Garbage collection algorithms are techniques used by the Java Virtual Machine (JVM) to reclaim memory occupied by objects that are no longer reachable or in use. Different garbage collection algorithms have been developed to address various requirements such as throughput, latency, and memory overhead. Here are some common garbage collection algorithms used in the JVM:
1. Mark and Sweep Algorithm
- Description:
- The Mark and Sweep algorithm is one of the simplest garbage collection algorithms.
- It consists of two phases: marking and sweeping.
- During the marking phase, the JVM traverses the object graph starting from the root objects and marks all reachable objects.
- In the sweeping phase, the JVM iterates through the entire heap and reclaims memory occupied by unmarked (unreachable) objects.
- Characteristics:
- Simple and straightforward implementation.
- Involves a stop-the-world pause during the marking and sweeping phases, which may result in longer pauses for large heaps.
2. Copying Collection Algorithms
- Description:
- Copying collection algorithms, such as the "Copying" or "Scavenge" algorithm, divide the heap into two semi-spaces: the "from" space and the "to" space.
- During garbage collection, live objects are copied from the "from" space to the "to" space, leaving behind only unreachable objects in the "from" space.
- After copying, the roles of the "from" and "to" spaces are swapped, and the "from" space becomes the "to" space for the next garbage collection cycle.
- Characteristics:
- Efficient for collecting short-lived objects (young generation) with high garbage collection throughput.
- Reduces memory fragmentation by compacting live objects.
3. Mark-Sweep-Compact Algorithm
- Description:
- The Mark-Sweep-Compact algorithm combines the marking and sweeping phases of the Mark and Sweep algorithm with the compacting phase.
- After marking and sweeping, the compacting phase moves live objects to one end of the heap, eliminating fragmentation and reclaiming memory.
- Compacting involves updating references to the moved objects to reflect their new locations.
- Characteristics:
- Reduces memory fragmentation and improves memory locality.
- Involves additional overhead for object relocation and reference updating.
4. Generational Garbage Collection
- Description:
- Generational garbage collection divides the heap into multiple generations based on the age of objects.
- Younger objects are allocated in the "young generation," while older objects are promoted to the "old generation" (also known as the tenured generation) after surviving multiple garbage collection cycles.
- Different garbage collection algorithms may be used for each generation, with copying collection algorithms often used for the young generation and mark-sweep algorithms for the old generation.
- Characteristics:
- Improves garbage collection efficiency by focusing on the most frequently collected objects (young generation).
- Reduces the frequency and duration of garbage collection pauses for long-lived objects (old generation).
Conclusion
Garbage collection algorithms play a crucial role in managing memory and ensuring the efficient utilization of resources in Java applications. By understanding the characteristics and trade-offs of different garbage collection algorithms, developers can tune garbage collection settings to meet the requirements of their applications in terms of throughput, latency, and memory overhead.
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