The Parallel Collector (original) (raw)

Parallel Collector Ergonomics

The parallel collector is selected by default on server-class machines. In addition, the parallel collector uses a method of automatic tuning that allows you to specify specific behaviors instead of generation sizes and other low-level tuning details. You can specify maximum garbage collection pause time, throughput, and footprint (heap size).

• Maximum Garbage Collection Pause Time: The maximum pause time goal is specified with the command-line option -XX:MaxGCPauseMillis= <N>. This is interpreted as a hint that pause times of <N> milliseconds or less are desired; by default, there is no maximum pause time goal. If a pause time goal is specified, the heap size and other parameters related to garbage collection are adjusted in an attempt to keep garbage collection pauses shorter than the specified value. These adjustments may cause the garbage collector to reduce the overall throughput of the application, and the desired pause time goal cannot always be met.
• Throughput: The throughput goal is measured in terms of the time spent doing garbage collection versus the time spent outside of garbage collection (referred to as application time). The goal is specified by the command-line option -XX:GCTimeRatio= <N>, which sets the ratio of garbage collection time to application time to 1 / (1 + <N> ).
  For example, -XX:GCTimeRatio=19 sets a goal of 1/20 or 5% of the total time in garbage collection. The default value is 99, resulting in a goal of 1% of the time in garbage collection.
• Footprint: Maximum heap footprint is specified using the option -Xmx <N>. In addition, the collector has an implicit goal of minimizing the size of the heap as long as the other goals are being met.

Generation Size Adjustments

The statistics such as average pause time kept by the collector are updated at the end of each collection. The tests to determine if the goals have been met are then made and any needed adjustments to the size of a generation is made. The exception is that explicit garbage collections (for example, calls to System.gc()) are ignored in terms of keeping statistics and making adjustments to the sizes of generations.

Growing and shrinking the size of a generation is done by increments that are a fixed percentage of the size of the generation so that a generation steps up or down toward its desired size. Growing and shrinking are done at different rates. By default a generation grows in increments of 20% and shrinks in increments of 5%. The percentage for growing is controlled by the command-line option -XX:YoungGenerationSizeIncrement= <Y> for the young generation and -XX:TenuredGenerationSizeIncrement= <T> for the tenured generation. The percentage by which a generation shrinks is adjusted by the command-line flag -XX:AdaptiveSizeDecrementScaleFactor= <D>. If the growth increment is X percent, then the decrement for shrinking is X/D percent.

If the collector decides to grow a generation at startup, then there is a supplemental percentage is added to the increment. This supplement decays with the number of collections and has no long-term effect. The intent of the supplement is to increase startup performance. There is no supplement to the percentage for shrinking.

If the maximum pause time goal is not being met, then the size of only one generation is shrunk at a time. If the pause times of both generations are above the goal, then the size of the generation with the larger pause time is shrunk first.

If the throughput goal is not being met, the sizes of both generations are increased. Each is increased in proportion to its respective contribution to the total garbage collection time. For example, if the garbage collection time of the young generation is 25% of the total collection time and if a full increment of the young generation would be by 20%, then the young generation would be increased by 5%.