Sun jdk 1.6内存管理 -实现篇 -毕玄

1. Sun JDK 1.6 内存管理实现篇毕玄 2010-10、2010-11

2. 目标 • 掌握内存管理的通常实现方法 • 了解Sun JDK是如何实现内存管理的，以及做了哪些优化 • 了解JRockit做了哪些优化

3. OS内存管理图片来源于JavaOne 2010 《where does all native memory go》 Session

8. 内存分配和回收 • allocate A B C D

9. 内存分配和回收 • allocate A B C B freelist

10. 内存分配和回收 • fragmenation A B C B freelist sorry,no space i need allocate E

11. 内存分配和回收 • Compaction A C B B free space pointer – need update pointer

12. Discussion • how to find garbage and claim it.. – let’s talk about this question first

13. GC Algorithm • Reference Counting • Tracing – Mark-Sweep – Mark-Compact – Copying

14. GC Algorithm Details • Reference Counting 图片来源于JavaOne 2010 《The Garbage Collection Mythbusters》 Session by tony,john

21. GC Algorithm Details • Reference Counting – extra space,time overhead – non-moving – cyclic garbage

22. GC Algorithm Details • Advance Reference Counting – two-bit reference counts • when max count(3) is reached,object becomes “sticky” – buffer reference updates – use a backup gc algorithm to handle cyclic garbage and “sticky” objects – complex,but still non-moving

23. GC Algorithm Details • Tracing – Mark-Sweep 图片来源于JavaOne 2010 《The Garbage Collection Mythbusters》 Session by tony,john

29. GC Algorithm Details • Tracing – Mark-Sweep – need scan all objects,so if java heap becomes larger,then gc slower – 可清除所有的Garbage – 内存碎片，分配低效 – 需要在一定的时机触发

30. GC Algorithm Details • Tracing – Mark-Compact 图片来源于JavaOne 2010 《The Garbage Collection Mythbusters》 Session by tony,john

31. GC Algorithm Details • Tracing – Mark-Compact – 没有内存碎片，分配高效 – 增加了回收需要耗费的时间 – 需要更新所有移动过的object的ref pointer – 需要在一定时机触发

32. GC Algorithm Details • Tracing – Copying(a special case) 图片来源于JavaOne 2010 《The Garbage Collection Mythbusters》 Session by tony,john

38. GC Algorithm Details • Tracing – Copying(a special case) – only need scan live objects,so gc speed only decided by lds – no fragmentation – need update object pointer – need keep an empty memory area

39. GC Algorithm Summary • Why gc need stop the world? – Mark-Sweep,Mark-Compact • if ref changes or new object created when marking; • compact need update pointer,so... – Copying • copying need update pointer,so...

41. GC in Hotspot • rootset – runtime stack – static or global variables – jni handles – jvm handles

42. GC in Hotspot • Generational GC – most objects are temp-lived • so JDK decide split heap into two generations to use gc algorithm properly Copying Mark-Sweep or Mark-Compact New Old

43. GC in Hotspot • how to stop the world – safepoint at ref change • safepoint is a memory page check • compile – when gc need execute,it submit “stop thread” request to jvm core,then jvm core set the memory page not readable – when code execute to safepoint, check the memory page if readable,if not then fail and stop

44. GC in Hotspot • YGC – allocate is slow,because need lock – how to make allocate faster? TLAB Use CAS replace lock • do u have any other ideas?

45. GC in Hotspot • YGC – how to find live objects faster? Live Maps Scan Algorithm • do u have any other ideas?

46. GC in Hotspot • YGC – how to handle object in old gen ref new gen object Write Barrier & Card table • do u have any other ideas?

47. GC in Hotspot • YGC – how to make scan and copying faster? Parallel Adaptive based on runtime feedback • do u have any other ideas?

48. GC in Hotspot • FGC – optimize for throughput • parallel – optimize for low latency • concurrent

49. GC in Hotspot • FGC – parallel – based on Mark-Compact – allocate use the bump pointer

50. GC in Hotspot • FGC – parallel – how to make mark phase faster? Parallel • do u have any other ideas?

51. GC in Hotspot • FGC – parallel – how to make compact phase faster? Partial Compact when use ParallelOldGC • do u have any other ideas?

52. GC in Hotspot • FGC – concurrent – based on mark-sweep – when concurrent gc executes,ref may be changed and new objects may be created,how to solve the question?

53. GC in Hotspot • FGC – concurrent – when concurrent gc executes,ref may be changed and new objects may be created,how to solve the question? Write Barrier & Card Table

54. GC in Hotspot • FGC – concurrent – how to mark remark phase faster? add a preclean phase • do u have any other ideas?

55. Futhermore – G1 • Region-based GC • how to behave better? garbage first hot regions

57. Futhermore – Open eyes • JRockit GC – YGC • pinned object for semi-long lived object • keep area – Mark • Two-Color for prefetching – Compaction • partial,for example some objects not be compacted because many objects ref them. – Adjust gens,size adaptive based on runtime feedback

59. how to research jvm • JDK Source codes • References

60. References • JVM References

Sun jdk 1.6内存管理 -实现篇 -毕玄

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