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Garbage Collector.md

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  • 标记清除（mark-and-sweep）
    • Bitmap marking
    • Lazy sweeping
  • 引用计数

碎片问题

  • Mark-Compact

  • Copying GC

  • Cheney

  • 三色标记

  • 分代

三色标记法

  • 黑色 (black)：表明对象被 collector 访问过，属于可到达对象
  • 灰色 (gray)：表明对象被访问过，但是它的子节点还没有被 scan 到
  • 白色 (white)：表明没有被访问到，如果在本轮遍历结束时还是白色，那么就会被收回

增加的中间状态灰色要求 mutator ==不会把黑色对象直接指向白色对象==（这称为三色不变性 tri-color invariant），collector 就能够认为黑色对象不需要在 scan，只需要遍历灰色对象即可。

image.png

上图描述了一个违法着色不变性的情况。假设 A 已经被完全地 scan，它本身被标为黑色，字节点被标为灰色，现在假设 mutator 交换了 A–>C 与 B–>D 的指针，现在指向 D 的指针只有 A，而 A 已经被完全地 scan 了，如果继续 scan 过程的话，B 会被置为黑色，C 会被重新访问，而 D 则不会被访问到，在本轮遍历后，D 由于是白色，会被错误的认为是垃圾并被回收掉。

分代 虽然对象的生命周期因应用而异，但对于大多数应用来说，80% 的对象在创建不久即会成为垃圾

Links:

  1. 垃圾回收的算法与实现
  2. Golang’s Real-time GC in Theory and Practice: https://making.pusher.com/golangs-real-time-gc-in-theory-and-practice/?spm=a2c4e.11153940.blogcont573819.32.b9e922bd8nwhPY
  3. Getting Started with the G1 Garbage Collector: https://www.oracle.com/technetwork/tutorials/tutorials-1876574.html
  4. Java HotSpot Garbage Collection: https://www.oracle.com/technetwork/java/javase/tech/index-jsp-140228.html
  5. HotSpot Virtual Machine Garbage Collection Tuning Guide: https://docs.oracle.com/en/java/javase/12/gctuning/introduction-garbage-collection-tuning.html#GUID-8A443184-7E07-4B71-9777-4F12947C8184
  6. Wilson P R. Uniprocessor garbage collection techniques[C]//International Workshop on Memory Management. Springer, Berlin, Heidelberg, 1992: 1-42.
  7. Printezis T, Detlefs D. A generational mostly-concurrent garbage collector[M]. ACM, 2000.
  8. Detlefs D, Flood C, Heller S, et al. Garbage-first garbage collection[C]//Proceedings of the 4th international symposium on Memory management. ACM, 2004: 37-48.
  9. Dijkstra E W, Lamport L, Martin A J, et al. On-the-fly garbage collection: an exercise in cooperation[J]. Communications of the ACM, 1978, 21(11): 966-975.
  10. Memory Management in the Java HotSpotTM Virtual Machine
  11. 深入浅出垃圾回收（四）分代式 GC: https://liujiacai.net/blog/2018/08/18/generational-gc/
  12. https://www.memorymanagement.org/glossary/t.html#term-tri-color-marking
  13. 关于Golang GC的一些误解--真的比Java算法更领先吗: https://mp.weixin.qq.com/s/eDd212DhjIRGpytBkgfzAg
  14. Go GC: Prioritizing low latency and simplicity: https://blog.golang.org/go15gc
  15. Hotspot的safe point: https://xhao.io/2018/03/safepoint-2/
  16. Safepoints: Meaning, Side Effects and Overheads: http://psy-lob-saw.blogspot.com/2015/12/safepoints.html
  17. Our Collectors: https://blogs.oracle.com/jonthecollector/the-unspoken-phases-of-cms
  18. The Unspoken - Phases of CMS: https://blogs.oracle.com/jonthecollector/our-collectors
  19. 深入探究 JVM | Safepoint 及 GC 的触发条件: https://www.sczyh30.com/posts/Java/jvm-gc-safepoint-condition/
  20. Hotspot的safe point: https://xhao.io/2018/03/safepoint-2/
  21. GC safe-point (or safepoint) and safe-region: http://xiao-feng.blogspot.com/2008/01/gc-safe-point-and-safe-region.html