This section describes how to adapt and tune the Garbage-First garbage collector (G1 GC) for evaluation, analysis and performance.

As described in the section Garbage-First Garbage Collector, the G1 GC is a regionalized and generational garbage collector, which means that the Java object heap (heap) is divided into a number of equally sized regions. Upon startup, the Java Virtual Machine (JVM) sets the region size. The region sizes can vary from 1 MB to 32 MB depending on the heap size. The goal is to have no more than 2048 regions. The eden, survivor, and old generations are logical sets of these regions and are not contiguous.

The G1 GC has a pause time target that it tries to meet (soft real time). During young collections, the G1 GC adjusts its young generation (eden and survivor sizes) to meet the soft real-time target. See the sections Pauses and Pause Time Goal in Garbage-First Garbage Collector for information about why the G1 GC takes pauses and how to set pause time targets.

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During mixed collections, the G1 GC adjusts the number of old regions that are collected based on a target number of mixed garbage collections, the percentage of live objects in each region of the heap, and the overall acceptable heap waste percentage.

The G1 GC reduces heap fragmentation by incremental parallel copying of live objects from one or more sets of regions (called Collection Sets (CSet)s) into one or more different new regions to achieve compaction. The goal is to reclaim as much heap space as possible, starting with those regions that contain the most reclaimable space, while attempting to not exceed the pause time goal (garbage first).

The G1 GC uses independent Remembered Sets (RSets) to track references into regions. Independent RSets enable parallel and independent collection of regions because only a region's RSet must be scanned for references into that region, instead of the whole heap. The G1 GC uses a post-write barrier to record changes to the heap and update the RSets.

Garbage Collection Phases

Apart from evacuation pauses (see the section Allocation (Evacuation) Failure in Garbage-First Garbage Collector) that compose the stop-the-world (STW) young and mixed garbage collections, the G1 GC also has parallel, concurrent, and multiphase marking cycles. G1 GC uses the snapshot-at-the-beginning (SATB) algorithm, which logically takes a snapshot of the set of live objects in the heap at the start of a marking cycle. The set of live objects also includes objects allocated since the start of the marking cycle. The G1 GC marking algorithm uses a pre-write barrier to record and mark objects that are part of the logical snapshot.

Mixed Garbage Collections

Upon successful completion of a concurrent marking cycle, the G1 GC switches from performing young garbage collections to performing mixed garbage collections. In a mixed garbage collection, the G1 GC optionally adds some old regions to the set of eden and survivor regions that will be collected. The exact number of old regions added is controlled by a number of flags (see 'Taming Mixed Garbage Collectors' in the section Recommendations). After the G1 GC collects a sufficient number of old regions (over multiple mixed garbage collections), G1 reverts to performing young garbage collections until the next marking cycle completes.

Important Defaults

The G1 GC is an adaptive garbage collector with defaults that enable it to work efficiently without modification. Table 10-1, 'Default Values of Important Options for G1 Garbage Collector' lists of important options and their default values in Java HotSpot VM, build 24. You can adapt and tune the G1 GC to your application performance needs by entering the options in Table 10-1, 'Default Values of Important Options for G1 Garbage Collector' with changed settings on the JVM command line.

How to Unlock Experimental VM Flags

To change the value of experimental flags, you must unlock them first. You can do this by setting -XX:+UnlockExperimentalVMOptions explicitly on the command line before any experimental flags. For example:

java -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=10 -XX:G1MaxNewSizePercent=75 G1test.jar

Recommendations

When you evaluate and fine-tune G1 GC, keep the following recommendations in mind:

• Young Generation Size: Avoid explicitly setting young generation size with the -Xmn option or any or other related option such as -XX:NewRatio. Fixing the size of the young generation overrides the target pause-time goal.

• Pause Time Goals: When you evaluate or tune any garbage collection, there is always a latency versus throughput trade-off. The G1 GC is an incremental garbage collector with uniform pauses, but also more overhead on the application threads. The throughput goal for the G1 GC is 90 percent application time and 10 percent garbage collection time. Compare this to the Java HotSpot VM parallel collector. The throughput goal of the parallel collector is 99 percent application time and 1 percent garbage collection time. Therefore, when you evaluate the G1 GC for throughput, relax your pause time target. Setting too aggressive a goal indicates that you are willing to bear an increase in garbage collection overhead, which has a direct effect on throughput. When you evaluate the G1 GC for latency, you set your desired (soft) real-time goal, and the G1 GC will try to meet it. As a side effect, throughput may suffer. See the section Pause Time Goal in Garbage-First Garbage Collector for additional information.

• Taming Mixed Garbage Collections: Experiment with the following options when you tune mixed garbage collections. See the section Important Defaults for information about these options:

  • -XX:InitiatingHeapOccupancyPercent: Use to change the marking threshold.

  • -XX:G1MixedGCLiveThresholdPercent and -XX:G1HeapWastePercent: Use to change the mixed garbage collection decisions.

  • -XX:G1MixedGCCountTarget and -XX:G1OldCSetRegionThresholdPercent: Use to adjust the CSet for old regions.

Overflow and Exhausted Log Messages

When you see to-space overflow or to-space exhausted messages in your logs, the G1 GC does not have enough memory for either survivor or promoted objects, or for both. The Java heap cannot because it is already at its maximum. Example messages:

• 924.897: [GC pause (G1 Evacuation Pause) (mixed) (to-space exhausted), 0.1957310 secs]

• 924.897: [GC pause (G1 Evacuation Pause) (mixed) (to-space overflow), 0.1957310 secs]

To alleviate the problem, try the following adjustments:

• Increase the value of the -XX:G1ReservePercent option (and the total heap accordingly) to increase the amount of reserve memory for 'to-space'.

• Start the marking cycle earlier by reducing the value of -XX:InitiatingHeapOccupancyPercent.

• Increase the value of the -XX:ConcGCThreads option to increase the number of parallel marking threads.

See the section Important Defaults for a description of these options.

Humongous Objects and Humongous Allocations

For G1 GC, any object that is more than half a region size is considered a humongous object. Such an object is allocated directly in the old generation into humongous regions. These humongous regions are a contiguous set of regions. StartsHumongous marks the start of the contiguous set and ContinuesHumongous marks the continuation of the set.

Before allocating any humongous region, the marking threshold is checked, initiating a concurrent cycle, if necessary.

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Dead humongous objects are freed at the end of the marking cycle during the cleanup phase and also during a full garbage collection cycle.

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To reduce copying overhead, the humongous objects are not included in any evacuation pause. A full garbage collection cycle compacts humongous objects in place.

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Because each individual set of StartsHumongous and ContinuesHumongous regions contains just one humongous object, the space between the end of the humongous object and the end of the last region spanned by the object is unused. For objects that are just slightly larger than a multiple of the heap region size, this unused space can cause the heap to become fragmented.

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If you see back-to-back concurrent cycles initiated due to humongous allocations and if such allocations are fragmenting your old generation, then increase the value of -XX:G1HeapRegionSize such that previous humongous objects are no longer humongous and will follow the regular allocation path.