Understanding (a bit of) G1 GC

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Understanding (a bit of) G1 GC Case Study: ETS Worker Miftah Fathudin Helmi Fitria Nugroho

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Runtime arguments

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G1GC (Garbage First Garbage Collector) is the low latency garbage collection algorithm included in recent versions of both OpenJDK and Oracle Java G1GC

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GC Epoch

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Generational collectors are based on the following statements both being mostly true: ● Data that is very young will not survive long ● Data that is old will continue to exist Generational Collector

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Heap Types

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Eden ● Consists of objects allocated since the current epoch started ● Resized for each epoch to be between -XX:G1NewSizePercent (default 5) and -XX:G1MaxNewSizePercent (default 60) ● All new objects are allocated in Eden, except Humongous objects ● Empty at the beginning of each epoch

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Survivor ● Survivor From consists of objects that have survived at least one epoch ● Survivor To is allocated but empty during the epoch ● Each object in survivor has a counter for the number of epochs survived ● Objects surviving long enough get promoted to Tenured ● Survivor To space is resized as a ratio (-XX:SurvivorRatio, default 8) of the current Eden size

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Tenure ● Consists of Working set + Humongous objects + fragmentation (Reclaimable and Un-reclaimable heap waste) ● Garbage-collected in batches during qualifying epochs

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Region ● Region sizes are not changeable and must be a power of two between 1MB - 32MB, inclusive ● If not explicitly set on the command line via -XX:G1HeapRegionSize=#m, the region size will be set such that there are the optimal 2k or more regions on startup

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Humongous Objects ● Humongous object size ≥ (G1HeapRegionSize/2) ● They’re allocated in contiguous regions of Free space ● They’re instantly added to Tenured

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Minor GC Event Lifecycle

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Triggers of GC Events ● Eden full ● Free space cannot accommodate a Humongous object ● Humongous object allocated successfully and GC event conditions met ● Externally initiated (jcmd, jmap, Runtime.gc())

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GC Event Types ● Minor: Eden + Survivor From -> Survivor To ● Mixed: Minor + (# reclaimable Tenured regions / -XX:G1MixedGCCountTarget) regions of Tenured ● Full GC: All regions evacuated ● Minor/Mixed + To-space exhaustion: Minor/Mixed + rollback + Full GC

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Conclusion ● Don’t just copy paste configs ● Load test not only throughput