Android Garbage Collection in a Nutshell

Have you ever wondered how Garbage Collection works under the hood in Android?

The term “under the hood” basically, boils down to these 4 simple questions:

1. What is GC?
2. How it works internally?
3. What happens when it fails?
4. Who is eligible for GC?

1. What is GC?

GC is a mechanism of collecting unused objects and freeing up (recycling ♺) the memory space for further usage.

2. How it works internally?

Till now, there were 5 versions of GC, in the Android OS, namely:

1. Dalvik GC (Until KitKat): “Stop the World GC”
2. ART GC (Lollipop and Marshmallow): “Generational GC”
3. ART GC (Nougat): ART/Dalvik Android Team rewrites the entire allocation process in assembly code.
4. ART GC (Oreo): “Concurrent Copying GC”
5. ART GC (Android 10 onwards): Re-introduced “Generational GC” as an extension of powerful “Concurrent Copying GC”!

Dalvik “Stop the World GC”

Until KitKat

This GC runs on : “Concurrent Mark-and-Sweep” (CMS) algorithm

Let’s understand this algorithm:

Step 1: Find Roots:

• The GC pauses all the threads in the system to find the roots (we will talk about the “roots” later in this course. At a very high level, for now, just think of it as a root node in a tree).
• This phase is not concurrent, so identifying roots takes time, and during this time the application cannot do anything.
• In this phase, one boolean flag “marked” is set to “false” by default for each and every object.

Step 2: Mark Reachable Objects:

• The objects which have a direct or indirect reference to the roots are marked as REACHABLE in this phase! (We’ll also talk about Object Reachability at later point of this article. Please, bear with me!