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2026-05-20 23:52:15 +09:00

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title

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

V8 young-gen scavenger (Node.js, Chrome).
OCaml minor heap collection.
SBCL, MLton 의 default GC.

💻 패턴

Core Cheney loop (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

// 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)

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)

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

// 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 · Node.js
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

4.6 KiB Raw Blame History Unescape Escape