Antigravity Agent
132 lines
4.1 KiB
Markdown
132 lines
4.1 KiB
Markdown
---
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id: wiki-2026-0508-write-barrier
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title: Write Barrier
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category: 10_Wiki/Topics
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status: verified
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canonical_id: self
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aliases: [write barrier, store barrier, GC write barrier]
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duplicate_of: none
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source_trust_level: A
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confidence_score: 0.9
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verification_status: applied
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tags: [garbage-collection, memory, runtime, gc, jvm]
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raw_sources: []
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last_reinforced: 2026-05-10
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github_commit: pending
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tech_stack:
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language: C++/Rust/Java
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framework: GC runtimes
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---
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# Write Barrier
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## 매 한 줄
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> **"매 reference store 의 inline 직전 추가 코드, 매 GC가 매 inter-generational pointer 또는 concurrent invariant 를 track 하기 위함"**. 매 generational / incremental / concurrent GC의 핵심 mechanism으로, 2026 modern runtime (HotSpot ZGC, OpenJDK G1, .NET, V8 Orinoco, Go) 모두 형태가 다른 write barrier를 사용.
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## 매 핵심
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### 매 왜 필요
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- Generational GC: old → young pointer 의 매 빠른 식별 (remembered set).
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- Concurrent GC: mutator + collector 동시 동작 시 invariant 유지 (SATB, Incremental Update).
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- 매 every reference store 마다 매 hooked code path.
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### 매 종류
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- **Card-marking** (G1, .NET): heap을 card (~512B) 로 분할, store 시 card byte를 dirty.
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- **Snapshot-At-The-Beginning (SATB)** (G1, ZGC): 매 overwrite 직전 old value를 mark queue에.
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- **Incremental Update** (CMS): 매 store 의 new value를 mark queue에.
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- **Read barrier** (ZGC, Shenandoah): 매 load 시점 hook (대조).
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### 매 응용
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1. 매 generational collector의 minor GC가 old gen scan 안 해도 됨.
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2. 매 concurrent marking의 lost-edge 방지.
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3. 매 region-based collector의 inter-region reference 추적.
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## 💻 패턴
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### Card-marking (pseudo C)
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```c
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// every "obj.field = val"
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void store_ref(Object* obj, int idx, Object* val) {
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obj->fields[idx] = val;
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// 매 barrier
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uintptr_t card = ((uintptr_t)obj) >> 9; // 512B
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card_table[card] = DIRTY;
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}
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```
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### SATB barrier
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```c
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void satb_store(Object* obj, int idx, Object* val) {
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Object* old = obj->fields[idx];
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if (concurrent_marking_active && old != NULL && !is_marked(old))
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enqueue_satb(old); // preserve snapshot
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obj->fields[idx] = val;
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}
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```
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### Incremental update barrier
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```c
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void inc_store(Object* obj, int idx, Object* val) {
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obj->fields[idx] = val;
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if (concurrent_marking_active && val != NULL && is_grey_or_white(val))
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enqueue_mark(val); // re-scan
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}
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```
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### Rust / unsafe 매 manual barrier (toy)
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```rust
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unsafe fn write_field(obj: *mut Obj, off: usize, val: *mut Obj) {
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let slot = (obj as *mut u8).add(off) as *mut *mut Obj;
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*slot = val;
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// card mark
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let card_idx = (obj as usize) >> 9;
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CARD_TABLE[card_idx].store(1, Ordering::Relaxed);
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}
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```
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### V8 (incremental marking write barrier)
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```cpp
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// simplified
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template<typename T>
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void StoreInObject(HeapObject host, T value) {
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host.SetSlot(offset, value);
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if (incremental_marking_running) {
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if (IsWhite(value)) MarkObject(value);
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}
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}
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```
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## 매 결정 기준
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| 상황 | GC choice |
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|---|---|
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| 매 latency-critical (low pause) | ZGC / Shenandoah (read+write barriers) |
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| 매 throughput-critical | Parallel GC (no concurrent barrier) |
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| 매 balanced | G1 (SATB + card) |
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| 매 manual memory | Rust / C++ — 매 barrier 의 X |
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**기본값**: 매 modern JVM = G1 (Java 9+ default), 매 ultra-low pause = ZGC.
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## 🔗 Graph
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- 부모: [[Garbage-Collection]] · [[Memory-Management]]
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- 변형: [[Read-Barrier]] · [[Card-Marking]] · [[SATB]]
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- 응용: [[G1-GC]] · [[ZGC]] · [[V8-Orinoco]] · [[.NET-GC]]
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- Adjacent: [[Generational-Hypothesis]] · [[Remembered-Set]] · [[Tri-Color-Marking]]
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## 🤖 LLM 활용
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**언제**: 매 GC tuning, 매 runtime 구현.
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**언제 X**: 매 application-level code — 매 mostly invisible.
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## ❌ 안티패턴
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- **매 barrier 비활성화 시도**: 매 GC invariant 깨짐 → silent heap corruption.
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- **매 barrier overhead 무시**: 매 hot loop의 매 store 마다 추가 instruction.
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## 🧪 검증 / 중복
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- Verified (Jones, Hosking, Moss — *The Garbage Collection Handbook* 2nd; OpenJDK ZGC docs 2026).
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- 신뢰도 A.
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## 🕓 Changelog
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| 날짜 | 변경 |
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|---|---|
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| 2026-05-08 | Phase 1 |
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| 2026-05-10 | Manual cleanup — write barrier types + GC examples |
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