Copy# Find all event registrations
grep -r "TickEvent\\.SERVER" common/src/main/java --include="*.java"

# Find boss room placers and spawners
find common/src/main/java -name "*BossRoom*.java" -o -name "*Spawner*.java"

# Find structure-related mixins
find common/src/main/java/*/mixin -name "*Structure*.java"

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Phase 2: Performance Analysis (T428)

2.1 Main Thread Blocking Detection

Critical patterns to find:

1. Large block scans on main thread:

   Copy// BAD: Scanning millions of blocks synchronously
   for (int x = minX; x <= maxX; x++) {
       for (int y = minY; y <= maxY; y++) {
           for (int z = minZ; z <= maxZ; z++) {
               BlockPos pos = new BlockPos(x, y, z);
               BlockState state = level.getBlockState(pos);
               // ... processing ...
           }
       }
   }
   

   Red flags:

   • Scan range > 50 blocks in any dimension
   • No chunking or pagination
   • Called every tick or frequently

2. Repeated full-area scans:

   Copy// BAD: Scanning same area multiple times
   removeWater();          // Scan 1
   placeTemplate();
   finalizeWaterlogging(); // Scan 2
   scheduledFinalize();    // Scan 3 (later)
   

   Red flags:

   • Same area scanned 3+ times
   • Delayed/scheduled rescans
   • No caching of positions

3. Single-tick batch operations:

   Copy// BAD: Processing all candidates in one tick
   for (PlacementCandidate candidate : allCandidates) {
       evaluateCandidate(candidate); // Heavy computation
   }
   

   Red flags:

   • Loop over large collection (10+ items)
   • Heavy computation per iteration
   • No async processing or spreading across ticks

2.2 Performance Impact Assessment

For each issue found, estimate:

Document findings:

Copy### Issue: [Description]
**File**: `path/to/File.java` (lines X-Y)
**Pattern**: [Large block scan / Repeated scan / Batch operation]
**Impact**:
- Blocks scanned: [count]
- Estimated pause: [ms]
- Frequency: Every [ticks]
**Severity**: [CRITICAL / HIGH / MEDIUM]

---

Phase 3: Thread Safety Analysis (T429)

3.1 Non-Thread-Safe Collection Detection

Critical patterns to find:

1. Static HashMap/HashSet in multi-threaded context:

   Copy// BAD: HashMap is NOT thread-safe
   private static final Map<ResourceLocation, Set<BlockPos>> map = new HashMap<>();
   

   Where to check:

   • Boss room placers
   • Entity spawners
   • Event handlers that process all dimensions
   • Mixin classes with instance variables

2. Non-atomic map operations:

   Copy// BAD: putIfAbsent + get is NOT atomic as two operations
   map.putIfAbsent(key, new HashSet<>());
   Set<BlockPos> set = map.get(key);  // Race condition here
   
   // BAD: get + increment + put is NOT atomic
   int count = map.get(key);
   count++;
   map.put(key, count);  // Lost updates possible
   

3. Mixin instance variables:

   Copy@Mixin(SomeClass.class)
   public class SomeMixin {
       // BAD: Instance variable in mixin (shared across threads)
       private final Set<BlockPos> positions = new HashSet<>();
   }
   

3.2 Thread Safety Verification

For each collection found, check:

Questions:

1. Is it accessed from multiple dimension threads? (Check event handler type)
2. Is it static? (Static = shared across all threads)
3. Is the collection type thread-safe? (HashMap/HashSet = NO, ConcurrentHashMap = YES)
4. Are operations atomic? (get+put = NO, compute() = YES)

Thread-safe patterns:

Copy// GOOD: ConcurrentHashMap for maps
private static final Map<K, V> map = new ConcurrentHashMap<>();

// GOOD: ConcurrentHashMap.newKeySet() for sets
private static final Set<BlockPos> set = ConcurrentHashMap.newKeySet();

// GOOD: Collections.newSetFromMap for sets
private static final Set<BlockPos> set = Collections.newSetFromMap(new ConcurrentHashMap<>());

// GOOD: Collections.synchronizedSet for sets
private final Set<BlockPos> set = Collections.synchronizedSet(new HashSet<>());

// GOOD: Atomic increment
map.compute(key, (k, v) -> (v == null ? 0 : v) + 1);

// GOOD: Atomic putIfAbsent for sets
map.computeIfAbsent(key, k -> ConcurrentHashMap.newKeySet());

Document findings:

Copy### Issue: Non-thread-safe [HashMap/HashSet/ArrayList]
**File**: `path/to/File.java` (lines X-Y)
**Pattern**:
```java
private static final Map<K, V> map = new HashMap<>();

Risk: [ConcurrentModificationException / Data corruption / Lost updates] Fix: Replace with ConcurrentHashMap Severity: [CRITICAL / HIGH / MEDIUM]

Copy
---

### Phase 4: Dimension Isolation Analysis (T430)

#### 4.1 Cross-Dimension Processing Detection

**Critical patterns to find**:

1. **Processing all dimensions without filtering**:
   ```java
   // BAD: Processes ALL dimensions
   TickEvent.SERVER_POST.register(server -> {
       for (ServerLevel level : server.getAllLevels()) {
           processLevel(level); // No dimension check!
       }
   });

1. Missing dimension filter in handlers:

   Copy// BAD: No early exit for wrong dimension
   public static void checkAndProcess(ServerLevel level) {
       // Immediately starts processing without dimension check
       for (...) { ... }
   }
   

GOOD patterns:

Copy// GOOD: Filter to specific dimension
TickEvent.SERVER_LEVEL_POST.register(level -> {
    if (!level.dimension().equals(ModDimensions.CHRONO_DAWN_DIMENSION)) {
        return; // Early exit
    }
    // ... process only Chrono Dawn
});

4.2 Dimension Context Verification

For each handler found, check:

Questions:

1. Which dimension(s) should this handler process?
2. Does it have a dimension filter?
3. Is it using SERVER_POST (global) or SERVER_LEVEL_POST (per-dimension)?
4. Does the structure/entity only spawn in one dimension?

Expected behavior:

• Boss room placers: Only Chrono Dawn dimension
• Boss spawners: Only Chrono Dawn dimension
• Time distortion: Only Chrono Dawn dimension (already has filter)
• Portal handlers: Multiple dimensions (OK to process all)

Document findings:

Copy### Issue: No dimension filtering
**File**: `path/to/File.java` (lines X-Y)
**Pattern**: Processes all dimensions without filter
**Impact**: Wasted CPU cycles in [Overworld/Nether/End]
**Expected**: Only process [Chrono Dawn] dimension
**Fix**: Add early-exit dimension filter
**Severity**: MEDIUM

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Phase 5: Generate Audit Report

5.1 Report Structure

Copy# Performance and Thread Safety Audit Report

**Date**: [YYYY-MM-DD]
**Branch**: [branch-name]
**Tasks**: T428 (Performance), T429 (Thread Safety), T430 (Dimension Isolation)

---

## Executive Summary

[Brief overview of findings, priority, risk]

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## T428: Main Thread Blocking in Structure Generation

### Findings

[Detailed findings from Phase 2]

### Root Cause Analysis

[Why these issues exist]

### Recommended Fixes

[Prioritized fix recommendations]

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## T429: Non-Thread-Safe Collection Usage

### Findings

[Detailed findings from Phase 3]

### Root Cause Analysis

[Why these issues exist]

### Recommended Fixes

[Prioritized fix recommendations]

---

## T430: Dimension Filtering in Chunk Processing

### Findings

[Detailed findings from Phase 4]

### Root Cause Analysis

[Why these issues exist]

### Recommended Fixes

[Prioritized fix recommendations]

---

## Summary of Issues Found

[Table of all issues with severity, files, priority]

---

## Next Steps

[Recommended actions in priority order]

---

## Estimated Effort

[Time estimates for fixes]

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## Risk Assessment

[What happens without fixes vs with fixes]

5.2 Severity Classification

CRITICAL:

• Main thread freeze > 500ms
• HashMap in multi-threaded context
• Race conditions causing data corruption

HIGH:

• Main thread freeze 200-500ms
• Non-atomic operations with high frequency
• Missing thread synchronization

MEDIUM:

• Main thread freeze 50-200ms
• Missing dimension filters (CPU waste)
• Inefficient algorithms

LOW:

• Main thread freeze < 50ms
• Minor optimization opportunities

---

Common Issues and Solutions

Issue 1: Large Block Scanning

Problem: Scanning millions of blocks on main thread Solution:

1. Chunk-based scanning: Process 1 chunk per tick
2. Marker caching: Cache marker positions after first scan
3. Pre-calculation: Calculate positions during structure generation

Example fix:

Copy// Before: Scan all at once
for (BlockPos pos : BlockPos.betweenClosed(min, max)) { ... }

// After: Process 1 chunk per tick
private static ChunkPos currentChunk = null;
private static Iterator<BlockPos> scanIterator = null;

public static void onTick(ServerLevel level) {
    if (scanIterator == null) {
        // Start new scan
        currentChunk = ...;
        scanIterator = BlockPos.betweenClosed(...).iterator();
    }

    int processed = 0;
    while (scanIterator.hasNext() && processed < 256) { // Limit per tick
        BlockPos pos = scanIterator.next();
        // ... process ...
        processed++;
    }

    if (!scanIterator.hasNext()) {
        scanIterator = null; // Done
    }
}

Issue 2: Thread-Unsafe Collections

Problem: HashMap/HashSet accessed from multiple threads Solution: Replace with ConcurrentHashMap

Example fix:

Copy// Before:
private static final Map<ResourceLocation, Set<BlockPos>> map = new HashMap<>();

// After:
private static final Map<ResourceLocation, Set<BlockPos>> map = new ConcurrentHashMap<>();

// Before (non-atomic):
map.putIfAbsent(key, new HashSet<>());
Set<BlockPos> set = map.get(key);

// After (atomic):
Set<BlockPos> set = map.computeIfAbsent(key, k -> ConcurrentHashMap.newKeySet());

// Before (non-atomic increment):
int count = map.get(key);
count++;
map.put(key, count);

// After (atomic increment):
int count = map.compute(key, (k, v) -> (v == null ? 0 : v) + 1);

Issue 3: Missing Dimension Filter

Problem: Processing all dimensions when structure only spawns in one Solution: Add early-exit dimension filter

Example fix:

Copy// Before:
public static void checkAndProcess(ServerLevel level) {
    ResourceLocation dimensionId = level.dimension().location();
    // ... process ...
}

// After:
public static void checkAndProcess(ServerLevel level) {
    // Only process Chrono Dawn dimension
    if (!level.dimension().equals(ModDimensions.CHRONO_DAWN_DIMENSION)) {
        return;
    }

    ResourceLocation dimensionId = level.dimension().location();
    // ... process ...
}

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Verification Checklist

After applying fixes:

• All modules compile successfully (:common:compileJava, :fabric:compileJava, :neoforge:compileJava)
• No new warnings introduced
• Thread-safe collections used everywhere
• Atomic operations for all shared state modifications
• Dimension filters added where appropriate
• Documentation updated (comments added explaining thread-safety)
• Performance logging added for slow operations
• Build test passes
• Manual testing in single-player (no freezes)
• Manual testing in multiplayer (no crashes)

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References

Related Documentation:

• CLAUDE.md → "Structure Worldgen Guidelines" (for structure generation best practices)
• .claude/skills/structure-worldgen/SKILL.md (for waterlogging prevention patterns)

Related Tasks:

• T428: Performance audit and optimization
• T429: Thread-safety audit and fixes
• T430: Dimension filtering audit

Example Audit Reports:

• See project root for audit_report_performance_thread_safety.md (example report)
• See project root for FIXES_APPLIED.md (example fixes summary)

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Last Updated: 2026-01-02 Author: Claude Sonnet 4.5