28.5.3. GC Phases

1. Mark

In the Mark phase, the garbage collector identifies which objects in memory are still in use by marking them. This involves traversing reachable objects starting from the root references—objects that are directly accessible by the running application, such as local variables in thread stacks or static fields in classes. During this process, any object that can be accessed is tagged as 'alive' or 'marked', while objects that are not accessible are left unmarked.

Think of the Mark phase as a librarian who needs to identify which books in a library are currently checked out. The librarian goes through the shelves and marks the books that have been taken out (active), while leaving those that are simply sitting on the shelves (inactive) unmarked.

2. Sweep

During the Sweep phase, the garbage collector scans through the heap memory and frees up space by removing unmarked objects—which are considered garbage. This essentially cleans the memory area by deallocating the space occupied by these unused objects, making it available for new object allocations. Since only the unmarked (unreachable) objects are cleared, this process helps manage memory effectively and prevents leaks.

This phase can be visualized as a cleanup crew that comes into the library after the librarian has marked the checked-out books. The crew will remove all the dusty, old books that no one has checked out in years, making space for new arrivals.

3. Compact

In the Compact phase, the garbage collector reorganizes the remaining live objects in memory to eliminate gaps left by the swept objects. This compaction makes the memory more contiguous, which can improve future allocation performance since new objects can be allocated in large blocks rather than scattered throughout memory. This phase minimizes fragmentation and optimizes memory space.

Imagine a multiplayer board game where players keep leaving and re-entering the game, creating gaps on the table. The Compact phase is akin to a player organizing the game board by moving all the active pieces closer together, thereby reducing clutter and making it easier for new players to join in.

4. Evacuate (in G1)

In the context of the G1 (Garbage First) garbage collector, the Evacuate phase refers to moving live objects from one region of the heap to another in order to collect garbage more efficiently. G1 divides the heap into regions and prioritizes regions with the most garbage during collection, promoting higher efficiency. When the Evacuate phase is triggered, the live objects in selected regions are copied to other areas. This phase helps maintain performance by ensuring that the active objects are kept together while also freeing up entire regions of memory.

Consider moving items from an overflowing closet to a better-organized storage unit where the most popular items are kept at the front for easy access. Here, the Evacuate phase ensures that the most frequently accessed items are readily available while efficiently clearing out the crowded areas.

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