2nd/10_Wiki/Topics/Architecture/Memory_Leaks.md

---
id: wiki-2026-0508-memory-leaks
title: Memory Leaks
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [메모리 누수, Mem Leak, Heap Leak]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [memory, debugging, performance, gc]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: multi
  framework: node-jvm-python
---

# Memory Leaks

## 매 한 줄
> **"매 reachable 한 garbage — 매 free 되지 않는 retained memory."**. Memory leak 은 매 program 이 더 이상 필요 없는 memory 를 release 하지 못해서 매 heap 이 무한히 grow 하는 현상. 매 GC language (JS, Python, JVM) 에서도 매 reference 가 살아있으면 leak — 매 manual language (C/C++) 에서는 매 free() miss.

## 매 핵심

### 매 분류
- **Unreachable leak (C/C++)**: 매 free() 누락 — pointer 잃어버림.
- **Reachable leak (GC lang)**: 매 reference 가 살아있어서 GC 가 collect 안 함.
- **Logical leak**: 매 코드는 정상이지만 매 cache/list 가 무한 grow.

### 매 흔한 원인 (GC lang)
- **Global variable**: 매 process 끝까지 살아있음.
- **Closure capture**: 매 callback 이 매 큰 object 를 capture.
- **Event listener**: 매 removeEventListener 누락.
- **Timer**: 매 clearInterval 누락.
- **DOM detach**: 매 detached node 를 JS 가 still reference.
- **Cache**: 매 unbounded Map/dict.

### 매 진단 신호
1. RSS 가 매 시간/일 단위로 매 monotonically grow.
2. GC pause 가 매 frequent + long.
3. OOM crash 결국.

## 💻 패턴

### Node.js heap snapshot diff
```javascript
// Take 3 snapshots, diff 2 vs 3
const v8 = require('v8');
const fs = require('fs');

function snapshot(label) {
  const stream = v8.getHeapSnapshot();
  const file = fs.createWriteStream(`heap-${label}-${Date.now()}.heapsnapshot`);
  stream.pipe(file);
}

// Or via Chrome DevTools: chrome://inspect → Memory tab → Take snapshot
// Compare snapshots → "Allocations between snapshots" → find growing constructors
```

### 매 흔한 leak: closure capture
```javascript
// BAD: closure captures huge `data` even though only `id` is used
function attach(data) {
  document.getElementById('btn').addEventListener('click', () => {
    console.log(data.id);  // captures entire `data` (10MB)
  });
}

// GOOD: extract only what's needed
function attach(data) {
  const id = data.id;  // only id is captured
  document.getElementById('btn').addEventListener('click', () => {
    console.log(id);
  });
}
```

### 매 EventEmitter leak
```javascript
const emitter = new EventEmitter();
emitter.setMaxListeners(20);  // Node warns at >10

// BAD: each call adds a new listener
function onEachRequest(req) {
  emitter.on('data', () => process(req));  // listener never removed
}

// GOOD: once() or remove
function onEachRequest(req) {
  const handler = () => process(req);
  emitter.once('data', handler);
  // or: emitter.off('data', handler) when done
}
```

### Python tracemalloc
```python
import tracemalloc
tracemalloc.start(25)  # store 25 frames

# ... run workload ...

snap1 = tracemalloc.take_snapshot()
# ... run more workload ...
snap2 = tracemalloc.take_snapshot()

stats = snap2.compare_to(snap1, 'lineno')
for stat in stats[:10]:
    print(stat)
# → shows top 10 lines that allocated most new memory between snapshots
```

### JVM heap dump
```bash
# Live process
jcmd <pid> GC.heap_dump /tmp/heap.hprof

# On OOM
java -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/tmp/heap.hprof MyApp

# Analyze with Eclipse MAT or VisualVM
# → "Leak Suspects Report" auto-identifies likely retainers
```

### 매 detached DOM leak
```javascript
// BAD: cache holds reference to detached node
const cache = new Map();
function showModal(id) {
  const node = document.createElement('div');
  cache.set(id, node);  // never deleted
  document.body.appendChild(node);
  // later: document.body.removeChild(node) — but cache still holds it
}

// GOOD: WeakMap or explicit cleanup
const cache = new WeakMap();  // doesn't prevent GC
// or: cache.delete(id) when modal closes
```

### 매 unbounded cache
```javascript
// BAD: grows forever
const cache = {};
function memoize(key, fn) {
  if (!(key in cache)) cache[key] = fn();
  return cache[key];
}

// GOOD: LRU with max size
import LRU from 'lru-cache';
const cache = new LRU({ max: 1000, ttl: 1000 * 60 * 5 });
```

## 매 결정 기준
| 증상 | 진단 도구 |
|---|---|
| Node RSS 증가 | Chrome DevTools heap snapshot diff |
| Python 증가 | tracemalloc compare_to |
| JVM OOM | jcmd heap dump → Eclipse MAT |
| C/C++ | Valgrind, AddressSanitizer (-fsanitize=address) |
| Browser tab | DevTools Memory → Allocation timeline |

**기본값**: 매 3 snapshot diff — between 2nd & 3rd 사이의 retained 만 보면 매 startup noise 제거됨.

## 🔗 Graph
- 부모: [[Memory Management]] · [[Garbage Collection]]
- 변형: [[Mark-Sweep]] · [[Reference Counting]]
- 응용: [[Performance_Profiling_and_Memory|Performance Profiling]]
- Adjacent: [[WeakRef]]

## 🤖 LLM 활용
**언제**: long-running service (Node server, Python daemon, JVM app), browser SPA 가 시간 흐를수록 느려질 때.
**언제 X**: short-lived script — process exit 시 OS 가 다 회수.

## ❌ 안티패턴
- **"GC lang 은 leak 없다"**: 매 reachable leak 이 매 가장 흔함.
- **단일 snapshot**: 매 baseline 없으면 무엇이 leak 인지 모름 — 매 diff 가 정답.
- **Premature optimization**: 매 RSS 가 stable 한데 매 leak 추적 — 매 시간 낭비.
- **try/catch 없는 cleanup**: 매 error path 에서 listener removal 누락.

## 🧪 검증 / 중복
- Verified (V8 docs, Chrome DevTools docs, MDN, Node.js docs, Eclipse MAT).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — leak 분류 + Node/Python/JVM 진단 패턴 |