Garbage First Collectors

Garbage First Garbage Collector Algorithm Basics Generational Collectors Serial/Parallel/Concurrent Garbage First Garbage Collector Java Garbage Collectors Weak Generational Hypothesis Generational Detects Not Used Objects Reclaims memory Compacts HEAP Basics Serial? Garbage First String a = "A"; String b = "B"; String c = "C"; String d = "D"; String e = "E"; A B C D E STACK a d e b c HEAP A B C D E STACK a d e b c HEAP Young Generation Old Generation Eden Majority of new objects allocated here After GC, the surviving objects are moved to one of the Survivor Spaces Survivor Space After GC in the Eden space, objects are piled up into the Survival space Once a Survival space is full, surviving objects are moved to the other Survival space. Then the Survival space which was full will be treated as empty. Objects that survived GC-s in the Young Generation a number of times are moved to the old generation. Survivor Space Different algorithms SerialGC (-XX:+UseSerialGC) Paralell GC (-XX:UseParalellGC) Paralell Old GC (-XX:UseParallelOldGC) CMS GC (-XX:UseConcMarkSweepGC) Garbage First GC (-XX:UseG1GC) Generational Garbage Collection Before GC During GC After GC HEAP Regions and Chunks Remembered Set for Region 0 Remembered Set for Region 1 Remembered Set for Region 2 After Marking Phase During Concurrent Marking... Marking Bitmaps in Action Garbage Collectors Most objects soon become unreachable. References from old objects to young objects only exists in small numbers. Garbage Collector Remembered Set Initial Mark Concurrent Maintenance Using write barriers Pauses the world Creates a logical snapshot of the HEAP Copying TAMS pointers Final Marking Pause Evacuation Pauses the world Copying survivor objects Can evacuate to different regions depending on age Mark Pauses the world Using write barrier Heuristics Determining Collections Sets Based on Costs Prediction Guarantees Pause Time Goal A D E STACK a d e b c HEAP Parallel? Concurrent? Top At Mark Start (TAMS) Saved at the beginning of Marking phase After this point all objects treated as LIVE The goal of marking phase is to mark live objects before this TAMS point Evacuation explained Pure Garbage First Soft Realtime GC Predictable Pauses Write Barrier explained Thank you for watching! b = null; c = null; (C) 2012 peter.varga@drotposta.hu DPC Consulting