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Garbage Collection — Refresher

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tagsjava, jvm, refresher, flashcards

2 min read

3-minute skim. Full detail → Garbage Collection — Full Notes.

The 10 facts

  1. GC decides garbage by reachability (trace from GC roots), NOT recency.
  2. GC roots: all threads’ stack locals, static fields, active threads, JNI.
  3. Generational hypothesis: most objects die young → young gen collected often/cheap.
  4. Young gen = Eden + 2 Survivors; survive tenuring threshold (max 15) → promoted to old gen.
  5. Mark → Sweep → Compact. Compaction removes fragmentation → fast bump-pointer allocation.
  6. Minor = young; Major = old; Full = whole heap + metaspace. Triggered by pressure, not a fixed count.
  7. STW = GC freezes all app threads. Big heap → long pause → why concurrent collectors exist.
  8. G1 = heap regions, collects most-garbage-first, pause target; default since JDK 9.
  9. ZGC (colored pointers, 64-bit, Oracle) vs Shenandoah (Brooks pointers, Red Hat) — both concurrent, sub-ms.
  10. PermGen → Metaspace (Java 8): metadata moved to auto-growing native memory; killed OutOfMemoryError: PermGen space.

Collectors by JDK

Serial (1.3) · Parallel (1.4, default 5–8) · CMS (1.4, removed 14) · G1 (default 9) · ZGC (11, prod 15) · Shenandoah (12, Red Hat) · Epsilon no-op (11).

Drill cards

How does GC decide what’s garbage?::Reachability — trace from GC roots; unreachable = garbage. Not recency.
Generational hypothesis?::Most objects die young → collect young gen often/cheap, promote rare survivors to old gen (collected rarely).
Mark-Sweep-Compact — why compact?::Compaction slides survivors together, removing fragmentation, so allocation is a fast bump pointer.
G1 vs ZGC?::Both region-based; G1’s compaction is stop-the-world (pauses grow with heap), ZGC compacts concurrently (sub-ms, any heap size).
ZGC vs Shenandoah?::ZGC = colored pointers (Oracle, 64-bit); Shenandoah = Brooks forwarding pointers (Red Hat). Both concurrent sub-ms.
Why Metaspace over PermGen?::PermGen was fixed-size and threw OOM on class-heavy apps; Metaspace uses auto-growing native memory. Doesn’t grow the heap.
Is GC needed with huge RAM?::Yes — it reclaims unreachable objects regardless of RAM; big heaps make low-pause concurrent collectors MORE necessary.