Mastering High Performance with Kotlin

Shallow and Retained Heap

Depending on the operating system architecture, an object needs to be either 32 bits or 64 bits per reference. This is related to the machine word, which is a minimal fixed-size piece of data that can be handled by a given CPU's instruction set. The Shallow Heap is the memory consumed by one object.

We already know that one object can hold references to other objects. This is how a reference tree is created. And when the object, X, which holds references to others objects, becomes unreachable and unreferenced, then objects that are held by object X can also become available for the GC if no other objects hold references to them. The following diagram shows this:

The set of objects, a, b, c, and d, represents a retained ...