One strength of the java 2 platform, standard edition is that it performs automatic memory management, thereby shielding the developer from the complexity of explicit memory management. This paper provides an overview of memory management in the java hotspot virtual machine. It describes the garbage collectors available to perform the memory management and gives some advice regarding choosing and configuring a collector and setting sizes for the memory areas on which the collector operates. java virtual machine definition continued: 133 for each separate platform. A java virtual machine makes this possible because it is aware of the specific instruction lengths and other particularities of the platform. The java virtual machine specification defines an abstract rather than a real machine or processor. The specification specifies an instruction set, a set of registers, a stack, a garbage heap, and a method area.

Once a java virtual machine has been implemented for a given platform, any java program which, after compilation, is called bytecode can run on that platform. 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. The g1 collector achieves high performance and pause time goals through several techniques. 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. It collects in these regions first, which usually yields a large amount of free space. As the name suggests, g1 concentrates its collection and compaction activity on the areas of the heap that are likely to be full of reclaimable objects, that is, garbage. G1 uses a pause prediction model to meet a user defined pause time target and selects the number of regions to collect based on the specified pause time target. The regions identified by g1 as ripe for reclamation are garbage collected using evacuation.

G1 copies objects from one or more regions of the heap to a single region on the heap, and in the process both compacts and frees up memory. This evacuation is performed in parallel on multi processors, to decrease pause times and increase throughput. Thus, with each garbage collection, g1 continuously works to reduce fragmentation, working within the user defined pause times. Parallelold garbage collection performs only whole heap compaction, which results in considerable pause times. It meets the set pause time target with high probability but not absolute certainty. Based on data from previous collections, g1 does an estimate of how many regions can be collected within the user specified target time. Thus, the collector has a reasonably accurate model of the cost of collecting the regions, and it uses this model to determine which and how many regions to collect while staying within the pause time target.

For more further information about using and configuring g1 please see the command line options. 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. Undesired long garbage collection or compaction pauses longer than 0.5 to 1 second g1 is planned as the long term replacement for the concurrent mark sweep collector cms.

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G1 compacts sufficiently to completely avoid the use of fine grained free lists for allocation, and instead relies on regions. This considerably simplifies parts of the collector, and mostly eliminates potential fragmentation issues. Also, g1 offers more predictable garbage collection pauses than the cms collector, and allows users to specify desired pause targets. If you are interested in helping improving g1, please try it and give feedback via openjdk and the hotspot gc [email protected] mailing list. Description of hotspot gcs: memory management in the java hotspot virtual machine white paper: link the original cms paper: printezis, t. In proceedings of the 2nd international symposium on memory management minneapolis, minnesota, united states, october 15 16, 20.

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