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10_Wiki/Topics/Topic_Programming/DevOps_and_Security/Scavenge.md

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+---
+id: wiki-2026-0508-scavenge
+title: Scavenge
+category: 10_Wiki/Topics
+status: verified
+canonical_id: self
+aliases: [Minor GC, Scavenger, Young Generation GC, Cheney Scavenger]
+duplicate_of: none
+source_trust_level: A
+confidence_score: 0.95
+verification_status: applied
+tags: [v8, garbage-collection, memory, runtime]
+raw_sources: []
+last_reinforced: 2026-05-10
+github_commit: pending
+tech_stack:
+  language: cpp
+  framework: v8-orinoco
+---
+
+# Scavenge
+
+## 매 한 줄
+> **"매 young object 의 매 빠른 die — 매 cheap 하게 collect"**. Scavenge 는 매 generational hypothesis (most objects die young) 의 매 exploit — 매 V8 young generation 을 매 from-space / to-space 로 나누고 매 live object 만 매 to-space 로 매 copy. 매 dead object 는 매 단순 abandon. 2026 V8 (Orinoco) 에서 매 parallel + concurrent 로 매 main-thread pause < 1ms.
+
+## 매 핵심
+
+### 매 Cheney's algorithm
+1. Allocate in to-space (linear bump pointer).
+2. When full → 매 swap roles. From-space = 매 old to-space.
+3. From roots, copy 매 reachable object to (new) to-space.
+4. Update 매 forwarding pointer in from-space slot.
+5. BFS through copied objects, copying 매 referenced objects.
+6. Done → from-space 매 entirely abandoned.
+
+### 매 V8-specific
+- Young gen = New Space ≈ 1–8 MB per worker.
+- Promotion: 매 survives 2 scavenges → 매 Old Space.
+- Parallel Scavenge (2018+): 매 multiple threads.
+- Concurrent (2021+): root marking on background.
+
+### 매 응용
+1. JS heap young gen.
+2. Java HotSpot Young Generation (Parallel Scavenge collector).
+3. .NET Gen 0/1.
+4. Erlang per-process heap.
+
+## 💻 패턴
+
+### Cheney scavenge (pseudo-C)
+```c
+typedef struct { intptr_t header; void* slots[]; } Object;
+char *from, *to, *alloc;
+
+void* scavenge_copy(Object *obj) {
+  if (is_forwarded(obj)) return forwarded_addr(obj);
+  size_t size = obj_size(obj);
+  Object *copy = (Object*)alloc;
+  memcpy(copy, obj, size);
+  alloc += size;
+  set_forwarded(obj, copy);
+  return copy;
+}
+
+void scavenge() {
+  swap(&from, &to);
+  alloc = to;
+  char *scan = to;
+  for (Root *r = roots; r; r = r->next) *r = scavenge_copy(*r);
+  while (scan < alloc) {
+    Object *o = (Object*)scan;
+    for (int i = 0; i < slot_count(o); i++)
+      o->slots[i] = scavenge_copy(o->slots[i]);
+    scan += obj_size(o);
+  }
+}
+```
+
+### Allocation (bump pointer)
+```c
+void* alloc_young(size_t bytes) {
+  if (alloc + bytes > to_end) trigger_scavenge();
+  void *p = alloc;
+  alloc += bytes;
+  return p;
+}
+```
+
+### Promotion check
+```c
+if (obj_age(o) >= 2) {
+  void *promoted = alloc_old(obj_size(o));
+  memcpy(promoted, o, obj_size(o));
+  // also update remembered set if old → young pointers exist
+} else {
+  scavenge_copy(o);
+  inc_age(o);
+}
+```
+
+### Remembered set (write barrier)
+```c
+void store_field(Object *obj, int slot, Object *val) {
+  obj->slots[slot] = val;
+  if (in_old_space(obj) && in_young_space(val))
+    remembered_set_add(&obj->slots[slot]);
+}
+```
+
+### V8 trace (observe scavenge)
+```bash
+node --trace-gc app.js
+# [12345:0x...] 100 ms: Scavenge 5.5 (6.7) -> 4.8 (7.7) MB, 0.4 / 0.0 ms
+```
+
+## 매 결정 기준
+| 상황 | Strategy |
+|---|---|
+| 매 short-lived alloc heavy | Larger young gen (--max-semi-space-size) |
+| 매 long-lived heavy | Smaller young, faster promotion |
+| 매 latency-critical | Concurrent scavenge enabled |
+| 매 throughput | Parallel scavenge |
+
+**기본값**: V8 default — auto-tuned per workload.
+
+## 🔗 Graph
+- 부모: [[Garbage Collection]] · [[V8 Engine]]
+- 변형: [[Mark-Sweep-Compact]] · [[Major GC]]
+- 응용: [[New Space(Young Generation)]] · [[Cheneys Algorithm]]
+- Adjacent: [[Orinoco GC]] · [[Stop-the-world]]
+
+## 🤖 LLM 활용
+**언제**: GC log analysis, allocation hotspot identification from heap snapshots, write-barrier overhead estimation.
+**언제 X**: 매 actual GC algorithm change — runtime team only.
+
+## ❌ 안티패턴
+- **Massive young alloc + immediate retain**: 매 promotion storm → 매 old space pressure.
+- **Linked list of small objects**: 매 scan cost 의 매 linear in slots → 매 use TypedArray.
+- **Disabling GC**: 매 --no-gc — 매 memory grows unbounded.
+
+## 🧪 검증 / 중복
+- Verified (V8 Orinoco docs, Cheney 1970 paper, "The Garbage Collection Handbook" 2nd Ed.).
+- 신뢰도 A.
+
+## 🕓 Changelog
+| 날짜 | 변경 |
+|---|---|
+| 2026-05-08 | Phase 1 |
+| 2026-05-10 | Manual cleanup — Cheney + V8 Orinoco parallel-concurrent state |