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J9 Balanced GC

Parameswaran Selvam
Parameswaran Selvam

Balanced GC aims to provide short and constant pause times even for large Java heaps over 20GB. It divides the heap into fixed size regions grouped into generations. Partial garbage collections (PGCs) occur when the Eden space is full and shift live objects to older generations using copy forward or mark compact. A global mark phase (GMP) identifies regions for PGCs to collect from and builds live object data for PGCs to use, allowing some marking to occur concurrently. Verbose GC logging and visualization tools can monitor Balanced GC execution.

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Balanced GC Why, What & How Parameswaran Selvam IBM Runtimes J9 VM J9 VM Knowledge Sharing Session
Balanced GC 則 WHY? Application Pause Time increases as Java Heap size increases with existing GC policies Applications with Heap sizes > 20 GB are common A GC policy with short & constant Pause time, though not guaranteed 2 512 1024 4096 8192 0 0.5 1 1.5 2 2.5 Reality Expected Heap Size PauseTime
Balanced GC 則 WHAT is? Heap Layout Region based GC method Divides Java Heap into fixed size regions Regions grouped/named into max 24 generations New Objects allocated/stored in regions tagged as 0th generation/Age 0 or Eden space Java Heap : Regions belong to different generations set 3
Balanced GC 則 WHAT is? GC Phases Partial Garbage Collect (PGC) When Eden space becomes full Global Mark Phase (GMP) When reaches certain threshold like free memory, allocation rate Global GC When PGC/GMP doesnt yield returns or When application explicitly invoked 4
Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Stop The World(STW) GC execution, hence application threads execution stopped Picks up the regions belong/tag to eden space + other regions(which are identified by GMP as regions contains more dead objects/memory reclaimed) 2 algorithms/methods used Copy Forward Mark & Compact PGC picks up a method based on heap availability 5
Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Copy Forward Same as nursery/scavenger GC in GENCON(Generational Concurrent) Scans through objects in collection regions set Shifting the live object to its next generation region promoting the object Eden full PGC triggered PGC shifts/moves object Before PGC PGC Copy Forward After PGC 6
Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Mark Compact Scans through objects in collection regions set Mark the live objects in collection set regions Compact the regions based on their generation set 7 Eden full PGC triggered PGC Mark Compact PGC compact Before PGC After PGC
Balanced GC 則 WHAT is? Global Mark Phase (GMP) Stop The World GC operation Scans through the entire Java heap to build up the live objects data PGC consumes the live objects data prepared by GMP once GMP completes Makes use of idle processor time to do concurrent mark Next PGC takes up GMP constructed live objects data set 8
Balanced GC 9 則 How to? Monitor / Study GC execution -verbose:gc enables verbose GC and writes data into JVM stderr stream -Xverbosgclog:<filename> to redirect the output to file Use Garbage Collection & Memory Visualizer tool to interpret the verbose GC data
Balanced GC 10 則 How to? 10
Q & A 13

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J9 Balanced GC

  • 1. Balanced GC Why, What & How Parameswaran Selvam IBM Runtimes J9 VM J9 VM Knowledge Sharing Session
  • 2. Balanced GC 則 WHY? Application Pause Time increases as Java Heap size increases with existing GC policies Applications with Heap sizes > 20 GB are common A GC policy with short & constant Pause time, though not guaranteed 2 512 1024 4096 8192 0 0.5 1 1.5 2 2.5 Reality Expected Heap Size PauseTime
  • 3. Balanced GC 則 WHAT is? Heap Layout Region based GC method Divides Java Heap into fixed size regions Regions grouped/named into max 24 generations New Objects allocated/stored in regions tagged as 0th generation/Age 0 or Eden space Java Heap : Regions belong to different generations set 3
  • 4. Balanced GC 則 WHAT is? GC Phases Partial Garbage Collect (PGC) When Eden space becomes full Global Mark Phase (GMP) When reaches certain threshold like free memory, allocation rate Global GC When PGC/GMP doesnt yield returns or When application explicitly invoked 4
  • 5. Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Stop The World(STW) GC execution, hence application threads execution stopped Picks up the regions belong/tag to eden space + other regions(which are identified by GMP as regions contains more dead objects/memory reclaimed) 2 algorithms/methods used Copy Forward Mark & Compact PGC picks up a method based on heap availability 5
  • 6. Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Copy Forward Same as nursery/scavenger GC in GENCON(Generational Concurrent) Scans through objects in collection regions set Shifting the live object to its next generation region promoting the object Eden full PGC triggered PGC shifts/moves object Before PGC PGC Copy Forward After PGC 6
  • 7. Balanced GC 則 WHAT is? Partial Garbage Collect (PGC) Mark Compact Scans through objects in collection regions set Mark the live objects in collection set regions Compact the regions based on their generation set 7 Eden full PGC triggered PGC Mark Compact PGC compact Before PGC After PGC
  • 8. Balanced GC 則 WHAT is? Global Mark Phase (GMP) Stop The World GC operation Scans through the entire Java heap to build up the live objects data PGC consumes the live objects data prepared by GMP once GMP completes Makes use of idle processor time to do concurrent mark Next PGC takes up GMP constructed live objects data set 8
  • 9. Balanced GC 9 則 How to? Monitor / Study GC execution -verbose:gc enables verbose GC and writes data into JVM stderr stream -Xverbosgclog:<filename> to redirect the output to file Use Garbage Collection & Memory Visualizer tool to interpret the verbose GC data