What is jvm heap size

The heap dump can show whether large objects were retained for a long time. Sometimes, to find the cause of retention, you need to understand the application code and the circumstances that caused the out-of-memory error. The problem might be caused by a memory leak in the application code; otherwise, the problem is probably inadequate memory allocated for the application's peak loads.

After you've identified the source of the large retention, view the paths from the garbage collection roots to see what is keeping the objects alive. The garbage collection roots are objects outside the heap and therefore are never collected. The path to the garbage collection roots shows the reference chain that prevents the object on the heap from being garbage collected. The article 10 Tips for using the Eclipse Memory Analyzer offers advice for analyzing heap dumps and memory leaks in the Eclipse Memory Analyzer.

Also, see How do I analyze a Java heap dump for more details on analyzing the heap dump. If the application code is not the problem, increase the size of the Java heap to meet the load requirement. Production environments often set the -Xms and -Xmx options to the same value so that the heap size is fixed and pre-allocated to the JVM.

See Oracle's list of java command options for more information about different JVM options and their use. Many programmers figure out the maximum heap value for their application's JVM correctly but discover that the JVM is using even more memory.

By Alvin Alexander. Last updated: August 23, Java memory test - How to consume all the memory RAM on a computer. Java stack and heap definitions.

How to show memory usage in a running Scala application. The mostly concurrent mark and sweep strategy often simply called concurrent garbage collection allows the Java threads to continue running during large portions of the garbage collection.

The threads must however be stopped a few times for synchronization. The mostly concurrent mark phase is divided into four parts:. The mostly concurrent sweep phase consists of four parts:. The parallel mark and sweep strategy also called the parallel garbage collector uses all available CPUs in the system for performing the garbage collection as fast as possible. All Java threads are paused during the entire parallel garbage collection.

The nursery, when it exists, is garbage collected with a special garbage collection called a young collection. A garbage collection strategy which uses a nursery is called a g enerational garbage collection strategy , or simply generational garbage collection. The young collector used in the JRockit JVM identifies and promotes all live objects in the nursery that are outside the keep area to the old space.

This work is done in parallel using all available CPUs. The Java threads are paused during the entire young collection. By default, the JRockit JVM uses a dynamic garbage collection mode that automatically selects a garbage collection strategy to use, aiming at optimizing the application throughput.

You can also choose between two other dynamic garbage collection modes or select the garbage collection strategy statically. The following dynamic modes are available:.

For more information on how to select the best mode or strategy for your application, see Selecting and Tuning a Garbage Collector. This means that the heap may become fragmented after a garbage collection, so that the free spaces in the heap are many but small, making allocation of large objects hard or even impossible.

Free spaces that are smaller than the minimum thread local area TLA size can not be used at all, and the garbage collector discards them as dark matter until a future garbage collection frees enough space next to them to create a space large enough for a TLA. To reduce fragmentation, the JRockit JVM compacts a part of the heap at every garbage collection old collection.

Compaction moves objects closer together and further down in the heap, thus creating larger free areas near the top of the heap. The size and position of the compaction area as well as the compaction method is selected by advanced heuristics, depending on the garbage collection mode used.

You may observe the decreasing number of objects when garbage collection occurs automatically or it is forced with the help of the corresponding toolbar button. All individual objects, as well as sets of objects have their shallow and retained sizes.

Shallow size of an object is the amount of allocated memory to store the object itself, not taking into account the referenced objects.

Shallow size of a regular non-array object depends on the number and types of its fields. Shallow size of an array depends on the array length and the type of its elements objects, primitive types. Shallow size of a set of objects represents the sum of shallow sizes of all objects in the set. Retained size of an object is its shallow size plus the shallow sizes of the objects that are accessible, directly or indirectly, only from this object.

In other words, the retained size represents the amount of memory that will be freed by the garbage collector when this object is collected. In general, retained size is an integral measure, which helps to understand the structure clustering of memory and the dependencies between object subgraphs, as well as find potential roots of those subgraphs.

Dead objects are shown only with shallow size, as they do not actually retain any other objects. YourKit uses cookies and other tracking technologies to improve your browsing experience on our website, to show you personalized content and targeted ads, to analyze our website traffic, and to understand where our visitors are coming from. By browsing our website, you consent to our use of cookies and other tracking technologies in accordance with the Privacy Policy.