2nd/10_Wiki/Topic_Programming/Architecture/Old_Space.md

---
id: wiki-2026-0508-old-space
title: Old Space (V8)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Old Generation, Tenured Space, V8 Old Space, Major GC heap]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [v8, gc, memory, javascript, runtime]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: javascript
  framework: v8
---

# Old Space (V8)

## 매 한 줄
> **"매 V8 heap의 long-lived object 영역 — Young Gen에서 2번 생존 후 promote, Major GC (Mark-Compact / Mark-Sweep / Concurrent Marking) 대상."**. Generational hypothesis 기반 (Ungar 1984). 2026 현재 V8 13.x · Node 24 · Chrome 130+에서 Orinoco의 concurrent / parallel / incremental marking + Pointer Compression으로 pause time <1ms.

## 매 핵심

### 매 V8 heap 구조
- **New Space (Young Gen)**: 1-8MB, semi-space (Cheney scavenger), allocation 빠름.
- **Old Space (Old Gen)**: 매 default unbounded (--max-old-space-size 제한), promote target.
- **Code Space**: JIT compiled code.
- **Map Space**: Hidden class (V8 maps).
- **Large Object Space**: ≥ ~256KB single object.
- **Read-Only Space**: immutable roots.

### 매 promotion 규칙
- 매 Young Gen에서 minor GC 2번 survive → Old Space promote.
- Large object (≥ kMaxRegularHeapObjectSize, ~256KB) → 직접 Old Space (LOS).

### 매 Major GC (Old Space) 알고리즘
- **Mark-Compact**: full pause, fragmentation 제거.
- **Concurrent Marking** (Orinoco): worker thread가 mark, main thread continue.
- **Incremental Marking**: chunk별 mark, 사이사이 mutator 실행.
- **Lazy Sweeping**: page별로 sweep on demand.

### 매 응용
1. Node.js server — heap profiling으로 leak 추적 (`--inspect`, heapdump).
2. Chrome — DevTools Memory tab의 retained size 분석.
3. Performance tuning — `--max-old-space-size=4096`로 limit 조정.

## 💻 패턴

### Heap snapshot 생성 (Node.js)
```javascript
import { writeHeapSnapshot } from 'node:v8';
import { performance } from 'node:perf_hooks';

process.on('SIGUSR2', () => {
  const file = `/tmp/heap-${Date.now()}.heapsnapshot`;
  writeHeapSnapshot(file);
  console.log('snapshot:', file);
});
// kill -USR2 <pid>
```

### Inspect heap stats
```javascript
import v8 from 'node:v8';
console.log(v8.getHeapSpaceStatistics());
// [ { space_name: 'old_space', space_size, space_used_size, ... } ]

console.log(v8.getHeapStatistics());
// { total_heap_size, used_heap_size, heap_size_limit, ... }
```

### Force GC for measurement (--expose-gc)
```javascript
// node --expose-gc app.js
function measureRetained(fn) {
  global.gc(); global.gc();
  const before = process.memoryUsage().heapUsed;
  const ref = fn();
  global.gc(); global.gc();
  const after = process.memoryUsage().heapUsed;
  return { ref, retained: after - before };
}
```

### Trace GC events
```bash
node --trace-gc --trace-gc-verbose app.js
# [12345:0x...] [GC] 100ms: 50.0 (60.0) -> 30.0 (60.0) MB (Mark-Sweep) ...
```

### Monitor old space in production (Prometheus)
```javascript
import client from 'prom-client';
import v8 from 'node:v8';

const oldSpaceUsed = new client.Gauge({
  name: 'nodejs_v8_old_space_used_bytes',
  help: 'V8 old space used',
});
setInterval(() => {
  const old = v8.getHeapSpaceStatistics()
    .find(s => s.space_name === 'old_space');
  oldSpaceUsed.set(old.space_used_size);
}, 5000);
```

### Avoid old-space leak — WeakRef / WeakMap for cache
```javascript
const cache = new WeakMap();  // key not retained
function compute(obj) {
  if (cache.has(obj)) return cache.get(obj);
  const v = expensive(obj);
  cache.set(obj, v);
  return v;
}
// obj GC'd → cache entry auto-removed
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Suspected memory leak | heap snapshot diff |
| OOM under load | `--max-old-space-size=` increase + investigate |
| Frequent major GC pause | reduce promotion rate (avoid long-lived churn) |
| Cache growing forever | WeakMap / LRU bound |
| Need pre-prod heap profile | `--inspect` + DevTools |

**기본값**: 매 default V8 settings — limit 변경은 측정 후.

## 🔗 Graph
- 부모: [[V8 엔진 힙 아키텍처|V8 Heap Architecture]] · [[Garbage_Collection]]
- 변형: [[Mark-Sweep]] · [[Incremental_Marking]] · [[Generational_Hypothesis]]
- 응용: [[Orinoco]] · [[Pointer_Compression]] · [[Memory_Leaks]]
- Adjacent: [[Write Barrier|Write_Barrier]] · [[Snapshots]] · [[Reachability_Analysis]]

## 🤖 LLM 활용
**언제**: Node.js memory issue 분석, heap profiling, V8 internals 학습, GC tuning.
**언제 X**: non-V8 runtime (Bun-JSC, Deno-V8 다르지만 비슷, Hermes는 별도), browser-only DOM leak (different territory).

## ❌ 안티패턴
- **Forgotten timer/listener**: 매 closure가 old space에 retain.
- **Global cache without bound**: 매 monotonic growth → OOM.
- **Large allocation in hot loop**: Young → Old promotion 폭증, frequent major GC.
- **String concat in loop without builder**: 매 intermediate retained.
- **`--max-old-space-size` blindly raised**: 매 leak hide, root cause 회피.

## 🧪 검증 / 중복
- Verified (V8 source `v8/src/heap/`, V8 blog Orinoco posts, Node.js v8 module docs 2026).
- 신뢰도 A.
- Duplicates: `Old_Space(Old_Generation).md` redirects here.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — canonical V8 Old Space doc + heap profiling patterns |