Android: what is the differences between shallow and retained heap

From Nikita Salnikov-Tarnovski's blog:

The shallow heap is easy – it consists of only the heap occupied by the object itself. There are some nuances to how to calculate it, but for the scope of this article we leave it as is. Stay tuned for future posts on the same topic.

The retained heap is in many ways more interesting. Only rarely are you interested in the shallow heap, in most cases your actual question can be translated to “If I remove this object from the memory, how much memory can now be freed by the garbage collector”.

Now, as we all remember, all Java garbage collection (GC) algorithms follow this logic:

1)There are some objects which are considered “important” by the GC. These are called GC roots and are (almost) never discarded. They are, for example, currently executing method’s local variables and input parameters, application threads, references from native code and similar “global” objects.

2)Any objects referenced from those GC roots are assumed to be in use and hence not discarded by the GC. One object can reference another in different ways in Java, in the most common case an object A is stored in a field of an object B. In such case we say “B references A”.

3)The process is repeated until all objects that can be transitively reached from GC roots are visited and marked as “in use”.

4)Everything else is unused and can be thrown away.

From eclipse memory analyser docs

Shallow vs. 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.

Retained set of X is the set of objects which would be removed by GC when X is garbage collected.

Retained heap of X is the sum of shallow sizes of all objects in the retained set of X, i.e. memory kept alive by X.

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

The retained set for a leading set of objects, such as all objects of a particular class or all objects of all classes loaded by a particular class loader or simply a bunch of arbitrary objects, is the set of objects that is released if all objects of that leading set become unaccessible. The retained set includes these objects as well as all other objects only accessible through these objects. The retained size is the total heap size of all objects contained in the retained set.

The Minimum Retained Size gives a good (under)estimation of the retained size which is calculated ways faster than the exact retained size of a set of objects. It only depends on the number of objects in the inspected set, not the number of objects in the heap dump.

In Simple words Shallow heap of an object is its size in the heap and retained size of the same object is the amount of heap memory that will be freed when the object is garbage collected.More Details