Monday, November 17, 2014

Eclipse Memory Analyzer: Shallow heap and Retained heap

Developers are more concerned about the size of objects when they are created. Eclipse memory analyzer gives us information about the size of the object in 2 calculations.

Shallow Heap
Retained Heap

Shallow heap is the memory consumed by one object. An object needs 32 or 64 bits (depending on the OS architecture) per reference, 4 bytes per Integer, 8 bytes per Long, etc. Depending on the heap dump format the size may be adjusted (e.g. aligned to 8, etc...) to model better the real consumption of the VM.

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

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

Dead objects are shown only with shallow size, as they do not actually retain any other objects.

Generally speaking, shallow size of an object is its "flat" size in the heap whereas the retained size of the same object is the amount of heap memory that will be freed when the object is garbage collected.

For example the shallow size of an instance of java.lang.String will not include the memory needed for the underlying char[] where as retained size includes.


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