The Garbage-First Garbage Collector

The Garbage-First (G1) garbage collector is fully supported in Oracle JDK 7 update 4 and later releases. The G1 collector is a server-style garbage collector, targeted for multi-processor machines with large memories. It meets garbage collection (GC) pause time goals with high probability, while achieving high throughput. Whole-heap operations, such as global marking, are performed concurrently with the application threads. This prevents interruptions proportional to heap or live-data size.

 The heap is partitioned into a set of equal-sized heap regions, each a contiguous range of virtual memory. G1 performs a concurrent global marking phase to determine the liveness of objects throughout the heap. After the mark phase completes, G1 knows which regions are mostly empty. It collects in these regions first, which usually yields a large amount of free space. This is why this method of garbage collection is called Garbage-First.


Recommended Use Cases for G1


The first focus of G1 is to provide a solution for users running applications that require large heaps with limited GC latency. This means heap sizes of around 6GB or larger, and stable and predictable pause time below 0.5 seconds. Applications running today with either the CMS or the ParallelOld garbage collector would benefit switching to G1 if the application has one or more of the following traits. More than 50% of the Java heap is occupied with live data. The rate of object allocation rate or promotion varies significantly. Undesired long garbage collection or compaction pauses (longer than 0.5 to 1 second)

Garbage First (G1) Garbage Collection Options
Option and Default ValueDescription
-XX:+UseG1GCUse the Garbage First (G1) Collector
-XX:MaxGCPauseMillis=nSets a target for the maximum GC pause time. This is a soft goal, and the JVM will make its best effort to achieve it.
-XX:InitiatingHeapOccupancyPercent=nPercentage of the (entire) heap occupancy to start a concurrent GC cycle. It is used by GCs that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (e.g., G1). A value of 0 denotes 'do constant GC cycles'. The default value is 45.
-XX:NewRatio=nRatio of old/new generation sizes. The default value is 2.
-XX:SurvivorRatio=nRatio of eden/survivor space size. The default value is 8.
-XX:MaxTenuringThreshold=nMaximum value for tenuring threshold. The default value is 15.
-XX:ParallelGCThreads=nSets the number of threads used during parallel phases of the garbage collectors. The default value varies with the platform on which the JVM is running.
-XX:ConcGCThreads=nNumber of threads concurrent garbage collectors will use. The default value varies with the platform on which the JVM is running.
-XX:G1ReservePercent=nSets the amount of heap that is reserved as a false ceiling to reduce the possibility of promotion failure. The default value is 10.
-XX:G1HeapRegionSize=nWith G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb.

 Links
http://www.oracle.com/technetwork/java/javase/tech/g1-intro-jsp-135488.html

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