---
id: wiki-2026-0508-cheneys-algorithm
title: Cheney's Algorithm
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Cheney GC, Semi-space Collector, Copying GC]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [gc, memory, algorithm, runtime]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: C/Rust
  framework: runtime/GC
---

# Cheney's Algorithm

## 매 한 줄
> **"매 stop-and-copy GC 의 BFS-style two-finger traversal"**. 1970년 C.J. Cheney 가 제시한 copying garbage collector 의 표준 algorithm — recursion 없이 queue-style 로 live object 를 from-space 에서 to-space 로 evacuate. 매 modern V8/SpiderMonkey young generation, OCaml minor heap, MLton 의 baseline.

## 매 핵심

### 매 semi-space 구조
- Heap 을 두 개의 equal-sized region 으로 split: from-space, to-space.
- Allocation 은 from-space 의 bump pointer 만 증가.
- GC 시 live object 를 to-space 로 copy 후 role swap.

### 매 two pointers
- `scan`: to-space 에서 아직 children 추적 안 한 boundary.
- `free`: to-space 의 next allocation slot.
- `scan == free` 이면 traversal 종료.

### 매 응용
1. V8 young-gen scavenger (Node.js, Chrome).
2. OCaml minor heap collection.
3. SBCL, MLton 의 default GC.

## 💻 패턴

### Core Cheney loop (C)
```c
void* to_space; size_t scan, free_;

void* copy(void* obj) {
    if (is_forwarded(obj)) return forward_addr(obj);
    size_t sz = size_of(obj);
    void* dst = (char*)to_space + free_;
    memcpy(dst, obj, sz);
    set_forward(obj, dst);
    free_ += sz;
    return dst;
}

void cheney_gc(void** roots, size_t n) {
    free_ = scan = 0;
    for (size_t i = 0; i < n; i++) roots[i] = copy(roots[i]);
    while (scan < free_) {
        void* obj = (char*)to_space + scan;
        for_each_pointer_field(obj, p) { *p = copy(*p); }
        scan += size_of(obj);
    }
    swap(from_space, to_space);
}
```

### Forwarding pointer trick
```c
// Object header overlap: live header OR forwarding pointer.
struct header { uintptr_t tag_or_fwd; };
#define IS_FWD(h) ((h)->tag_or_fwd & 1)
#define FWD_PTR(h) ((void*)((h)->tag_or_fwd & ~1))
#define SET_FWD(h, dst) ((h)->tag_or_fwd = (uintptr_t)(dst) | 1)
```

### Allocation (post-GC)
```c
void* alloc(size_t sz) {
    if (free_ + sz > SEMI_SIZE) cheney_gc(roots, n_roots);
    if (free_ + sz > SEMI_SIZE) abort(); // OOM
    void* p = (char*)to_space + free_;
    free_ += sz;
    return p;
}
```

### V8-style scavenger (simplified)
```cpp
void Scavenger::Process() {
    while (!worklist_.empty()) {
        HeapObject obj = worklist_.Pop();
        obj->IterateBody(this);  // visits each pointer field
    }
}
void Scavenger::VisitPointer(Object** slot) {
    HeapObject obj = HeapObject::cast(*slot);
    if (Heap::InFromSpace(obj)) {
        HeapObject target = EvacuateObject(obj);
        *slot = target;
    }
}
```

### Generational tweak
```c
// Young gen uses Cheney; old gen uses mark-sweep.
// Promotion: if object survives N scavenges, copy to old-gen instead of to-space.
if (age(obj) >= PROMOTION_THRESHOLD) dst = old_gen_alloc(sz);
else dst = (char*)to_space + free_;
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Short-lived allocation 多 | Cheney (semi-space) — fast bump alloc |
| Large heap, low live ratio | Cheney 우수 (cost ∝ live, not heap) |
| Mostly-live mature data | Mark-sweep / mark-compact |
| Real-time constraints | Incremental / concurrent GC (Shenandoah, ZGC) |
| Memory tight (mobile) | Mark-sweep (no 2× overhead) |

**기본값**: Young generation 에 Cheney, old generation 에 mark-compact (generational hypothesis).

## 🔗 Graph
- 부모: [[Garbage Collection]] · [[Memory Management]]
- 변형: [[Mark-Sweep]]
- 응용: [[V8 Engine]] · [[Nodejs]]
- Adjacent: [[Write Barrier]]

## 🤖 LLM 활용
**언제**: GC 설명, runtime internals 분석, language implementation 설계 시.
**언제 X**: Application-level memory tuning (use language-specific profiler 대신).

## ❌ 안티패턴
- **Naive recursive copy**: stack overflow 가능 — Cheney 의 queue 방식 사용.
- **Forgetting forward check**: 동일 object 두 번 copy → 데이터 corrupt.
- **Pointer 누락**: stack/register/global root scan 빠짐 → dangling pointer.
- **Pinning ignored**: native pointer 가 from-space object 가리키는 동안 GC → crash.

## 🧪 검증 / 중복
- Verified (Cheney 1970 CACM paper, V8/SpiderMonkey source).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Cheney GC algorithm 의 BFS copy + V8 scavenger 패턴 |