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

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-old-space	Old Space (V8)	10_Wiki/Topics	verified	self	Old Generation Tenured Space V8 Old Space Major GC heap	none	A	0.95	applied	v8 gc memory javascript runtime		2026-05-10	pending	language framework javascript 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=4096로 limit 조정.

💻 패턴

Heap snapshot 생성 (Node.js)

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

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)

// 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

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)

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

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 · Garbage_Collection
• 변형: Mark-Sweep · Incremental_Marking · Generational_Hypothesis
• 응용: Orinoco · Pointer_Compression · Memory_Leaks
• Adjacent: 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