2nd/10_Wiki/Topic_Programming/Frontend/Scavenger 알고리즘.md

---
id: wiki-2026-0508-scavenger-알고리즘
title: Scavenger 알고리즘
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [V8 Scavenger, Minor GC, Young Generation GC, Cheney Algorithm]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [v8, javascript-engine, garbage-collection, gc, performance]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: JavaScript
  framework: V8 / Node.js
---

# Scavenger 알고리즘

## 매 한 줄
> **"매 Scavenger 는 V8 의 minor GC — semi-space copying collector for short-lived objects"**. Cheney's algorithm 기반, young generation (new space) 을 from-space / to-space 로 나눠 매 minor GC 마다 live object 만 to-space 로 복사. Survivor 는 old space 로 promote. 매 fast (수 ms), 매 web app 의 hot path. Major GC (Mark-Sweep-Compact) 와 분리.

## 매 핵심

### 매 generational hypothesis
- 대부분 객체는 young 일 때 die → 매 young 만 자주 GC 하면 효율
- V8 heap: new space (young, ~16MB) + old space (long-lived) + large object space + code/map space

### 매 Cheney's copying
- new space = from-space + to-space (반반)
- minor GC: from-space 의 live root → to-space 로 BFS-copy
- 매 dead object 은 자동 폐기 (sweep 불필요)
- 두번째 survival 시 old space 로 promote

### 매 비용
- O(live data) — dead 가 많으면 매우 빠름
- pause time: 매 1-5ms (web app)
- write barrier: old → new pointer 추적 (remembered set)

### 매 응용
1. Allocation-heavy code path 의 GC pressure 분석.
2. Hot loop 에서 전혀 다른 object 양산 회피.
3. Node.js memory profile (`--trace-gc`).

## 💻 패턴

### Trace GC events
```bash
node --trace-gc app.js
# [pid] 12 ms: Scavenge 4.5 (5.7) -> 0.8 (5.7) MB, 0.3 / 0.0 ms
# [pid] 45 ms: Mark-sweep 5.0 (10.2) -> 2.1 (10.2) MB, 4.8 / 0.0 ms
```

### Detailed GC trace
```bash
node --trace-gc-verbose --trace-gc-nvp app.js
```

### Heap snapshot (Chrome DevTools 연결)
```typescript
import { writeHeapSnapshot } from "node:v8";
const file = writeHeapSnapshot();
console.log(`매 snapshot at ${file}`);
// load in DevTools → Memory tab → Compare snapshots
```

### Avoid allocation in hot loop
```typescript
// 매 BAD — 매 iteration creates objects
function sumPoints(points: { x: number; y: number }[]): number {
  return points
    .map((p) => ({ ...p, sum: p.x + p.y }))  // new objects
    .reduce((a, b) => a + b.sum, 0);
}

// 매 GOOD — 매 no allocation in loop
function sumPointsFast(points: { x: number; y: number }[]): number {
  let s = 0;
  for (let i = 0; i < points.length; i++) s += points[i].x + points[i].y;
  return s;
}
```

### Object pool to bypass GC
```typescript
class Vec3Pool {
  private pool: { x: number; y: number; z: number }[] = [];
  acquire(x = 0, y = 0, z = 0) {
    const v = this.pool.pop() ?? { x: 0, y: 0, z: 0 };
    v.x = x; v.y = y; v.z = z;
    return v;
  }
  release(v: { x: number; y: number; z: number }) {
    this.pool.push(v);
  }
}
const pool = new Vec3Pool();
const v = pool.acquire(1, 2, 3);
pool.release(v);
```

### Measure GC pause via PerformanceObserver
```typescript
import { PerformanceObserver } from "node:perf_hooks";

const obs = new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    console.log(`매 GC ${entry.detail?.kind} took ${entry.duration.toFixed(2)}ms`);
  }
});
obs.observe({ entryTypes: ["gc"], buffered: true });
```

### Tune new space size
```bash
node --max-semi-space-size=64 app.js   # default ~16MB, increase for alloc-heavy
node --max-old-space-size=4096 app.js  # old gen
```

### Detect promotion pressure
```typescript
import v8 from "node:v8";
setInterval(() => {
  const stats = v8.getHeapSpaceStatistics();
  for (const s of stats) {
    console.log(`${s.space_name}: ${(s.space_used_size / 1e6).toFixed(1)} MB`);
  }
}, 5000);
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Short-lived 객체 많음 (정상) | Scavenger 가 처리 — 신경 X |
| Hot loop allocation 폭주 | 매 reuse / pool / typed arrays |
| Old space 증가 (leak 의심) | heap snapshot + retainer 분석 |
| Long pause (>50ms) | major GC 문제 — incremental marking 확인 |
| Alloc-heavy server | `--max-semi-space-size` 증가 |

**기본값**: 매 normal code 는 Scavenger 가 자동 처리. 매 hot path 는 alloc-free 하게 작성.

## 🔗 Graph
- 부모: [[V8 Engine]] · [[Garbage Collection]]
- 변형: [[Mark-Sweep-Compact]] · [[Incremental_Marking|Incremental Marking]] · [[Concurrent Marking]] · [[Orinoco]]
- 응용: [[Node.js Performance]]
- Adjacent: [[Cheney Algorithm]]

## 🤖 LLM 활용
**언제**: V8 GC 설명, Node.js memory tuning, hot-path optimization. 매 `--trace-gc` output 해석.
**언제 X**: 매 일반 web/app code review (premature opt 우려). 매 non-V8 runtime (Bun is JSC fork? actually JSCore — 다른 GC).

## ❌ 안티패턴
- **GC tuning premature**: 매 measure 없이 flag 변경. 매 profile 먼저.
- **Manual `global.gc()`**: 매 production 에서 `--expose-gc` 의존 — 매 anti-pattern.
- **Object literal in hot loop**: GC pressure 증가. 매 reuse / pool.
- **Megamorphic shapes**: 매 hidden class 변형 → IC miss → 추가 alloc. 매 shape 일정.
- **Closures in loop**: 매 iteration 마다 closure 생성 → young heap pressure.

## 🧪 검증 / 중복
- Verified (V8 blog, "Trash talk: the Orinoco garbage collector", 2016 + later updates).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — V8 Scavenger algorithm full content |