What are the features of the Garbage Collector?

Garbage collection (GC) is an automatic memory management process that frees memory occupied by objects that are no longer in use by the program.

Main methods of garbage collection:

1. Reference counting:

   • Each object has a counter that tracks the number of references to it.
   • When a reference is created, the counter increases.
   • When a reference is removed, the counter decreases.
   • When the counter reaches zero, the object is unreachable and its memory can be reclaimed.
   • Problems: Cannot handle cyclic references.

2. Tracing collectors:

   • Works in two phases: marking and sweeping or compacting.
   • Marking: Starts from root set (global variables, stack objects), marks all reachable objects.
   • Sweeping: Unmarked objects are considered garbage and their memory is reclaimed.
   • Compacting: Moves live objects to one end to reduce fragmentation.
   • Advantages: Handles cyclic references.
   • Disadvantages: Can cause pause times during collection.

Common algorithms:

• Mark-and-sweep: Mark live objects, then sweep.
• Mark-and-compact: Mark, then move objects.
• Copying: Divide heap into two halves, copy live objects from one to the other.
• Generational: Divide heap into young and old generations, collect young more frequently.

Modern GC implementations (like G1, ZGC) aim to minimize pause times.

The choice of algorithm affects performance and pause times. Most modern languages and runtimes (Java, C#) use generational, tracing collectors.