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Balanced GC – Why, What & How
Parameswaran Selvam
IBM Runtimes – J9 VM
J9 VM Knowledge Sharing Session
Balanced GC
§WHY?
– Application Pause Time increases as Java Heap size increases with existing GCpolicies
● Applications with Heap sizes > 20 GB are common
– A GC policy with short & constant Pause time, though not guaranteed
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0.5
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RealityExpected
Heap Size
Pau
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ime
Balanced GC
§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 orEden space
Java Heap : Regions belong to different generations set
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Balanced GC
§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 doesn’t yield returns or When application explicitlyinvoked
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Balanced GC
§WHAT is?
– Partial Garbage Collect (PGC)
● Stop The World(STW) GC execution, hence application threads executionstopped
● Picks up the regions belong/tag to eden space + other regions(which areidentified by GMP as regions contains more dead objects/memoryreclaimed)
● 2 algorithms/methods used
– Copy Forward– Mark & Compact
● PGC picks up a method based on heap availability
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Balanced GC
§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 PGC6
Balanced GC
§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
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Eden full – PGC triggered
PGC – Mark Compact
PGC compact
Before PGC
After PGC
Balanced GC
§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 GMPcompletes
● Makes use of idle processor time to do concurrent mark
Next PGC takes up GMPconstructed live objects dataset
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Balanced GC
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§ 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 verboseGC data
Balanced GC
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§ How to?
10
Q & A
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