Understanding Garbage Collection Using Automatic Memory Management
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Understanding Garbage Collection Using Automatic Memory Management by Douglas Hawkins. Find him on Twitter at @dougqh and on the web at http://www.dougqh.net/. For all of your openJDK™, Java, and Azul Systems information, please find us on the web at: http://www.zuluJDK.org http://www.azulsystems.com @zuluJDK.org @azulsystems
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Understanding Garbage Collection Using Automatic Memory Management
- 1. Understanding Garbage Collection (aka Automatic Memory Management) Douglas Q. Hawkins http://www.dougqh.net dougqh@gmail.com
- 2. Why?
- 3. Leaky Abstraction 2 of 3 Optimization Flags Are For Memory Management
- 4. but not about tuning per 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 Collection Strategies & Terminology Garbage Collection in HotSpot Garbage Collection in J9 and JRockit Alternatives to Garbage Collection Azul C4
- 7. Fundamentals
- 8. Tasks for a Tracing GC Identify Live Objects Remove Dead Objects Optionally, Coalesce Free Space (Compaction)
- 9. Stack Frames Mark & Sweep Static Variables System.in System.out
- 10. Mark Compact
- 11. Allocation 4 8
- 12. Copy Collector
- 13. Tracing Problem Random Walk of Heap Cache Hostile Bad Idea on Large Heap
- 15. Generational Young Old / Tenured Stack Frames Static Variables System.in System.out
- 16. Remembered Set Young Old Stack Frames Static Variables System.in System.out
- 17. GC in Java VMs
- 18. HotSpot Garbage Collector Young Generation Old Generation Eden S1S0
- 19. Young vs Tenured CollectionsBytesSurviving Bytes Allocated Young Tenured
- 20. Which Tracing Strategy? Low (Young Region) Copy High (Tenured Region) Mark & Sweep OR Mark Compact Liveness Strategy
- 21. Object Allocation Young Generation Old Generation Eden S1S0 TLAB
- 22. Minor Collection Young Generation Old Generation Eden S1S0 TLAB 1 2
- 23. Major Collection Young Generation Old Generation Eden S1S0
- 24. Permanent Generation Young Generation Old Generation Eden S1S0 Permanent Generation Bootstrap App Server Web Application C C C C C C C C
- 26. Tuning Newer JVMs can adaptively tune, but you may not want to rely on that. Steady State
- 27. 8x Newer JVMs can automatically 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 during Minor 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 which would 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. Stop the World Concurrent Compacting Non-compacting VS. Full VS. Incremental
- 32. Application Thread GC Thread ParallelSerial Stop the World!
- 33. Why Stop the World? 0x0C 0x0C 0x0C 0x04 0x04 0x04
- 34. BJL
- 35. Parallel Mark Compact Application Thread GC 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 GC Thread Throughput vs Latency
- 38. If Latency is Too 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!
- 42. Hybrid Mark Sweep & Copy To From
- 43. New GCs in Java VMs
- 44. G1:Garbage First & Balanced
- 45. G1 & Balanced GC Unused Young Old
- 46. G1 - Minor Collection Unused Young Old
- 47. Unused Young Old G1 - Major Collection
- 48. Azul C4 Continuously Concurrent Compacting Collector To From http://www.infoq.com/articles/azul_gc_in_detail
- 49. Alternatives to Hard References & GC
- 50. WeakReference WR WeakReference<Foo> fooRef = new WeakReference<Foo>(foo); Foo foo = fooRef.get();
- 51. SoftReference SR SoftReference<Foo> fooRef = new SoftReference<Foo>(foo); Foo foo = fooRef.get(); Garbage Collected if Low on Space
- 52. Be Careful With References 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
- 55. Direct ByteBuffers ByteBuffer buffer = ByteBuffer.allocateDirect(...);
- 56. Additional Reading The Garbage Collection 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 fill out your evaluations! Douglas Q. Hawkins http://www.dougqh.net dougqh@gmail.com