Java Garbage collectors Practical Guide

Java Garbage collection process

The Java Garbage Collection process is responsible for automatically reclaiming memory occupied by objects that are no longer in use by the application. The process is performed by the Java Virtual Machine (JVM) and involves several steps. Here is a general overview of the Java Garbage Collection process:

1. Allocation: The JVM allocates memory to objects dynamically as they are created using the new keyword or other object creation mechanisms. The JVM divides the memory into different regions, such as the young generation and the old generation.
2. Reachability Analysis: The Garbage Collector determines which objects are reachable and which are not. It starts by considering all objects in the heap as potentially reachable and then recursively traces object references to determine their reachability.
3. Marking: In this phase, the Garbage Collector marks all reachable objects, typically starting from the root objects (e.g., static variables, method local variables, and threads’ stack frames) and following object references to mark other reachable objects. Unreachable objects remain unmarked.
4. Sweeping: The Garbage Collector sweeps through the heap, identifying and reclaiming memory occupied by unmarked (unreachable) objects. The memory is then marked as available for future allocations.
5. Compaction: In some Garbage Collection algorithms, a compaction phase may be performed after sweeping. It involves moving the remaining live objects closer together to reduce memory fragmentation and optimize memory usage.
6. Object Finalization: Objects that have a finalize() method are put in a queue for finalization. Finalization is the process of performing cleanup operations before the object is garbage collected. Finalized objects are usually collected in subsequent Garbage Collection cycles.
7. Deallocation: After the Garbage Collection process is complete, the memory occupied by unreachable objects is deallocated, making it available for future allocations.

It’s important to note that different Garbage Collection algorithms and JVM implementations may have variations in the specifics of the process. The JVM employs various Garbage Collection algorithms, such as the Serial, Parallel, CMS, and G1, which have different strategies and optimizations for efficient memory management.

JVM Garbage Collectors

Java Virtual Machine (JVM) Garbage Collectors are responsible for automatic memory management in Java applications. They reclaim memory occupied by objects that are no longer in use and deallocate them, freeing up resources for future use. There are several garbage collectors available in the JVM, each with its own characteristics and behaviors. Here are some commonly used garbage collectors:

• Serial Garbage Collector (Serial GC):
  • A single-threaded collector that performs garbage collection using a stop-the-world approach.
  • Suitable for small applications or simple command-line tools with low memory requirements.
  • It stops all application threads during garbage collection.
• Parallel Garbage Collector (Parallel GC):
  • Similar to the Serial GC, but it uses multiple threads for garbage collection, resulting in shorter pause times.
  • Suitable for applications that can benefit from parallelism and have larger heap sizes.
  • The application threads are stopped during garbage collection.
• Concurrent Mark Sweep (CMS) Garbage Collector:
  • Designed to minimize pause times by performing most of the garbage collection work concurrently with the application threads.
  • Suitable for applications where low pause times are critical, but not ideal for very large heaps or applications with limited CPU resources.
  • It uses multiple threads for the initial marking and concurrent sweeping phases.
• Garbage-First (G1) Garbage Collector:
  • Introduced in Java 7, G1 is a low-pause, server-style collector designed for large heaps.
  • It divides the heap into regions and performs garbage collection on a subset of regions, called “garbage-first” regions.
  • G1 aims to achieve both low pause times and high throughput by dynamically adapting its behavior based on the application’s needs.
• Z Garbage Collector (ZGC):
  • Introduced in Java 11, ZGC is a scalable, low-latency garbage collector designed for applications that require very short pause times.
  • It performs garbage collection concurrently with the application threads and dynamically adjusts the pause time based on the application’s needs.
  • ZGC is suitable for applications with very large heaps and stringent latency requirements.

Serial Garbage Collector (Serial GC)

The Serial Garbage Collector (Serial GC) is a simple and basic garbage collector available in the Java Virtual Machine (JVM). It is a single-threaded collector that performs garbage collection using a stop-the-world approach, meaning it pauses all application threads during garbage collection.

Here are some key details about the Serial GC:

• Use Case: The Serial GC is suitable for small applications or simple command-line tools with low memory requirements. It may not be ideal for large, multi-threaded applications or systems with strict performance requirements.
• Young Generation Collection: The Serial GC divides the heap into two generations: the young generation and the old generation. It uses a copying algorithm for the young generation. When a minor garbage collection occurs in the young generation, live objects are copied to a new space, leaving behind the dead objects.
• Stop-the-World Pauses: During garbage collection, the Serial GC pauses all application threads, including the main thread, while it performs the garbage collection tasks. This pause can result in noticeable application freezes or delays, especially in larger applications or when the heap size is large.
• Heap Size Limitations: The Serial GC may not be suitable for applications with very large heaps because the stop-the-world pauses can become significant and impact application responsiveness.
• Configuration: To enable the Serial GC explicitly, you can use the JVM command-line option -XX:+UseSerialGC. This option instructs the JVM to use the Serial GC as the garbage collector.

Here’s an example of how to enable the Serial GC in a Java application using the JVM command-line option:

java -XX:+UseSerialGC app.jar

By specifying -XX:+UseSerialGC, you instruct the JVM to utilize the Serial GC for garbage collection.

It’s important to note that the Serial GC is considered a simple and less efficient garbage collector compared to more advanced collectors like the Parallel GC, CMS, or G1. However, it can still be useful for small applications or situations where simplicity and predictable behavior are prioritized over performance.

When developing applications, it’s recommended to analyze the specific requirements and characteristics of your application to determine the most appropriate garbage collector to use. Additionally, newer versions of the JVM may have different default garbage collectors, so it’s important to consider the default behavior of the JVM you are using.

Parallel Garbage Collector (Parallel GC)

The Parallel Garbage Collector (Parallel GC) is a garbage collector available in the Java Virtual Machine (JVM) that is designed to improve garbage collection performance by utilizing multiple threads for garbage collection tasks. It is suitable for applications with larger heap sizes and can provide better throughput compared to the Serial GC. Here are some complete details about the Parallel GC:

• Use Case: The Parallel GC is suitable for applications that can benefit from parallelism and have larger heap sizes. It is often used in server-side applications where throughput is a priority over low pause times.
• Young Generation Collection: The Parallel GC uses a copying algorithm for the young generation, similar to the Serial GC. During a minor garbage collection, live objects in the young generation are copied to a new space, leaving behind the dead objects.
• Stop-the-World Pauses: Like the Serial GC, the Parallel GC also uses a stop-the-world approach for major garbage collection. During major garbage collection, all application threads, including the main thread, are paused while the garbage collection tasks are performed.
• Parallel Processing: The Parallel GC utilizes multiple threads for garbage collection tasks, which can help reduce the duration of stop-the-world pauses. By using parallelism, it can take advantage of multiple CPU cores to perform garbage collection tasks concurrently.
• Throughput Oriented: The primary goal of the Parallel GC is to achieve higher throughput by maximizing CPU utilization for garbage collection. It is optimized for applications that prioritize overall application throughput over low pause times.
• Configuration: By default, the JVM uses the Parallel GC as the garbage collector when multiple CPUs or CPU cores are available. However, you can explicitly enable the Parallel GC using the JVM command-line option -XX:+UseParallelGC.

Here’s an example of how to enable the Parallel GC in a Java application using the JVM command-line option:

java -XX:+UseParallelGC app.jar

By specifying -XX:+UseParallelGC, you instruct the JVM to utilize the Parallel GC for garbage collection.

The Parallel GC is considered a generational garbage collector, focusing on optimizing garbage collection in the young generation. It can provide good performance for applications that have large amounts of short-lived objects.

When selecting a garbage collector, it’s important to consider the specific requirements and characteristics of your application. Factors such as application size, performance requirements, available CPU resources, and latency tolerance should be taken into account. Additionally, newer versions of the JVM may introduce different garbage collectors or enhancements to existing ones, so it’s worth considering the default behavior of the JVM you are using.

Concurrent Mark Sweep (CMS) Garbage Collector

The Concurrent Mark Sweep (CMS) Garbage Collector is a garbage collector available in the Java Virtual Machine (JVM) that aims to minimize pause times by performing most of the garbage collection work concurrently with the application threads. It is designed for applications that require low pause times and responsiveness. Here are complete details about the CMS Garbage Collector:

• Use Case: The CMS Garbage Collector is suitable for applications where low pause times are critical, such as interactive applications or systems that require high responsiveness. It is commonly used in server-side applications.
• Generational Collection: The CMS Garbage Collector divides the heap into two generations: the young generation and the old generation. It uses a copying algorithm for the young generation, similar to the Serial and Parallel GCs. For the old generation, CMS uses a concurrent marking and sweeping algorithm to minimize pause times.
• Concurrent Marking: CMS performs concurrent marking by using multiple threads alongside the application threads. It marks objects that are reachable, identifying the live objects. Concurrent marking allows the application threads to continue executing while marking is in progress, reducing the impact on application responsiveness.
• Concurrent Sweeping: After the concurrent marking phase, CMS performs the sweeping phase concurrently with the application threads. It frees up memory occupied by the garbage objects, making it available for future allocation. Concurrent sweeping further minimizes pause times.
• Possible Fragmentation: CMS Garbage Collector can introduce heap fragmentation due to the concurrent nature of its operations. Fragmentation occurs when the free memory is divided into small non-contiguous blocks, making it challenging to allocate large objects.
• Configuration: To enable the CMS Garbage Collector explicitly, you can use the JVM command-line option -XX:+UseConcMarkSweepGC. This option instructs the JVM to use the CMS Garbage Collector.

Here’s an example of how to enable the CMS Garbage Collector in a Java application using the JVM command-line option:

java -XX:+UseConcMarkSweepGC app.jar

By specifying -XX:+UseConcMarkSweepGC, you instruct the JVM to utilize the CMS Garbage Collector for garbage collection.

The CMS Garbage Collector provides reduced pause times compared to the Serial and Parallel GCs. However, it may not be suitable for applications with very large heaps or applications that have limited CPU resources since concurrent garbage collection requires additional CPU overhead.

It’s worth noting that the CMS Garbage Collector has been deprecated starting from JDK 9 in favor of the Garbage-First (G1) Garbage Collector. G1 offers better overall performance and lower pause times compared to CMS. Therefore, it’s recommended to consider using G1 or other garbage collectors available in newer versions of the JVM.

When selecting a garbage collector, consider the specific requirements of your application and choose the appropriate one based on factors such as pause time requirements, throughput, available CPU resources, and heap size.

Garbage-First (G1) Garbage Collector

The Garbage-First (G1) Garbage Collector is a garbage collector available in the Java Virtual Machine (JVM) that is designed to provide high throughput and low pause times for large heap applications. It is a generational garbage collector that divides the heap into multiple regions and performs garbage collection concurrently. Here are complete details about the G1 Garbage Collector:

• Use Case: The G1 Garbage Collector is suitable for applications that require low pause times and have large heaps (typically several gigabytes in size). It is designed to handle large applications with varying object lifetimes efficiently.
• Region-Based Memory Management: G1 divides the heap into a set of equal-sized regions, where each region can be either young or old generation. The region size can vary, but it is typically a power of two and ranges from 1 MB to 32 MB.
• Generational Collection: G1 uses a generational approach similar to other garbage collectors. It performs young generation collection using a copying algorithm, and old generation collection using concurrent marking and sweeping.
• Concurrent and Incremental: G1 performs garbage collection concurrently, meaning it can run in parallel with the application threads. It also uses incremental techniques to break the garbage collection work into smaller parts, further reducing pause times.
• Region-Based Marking: G1 uses a region-based marking algorithm during the concurrent marking phase. It divides the heap into a set of regions and marks live objects in those regions. This allows for better control over pause times and reduces the need for a complete stop-the-world pause.
• Adaptive Collection: G1 dynamically adjusts the amount of work performed by the garbage collector based on application behavior and available system resources. It aims to meet specific pause time goals while maintaining high throughput.
• Configuration: The G1 Garbage Collector is enabled by default in recent versions of the JVM (JDK 9 onwards). However, you can explicitly enable it using the JVM command-line option -XX:+UseG1GC.

Here’s an example of how to enable the G1 Garbage Collector in a Java application using the JVM command-line option:

java -XX:+UseG1GC app.jar

By specifying -XX:+UseG1GC, you instruct the JVM to utilize the G1 Garbage Collector for garbage collection.

The G1 Garbage Collector aims to achieve high throughput while keeping pause times predictable and relatively short. It automatically adapts to the application’s behavior and dynamically adjusts its garbage collection strategy to meet pause time goals.

When selecting a garbage collector, consider the specific requirements and characteristics of your application. Factors such as pause time requirements, throughput, heap size, and available CPU resources should be taken into account. It’s recommended to use the default garbage collector or consider the G1 Garbage Collector if you have a large heap application with low pause time requirements.

Z Garbage Collector (ZGC)

The Z Garbage Collector (ZGC) is a garbage collector available in the Java Virtual Machine (JVM) that focuses on low pause times and scalability. It is designed to handle very large heaps (up to 16 terabytes) and provide consistent pause times, even with large heaps and high allocation rates. Here are complete details about the ZGC:

• Use Case: The ZGC is suitable for applications that require extremely low pause times and high scalability. It is designed for modern, memory-intensive applications that need to handle large heaps with high allocation rates.
• Region-Based Memory Management: Similar to the G1 Garbage Collector, ZGC divides the heap into a set of equal-sized regions. However, the size of the regions in ZGC is fixed and generally larger compared to G1. ZGC uses a 4KB region size by default.
• Concurrent Marking and Relocation: ZGC performs concurrent marking and relocation of objects. It concurrently marks live objects and relocates them to free up memory space. This concurrent approach allows the application threads to continue running with minimal pause times.
• Colored Pointers: ZGC uses colored pointers to distinguish between old and new object references. This helps in maintaining a consistent view of the heap during concurrent operations and enables efficient evacuation of objects during relocation.
• Concurrent Compaction: ZGC also performs concurrent compaction, which involves moving objects to reduce fragmentation and make better use of memory. By compacting the heap concurrently, ZGC avoids significant pauses that are typically associated with compaction in other garbage collectors.
• Low Pause Times: The primary focus of ZGC is to keep pause times consistently low, even with large heaps and high allocation rates. It achieves this by performing concurrent operations and employing techniques like load barriers to ensure correct memory access during concurrent operations.
• Configuration: The ZGC is available as an experimental feature in JDK 11 and later versions. To enable it, you can use the JVM command-line option -XX:+UnlockExperimentalVMOptions -XX:+UseZGC.

Here’s an example of how to enable the ZGC in a Java application using the JVM command-line options:

java -XX:+UnlockExperimentalVMOptions -XX:+UseZGC app.jar

By specifying -XX:+UnlockExperimentalVMOptions -XX:+UseZGC, you instruct the JVM to enable the ZGC as the garbage collector.

The Z Garbage Collector is designed to provide low pause times and scalability for applications with large heaps and high allocation rates. It is particularly well-suited for modern, memory-intensive applications. However, since it is an experimental feature, it’s important to carefully evaluate its performance and stability in your specific application context.

When selecting a garbage collector, consider the specific requirements and characteristics of your application. Factors such as pause time requirements, heap size, allocation rates, and available system resources should be taken into account. It’s recommended to thoroughly test and benchmark different garbage collectors to determine the most suitable one for your application.