Understanding Garbage Collection

What is Garbage Collection?

In programming, memory management is crucial. When a program runs, it allocates memory for variables, objects, and data structures. If this memory isn't properly released when no longer needed, it can lead to memory leaks, eventually slowing down or crashing the application.

Garbage Collection (GC) is an automatic memory management process that reclaims memory occupied by objects that are no longer reachable or "in use" by the program. Instead of developers manually allocating and deallocating memory (like in C or C++), languages with GC (like Java, Python, JavaScript, C#) handle this automatically, simplifying development and reducing common memory-related bugs.

Memory Basics: Heap vs. Stack

To understand GC, it's helpful to know about two primary memory areas:

• Stack: Used for static memory allocation. It stores local variables, function call information, and primitive data types. Memory is allocated and deallocated in a LIFO (Last-In, First-Out) manner.
• Heap: Used for dynamic memory allocation. Objects and complex data structures are typically stored here. Memory on the heap must be explicitly managed (either manually or by a GC) because its lifetime is not tied to function calls.

The Core Idea: Reachability

Garbage collectors operate on the principle of reachability. An object is considered "live" (and thus should not be collected) if it can be reached by the program through a chain of references starting from a "root" set.

The root set typically includes:

• Local variables on the stack.
• Static variables (global variables).
• CPU registers.

Any object that is not reachable from this root set is considered "garbage" and eligible for collection.

Garbage Collection Lifecycle (Mark-and-Sweep)

While there are several GC algorithms, the fundamental "Mark-and-Sweep" algorithm illustrates the core lifecycle phases:

1. Mark Phase

The GC starts from the root set and traverses all reachable objects by following references. It "marks" all objects it encounters as live. Unmarked objects are considered unreachable.

2. Sweep Phase

After marking, the GC iterates through the entire heap. Any object that was not marked as live during the mark phase is considered garbage. Its memory is reclaimed and added to a list of free memory blocks, making it available for future allocations.

3. Compaction Phase (Optional but Important)

After sweeping, the heap can become fragmented, meaning live objects are scattered, leaving small, unusable gaps of free memory. Compaction moves live objects together, defragmenting the heap and creating larger contiguous blocks of free space. This improves allocation efficiency and can reduce memory usage.

Garbage Collection Process Flow

This flowchart summarizes the typical steps involved in a garbage collection cycle.