Garbage Collection

Garbage collection (GC) is a form of automatic memory management that frees memory allocated to objects that are no longer needed by a program.

Instead of requiring the programmer to manually allocate and deallocate memory, languages such as Java, C#, Python, and JavaScript include garbage collectors that track which objects are still in use and reclaim the memory of objects that have become unreachable.

By handling memory allocation and release automatically, GC eliminates common programming problems such as memory leaks, forgetting to free memory, double-freeing objects, or leaving dangling pointers.

How Garbage Collection Works

Modern garbage collectors follow a set of general principles:

• Object Allocation on the Heap: When a program creates new objects, memory is allocated from a region called the heap. In managed runtimes, a contiguous block of memory is reserved for the managed heap, and objects are allocated sequentially.
• Roots and Reachability: Garbage collectors determine which objects are still needed by examining roots—references in static fields, local variables, and registers. Starting from these roots, the collector traces through all references to mark objects that are still reachable.
• Identifying Unreachable Objects: Objects that are not reachable from any root are considered garbage and are eligible for collection. Because the general problem of determining whether memory is truly needed is computationally undecidable, garbage collectors use heuristic algorithms that approximate this decision.
• Reclaiming Memory: Once unreachable objects are identified, their memory is reclaimed. In some systems, reachable objects are compacted to remove the gaps left by freed objects, which improves the locality of reference and speeds up future allocations.

Garbage collection runs periodically or when certain memory thresholds are reached. For instance, Python triggers GC when the number of allocations exceeds a threshold for each generation, while .NET performs a collection when memory usage surpasses an acceptable limit or when low memory notifications occur.

Garbage Collection Examples and Use Cases

GC is a core feature of many languages and runtimes:

• Java Virtual Machine (JVM): The JVM automatically runs a garbage collector that finds unused objects in the heap and deletes them. Java developers do not need to explicitly free memory; the GC runs as a background thread, removing objects that are no longer referenced.
• .NET Common Language Runtime (CLR): The CLR’s garbage collector manages the allocation and release of memory for managed code. It examines roots, builds a graph of reachable objects, frees unreachable objects, and compacts the managed heap.
• Python: Python uses a combination of reference counting and a generational garbage collector. Each object tracks the number of references pointing to it, and when this count drops to zero, the object is immediately deallocated. To handle cyclic references (objects that reference each other, preventing their reference counts from ever reaching zero), Python includes a generational garbage collector that detects and cleans them up.
• JavaScript Engines: Modern JavaScript engines use a mark-and-sweep garbage collector. They periodically start from a global object (the root), mark all reachable objects, and then collect all the objects that remain unmarked.

Types of Garbage Collection Algorithms

Several algorithms are used to implement garbage collection, each with different strategies and trade-offs:

Reference Counting: In this simple algorithm, each object maintains a counter of how many references point to it. When a reference is created, the counter increments; when a reference is removed, the counter decrements. When the count reaches zero, the object is immediately deallocated. While simple, this algorithm fails to reclaim cyclic references.

Mark-and-Sweep: This tracing algorithm operates in two phases. A mark phase starts from root references and traverses the entire object graph, marking each reachable object. A sweep phase then scans the heap for objects that remain unmarked and frees their memory. Mark-and-sweep correctly handles cycles and forms the basis of many modern collectors.

Generational Garbage Collection: This algorithm is based on the observation that most objects die young. It divides the heap into generations (e.g., young and old) and collects them separately. Young objects are collected frequently (minor GC), while older objects are collected less often (major GC), which improves performance by focusing on the regions where garbage is most likely to accumulate.

Incremental and Concurrent GC: To reduce “pause times” where an application must stop for garbage collection, some collectors operate incrementally or concurrently with program execution. Incremental collectors break the GC work into small chunks, while concurrent collectors perform most of their work without stopping the application. These techniques often build on mark-and-sweep or generational algorithms.

Related Concepts

• Manual Memory Management: In languages like C or C++, programmers manually allocate and free memory. Forgetting to free memory leads to memory leaks, while freeing memory too early creates dangling pointers. Garbage collection automates these tasks.
• Weak References: To prevent objects from being held in memory unnecessarily, some languages support weak references, which do not count toward an object’s reference count. Java’s WeakReference and JavaScript’s WeakMap and WeakSet allow objects to be referenced without preventing their collection.
• Finalizers and Destructors: Some runtimes allow objects to perform cleanup actions just before they are collected. Because finalizers can introduce unpredictability, they are typically used sparingly, and disposing of resources is often handled explicitly.

Conclusion

Garbage collection is a crucial automatic memory management process that tracks program references, identifies objects that are no longer reachable, and reclaims their memory. This frees developers from the complex and error-prone task of manual memory management.

Implementations vary across languages but generally involve tracing reachable objects (e.g., mark-and-sweep), reclaiming the rest, and sometimes organizing objects into generations to optimize performance.

Understanding garbage collection helps computer science students appreciate how high-level languages manage memory, what algorithms underlie GC, and how to write memory-efficient applications that cooperate with the collector.