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A look on Java Garbage Collector..

A garbage collector is responsible for
• allocating memory
• ensuring that any referenced objects remain in memory, and
• recovering memory used by objects that are no longer reachable from references in executing code.
Objects that are referenced are said to be live. Objects that are no longer referenced are considered dead and are termed garbage. The process of finding and freeing (also known as reclaiming) the space used by these objects is known as garbage collection.

Garbage collection solves many, but not all, memory allocation problems. You could, for example, create objects indefinitely and continue referencing them until there is no more memory available. Garbage collection is also a complex task taking time and resources of its own.
The precise algorithm used to organize memory and allocate and deallocate space is handled by the garbage collector and hidden from the programmer. Space is commonly allocated from a large pool of memory referred to as the heap.
The timing of garbage collection is up to the garbage collector. Typically, the entire heap or a subpart of it is collected either when it fills up or when it reaches a threshold percentage of occupancy.
The task of fulfilling an allocation request, which involves finding a block of unused memory of a certain size in the heap, is a difficult one.
We have already discussed regarding the Java Heap Memory Sizing in my previous posts, moving ahead lets discuss regarding the Java Garbage Collector Sizing for Hyperion Applications in Windows and Linux
Before you start to tune the command line arguments for Java be aware that Sun’s HotSpot™ Java Virtual Machine has incorporated technology to begin to tune itself. This smart tuning is referred to as Ergonomics. Most computers that have at least 2 CPU’s and at least 2 GB of physical memory are considered server-class machines which means that by default the settings are:
·         The -server compiler
·         The -XX:+UseParallelGC parallel (throughput) garbage collector
·         The -Xms initial heap size is 1/64th of the machine’s physical memory
·         The -Xmx maximum heap size is 1/4th of the machine’s physical memory (up to 1 GB max).
Please note that 32-bit Windows systems all use the -client compiler by default and 64-bit Windows systems which meet the criteria above will be be treated as server-class machines.
Garbage Collector Policy
The Java™ Platform offers a choice of Garbage Collection algorithms. For each of these algorithms there are various policy tunables. Instead of repeating the details of the Tuning Garbage Collection document here suffice it to say that first two choices are most common for large server applications:
·         The -XX:+UseParallelGC parallel (throughput) garbage collector, or
·         The -XX:+UseConcMarkSweepGC concurrent (low pause time) garbage collector (also known as CMS)
·         The -XX:+UseSerialGC serial garbage collector (for smaller applications and systems)

For more details please refer : http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html


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