Types of garbage collectors in HotSpot?

The HotSpot JVM offers several garbage collectors (GCs), each with different characteristics and trade-offs. Understanding these collectors is crucial for optimizing application performance. This tutorial explores the most common garbage collectors in HotSpot, their workings, and when to use them.

Introduction to HotSpot Garbage Collectors

HotSpot provides a range of garbage collectors to suit different application needs. These collectors vary in their performance characteristics, such as throughput (amount of work done per unit of time), latency (pause times), and memory footprint. Selecting the right collector is key to achieving optimal performance. The main types include:

• Serial Garbage Collector: Simple, single-threaded GC best suited for small, single-processor machines.
• Parallel Garbage Collector (Throughput Collector): Uses multiple threads for garbage collection, maximizing throughput at the expense of pause times.
• Concurrent Mark Sweep (CMS) Collector: Attempts to minimize pause times by performing most garbage collection work concurrently with the application.
• G1 (Garbage-First) Collector: Balances throughput and pause times by dividing the heap into regions and collecting the regions containing the most garbage first.
• Z Garbage Collector (ZGC): A low-latency collector designed for applications with very large heaps and strict pause time requirements.
• Shenandoah Garbage Collector: Another low-latency collector similar to ZGC.

Serial Garbage Collector

The Serial Garbage Collector is the simplest garbage collector. It uses a single thread to perform all garbage collection activities. Because of its single-threaded nature, it pauses the application completely during garbage collection cycles. This is known as a 'stop-the-world' (STW) event. It's suitable for small applications or single-processor machines where pause times are less critical.

How it works: The Serial GC uses a simple mark-and-copy algorithm. It identifies live objects (marking phase) and then copies them to a new memory area, leaving the garbage behind (copying phase).

Enabling it: Use the JVM option -XX:+UseSerialGC

Parallel Garbage Collector (Throughput Collector)

Also known as the Throughput Collector, this GC uses multiple threads to perform garbage collection. This significantly improves throughput compared to the Serial GC. However, pause times are still relatively long because it's still a stop-the-world collector. It's a good choice for applications where high throughput is more important than low pause times.

How it works: It divides the heap into generations (young and old) and uses parallel threads to collect garbage in each generation. The young generation collection (Minor GC) is typically faster than the old generation collection (Major GC or Full GC).

Enabling it: This is the default garbage collector in many modern JVMs. Explicitly enable it using -XX:+UseParallelGC

Tuning: The number of threads used by the Parallel GC can be adjusted using the -XX:ParallelGCThreads=n option, where 'n' is the number of threads.

Concurrent Mark Sweep (CMS) Collector

The CMS collector aims to minimize pause times by performing most of its work concurrently with the application. It's a more complex collector than the Serial and Parallel GCs. However, it can still experience relatively long pause times, especially during the final 'remark' phase and during concurrent mode failures (when the heap fills up before the concurrent collection completes).

How it works: CMS involves several phases:

1. Initial Mark: A short stop-the-world pause to mark objects directly reachable from the root set.
2. Concurrent Mark: Traverse the object graph concurrently with the application to find all reachable objects.
3. Remark: A short stop-the-world pause to re-mark objects that were modified during the concurrent mark phase.
4. Concurrent Sweep: Reclaim the space occupied by unreachable objects concurrently with the application.
5. Concurrent Reset: Reset internal data structures to prepare for the next GC cycle.

Enabling it: Use the JVM option -XX:+UseConcMarkSweepGC

Note: CMS is deprecated in Java 9 and later and removed in Java 14. Consider using G1 or ZGC/Shenandoah instead.

Garbage-First (G1) Collector

The G1 collector is designed to provide a balance between throughput and pause times. It's a regionalized garbage collector, meaning it divides the heap into smaller regions. It then prioritizes collecting regions that contain the most garbage, hence the name 'Garbage-First'. It aims to meet specified pause time goals.

How it works: G1 performs garbage collection in the following phases:

1. Initial Mark: Marks root objects.
2. Concurrent Marking: Traverses the object graph concurrently.
3. Remark: Finalizes the marking process.
4. Cleanup: Identifies regions that are mostly free.
5. Copying: Copies live objects from regions with high garbage density to new regions. This is done in parallel.

Enabling it: Use the JVM option -XX:+UseG1GC. G1 is the default garbage collector in Java 9 and later in many configurations.

Tuning: A key parameter is -XX:MaxGCPauseMillis=200, which sets a target for the maximum GC pause time in milliseconds.

Z Garbage Collector (ZGC)

ZGC is a low-latency garbage collector designed for applications with very large heaps and strict pause time requirements (typically less than 10ms). It's a concurrent collector, meaning most of its work is done while the application is running. ZGC uses colored pointers and load barriers to achieve these low pause times.

How it works: ZGC uses colored pointers and load barriers to track object liveness and move objects in the heap without stopping the application for long periods. It performs concurrent marking, relocation, and compaction.

Enabling it: Use the JVM option -XX:+UseZGC.

Suitable for: Applications requiring very low latency, even with very large heaps (e.g., hundreds of gigabytes or terabytes).

Shenandoah Garbage Collector

Shenandoah is another low-latency garbage collector, similar to ZGC. It aims to keep pause times consistently low, regardless of heap size. It performs most of its garbage collection work concurrently with the application, minimizing disruption.

How it works: Shenandoah uses a technique called 'concurrent evacuation', where live objects are moved concurrently with application execution. This significantly reduces pause times.

Enabling it: Use the JVM option -XX:+UseShenandoahGC.

Suitable for: Applications with strict latency requirements and large heaps.

When to Use Them

Choosing the right garbage collector depends on your application's specific needs:

• Serial GC: Small, single-processor applications where pauses are not critical.
• Parallel GC: Applications where high throughput is more important than low pause times.
• CMS GC: (Deprecated) Applications requiring lower pause times than Parallel GC, but not as strict as G1, ZGC or Shenandoah.
• G1 GC: Most applications benefiting from balanced throughput and pause times. Good starting point.
• ZGC & Shenandoah: Applications with very large heaps and extremely strict latency requirements.

Real-Life Use Case Section

Use Case 1: High-Throughput Batch Processing

An application that processes large amounts of data in batch mode (e.g., overnight data warehousing) would benefit from the Parallel GC. High throughput is more important than occasional long pauses.

Use Case 2: Interactive Web Application

An interactive web application needs to respond quickly to user requests. Long pauses can lead to a poor user experience. G1 or ZGC/Shenandoah would be suitable choices to minimize pause times.

Use Case 3: Financial Trading Platform

A financial trading platform has extremely strict latency requirements. Even short pauses can lead to significant financial losses. ZGC or Shenandoah would be the preferred options.

Best Practices

Monitoring: Monitor garbage collection activity to understand how the GC is performing and identify potential bottlenecks. Tools like VisualVM, JConsole, and Java Mission Control (JMC) can provide valuable insights.

Tuning: Tune the garbage collector parameters based on your application's specific needs. Start with reasonable defaults and gradually adjust the parameters to optimize performance.

Heap Size: Set the initial and maximum heap sizes appropriately. A larger heap can reduce the frequency of garbage collections, but it can also increase pause times.

Object Pooling: Consider using object pooling to reduce the creation and destruction of objects, which can reduce the load on the garbage collector.

Avoid Excessive Object Creation: Minimize the creation of short-lived objects, as they contribute to frequent garbage collections.

Interview Tip

When discussing garbage collectors in an interview, demonstrate an understanding of the trade-offs between throughput and latency. Be able to explain the key characteristics of each collector and when to use them. Also, mention that CMS is deprecated and should be avoided in newer applications.