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Understanding Garbage Collection
This document discusses garbage collection and automatic memory management in Java. It covers the basic strategies used in garbage collection like mark and sweep. It also discusses the different garbage collectors used in HotSpot like SerialGC, ParallelGC, ConcMarkSweepGC and G1. It provides an overview of garbage collection in J9 and JRockit as well. The document also touches on alternatives to garbage collection like weak and soft references.
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Understanding Garbage Collection
- 1. Understanding Garbage Collection (aka AutomaticMemory Management) Douglas Q. Hawkins http://www.dougqh.net dougqh@gmail.com
- 2.
- 3. Leaky Abstraction 2 of3 Optimization Flags Are For Memory Management
- 4. but not about tuningper se ... ...
- 5. HotSpot Collectors Young Old -XX:+UseSerialGC -XX:+UseParNewGC -XX:+UseParallelGC -XX:+UseConcMarkSweepGC-XX:+UseParallelOldGC Serial Parallel Scavenge Serial Old (Mark Compact) Concurrent Mark & Sweep Parallel Old (Mark Compact) Parallel
- 6. Topics Basic Garbage CollectionStrategies & Terminology Garbage Collection in HotSpot Garbage Collection in J9 and JRockit Alternatives to Garbage Collection Azul C4
- 7.
- 8. Tasks for aTracing GC Identify Live Objects Remove Dead Objects Optionally, Coalesce Free Space (Compaction)
- 9. Stack Frames Mark &Sweep Static Variables System.in System.out
- 10.
- 11.
- 12.
- 13. Tracing Problem Random Walkof Heap Cache Hostile Bad Idea on Large Heap
- 14.
- 15. Generational Young Old / Tenured StackFrames Static Variables System.in System.out
- 16. Remembered Set Young Old Stack Frames StaticVariables System.in System.out
- 17.
- 18. HotSpot Garbage Collector YoungGeneration Old Generation Eden S1S0
- 19. Young vs TenuredCollectionsBytesSurviving Bytes Allocated Young Tenured
- 20. Which Tracing Strategy? Low (YoungRegion) Copy High (Tenured Region) Mark & Sweep OR Mark Compact Liveness Strategy
- 21. Object Allocation Young Generation OldGeneration Eden S1S0 TLAB
- 22. Minor Collection Young Generation OldGeneration Eden S1S0 TLAB 1 2
- 23. Major Collection Young Generation OldGeneration Eden S1S0
- 24. Permanent Generation Young Generation OldGeneration Eden S1S0 Permanent Generation Bootstrap App Server Web Application C C C C C C C C
- 25.
- 26. Tuning Newer JVMs canadaptively tune, but you may not want to rely on that. Steady State
- 27. 8x Newer JVMs canautomatically tune, but you may not want to rely on that. -XX:MaxGCPauseMillis -XX:GCTimeRatio Meets these goals by adjusting the heap size. Adaptive Tuning
- 28. Maximize Collection duringMinor GCs Avoid Major GCs at much as possible Promote as little as possible Maximize Available Memory Adjust for 2 of 3 GC Goals Throughput Latency Footprint Basic Principles
- 29. Stops region resizing whichwould require a full GC Set Xms = Xmxinitial heap size max heap size 3-4x Young (Xmn): 1-1.5x Old: 2-3x
- 30. Set XX:PermSize = XX:MaxPermSize Again,stops region resizing which would require a full GC 1.2-1.5x
- 31. GC Choices Serial ParallelVS. Stopthe World Concurrent Compacting Non-compacting VS. Full VS. Incremental
- 32. Application Thread GCThread ParallelSerial Stop the World!
- 33. Why Stop theWorld? 0x0C 0x0C 0x0C 0x04 0x04 0x04
- 34.
- 35. Parallel Mark Compact Application ThreadGC Thread Concurrent Mark & Sweep Concurrent Marking Remark Concurrent Sweep
- 36. HotSpot Collectors Young Old -XX:+UseSerialGC -XX:+UseParNewGC -XX:+UseParallelGC -XX:+UseConcMarkSweepGC-XX:+UseParallelOldGC Serial Parallel Scavenge Serial Old (Mark Compact) Concurrent Mark & Sweep Parallel Old (Mark Compact) Parallel
- 37. Application Thread GCThread Throughput vs Latency
- 38. If Latency isToo High, Switch to ConcMarkSweep CMS kicks in when old generation is 65% full. Increase Old Generation an additional 25-30% When compaction is needed it is serial, so avoid compaction at all costs!
- 39.
- 40.
- 41.
- 42. Hybrid Mark Sweep& Copy To From
- 43.
- 44. G1:Garbage First &Balanced
- 45. G1 & BalancedGC Unused Young Old
- 46. G1 - MinorCollection Unused Young Old
- 47. Unused Young Old G1- Major Collection
- 48. Azul C4 Continuously ConcurrentCompacting Collector To From http://www.infoq.com/articles/azul_gc_in_detail
- 49.
- 50. WeakReference WR WeakReference<Foo> fooRef = newWeakReference<Foo>(foo); Foo foo = fooRef.get();
- 51. SoftReference SR SoftReference<Foo> fooRef = newSoftReference<Foo>(foo); Foo foo = fooRef.get(); Garbage Collected if Low on Space
- 52. Be Careful WithReferences Reference<User> ref = ... if ( ref.get() != null ) { System.out.println(ref.get().getName()); } Possible NullPointerException Reference<User> ref = ... User user = ref.get(); if ( user != null ) { System.out.println(user.getName()); }
- 53. ConcurrentMap<Key, Graph> graphs= new MapMaker() .concurrencyLevel(4) .weakKeys() .maximumSize(10000) .expireAfterWrite(10, TimeUnit.MINUTES) .makeComputingMap( new Function<Key, Graph>() { public Graph apply(Key key) { return createExpensiveGraph(key); } }); Guava MapMaker & CacheBuilder
- 54.
- 55. Direct ByteBuffers ByteBuffer buffer= ByteBuffer.allocateDirect(...);
- 56. Additional Reading The GarbageCollection Handbook By Richard Jones, Antony Hoskin, Eliot Moss http://www.amazon.com/The-Garbage-Collection-Handbook-Management/ Memory Management in the HotSpot Java Virtual Machine http://java.sun.com/j2se/reference/whitepapers/memorymanagement_whitepaper.pdf By Charlie Hunt and Binu John Java Performance http://www.amazon.com/Java-Performance-Charlie-Hunt/ http://www.infoq.com/presentations/JVM-Performance-Tuning-twitter By Attila Szegedi Everything I Ever Learned About JVM Performance Tuning
- 57. Questions? Thanks, please fillout your evaluations! Douglas Q. Hawkins http://www.dougqh.net dougqh@gmail.com