How Does Java Manage Garbage Collection, and What Are the Differences Between G1 GC and CMS?

Summary

• Introduction
• How Java Manages Garbage Collection
• Comparing G1 GC and CMS
• Key Differences
• Which One Should You Use?
• Conclusion

Introduction

Let's talk about garbage collection in Java. It's really about how Java automatically handles memory, which is crucial for preventing those pesky memory leaks and keeping our applications running smoothly. The Java Virtual Machine, or JVM, offers us different 'garbage collectors' to do this job. Each one is a bit like a specialized tool, designed to balance how quickly it works, how much it can handle, and how long it might pause your application.

Today, we're going to focus on two popular options: G1 GC, which stands for Garbage-First Garbage Collector, and CMS, or Concurrent Mark-Sweep. We'll look at how Java generally manages this process of garbage collection, and then we'll dive into the key differences between these two collectors, so you can understand which one might be best for your specific application.

How Java Manages Garbage Collection

Java’s garbage collection process primarily revolves around the Generational Heap Model, which divides memory into:

1. Young Generation: Contains newly allocated objects. It consists of:
   • Eden Space (where new objects are created).
   • Survivor Spaces (S0, S1) (where surviving objects from minor GCs are moved).
2. Old Generation (Tenured Space): Holds long-lived objects that survived multiple minor GCs.
3. Permanent Generation (Metaspace in Java 8+): Stores class metadata and other runtime structures.

Garbage collection involves:

• Minor GC: Cleans up the Young Generation.
• Major (Full) GC: Cleans up the Old Generation.
• Compacting and Defragmenting: Rearranging memory to reduce fragmentation.

Comparing G1 GC and CMS

G1 GC (Garbage-First Garbage Collector)

Introduced in Java 7, G1 GC is designed to minimize long pause times and provide predictable performance.

• Region-based memory layout: Instead of using fixed generational areas, G1 GC structures the heap as a collection of equally sized regions, which it then dynamically assigns as Eden, Survivor, or Old.
• Incremental and concurrent: G1 GC leverages parallel processing to reclaim memory, executing garbage collection tasks simultaneously to minimize application pauses.
• Predictable pause times: Developers can specify the maximum pause time target (-XX:MaxGCPauseMillis), and G1 GC attempts to meet that goal.
• Automatic compaction: Unlike CMS, G1 GC performs compaction during collection to reduce fragmentation.
• Better suited for large heaps: G1 GC excels in applications with heap sizes above 4GB.

CMS (Concurrent Mark-Sweep)

CMS was designed to minimize stop-the-world pauses by performing most of its work concurrently with the application threads. However, it comes with trade-offs:

• Four-phase process:
  1. Initial Mark (stop-the-world pause to mark directly reachable objects).
  2. Concurrent Mark (runs alongside application threads, marking reachable objects).
  3. Remark (short stop-the-world phase to finalize marking).
  4. Sweep (frees unused objects while the application runs).
• Lower latency but higher CPU usage: CMS trades CPU cycles for lower pause times.
• Does not compact memory: Can lead to fragmentation issues, requiring Full GC to compact memory.
• Better suited for applications with low-latency requirements, such as real-time applications.

Key Differences

Which One Should You Use?

• Use G1 GC if you need predictable pause times, better compaction, and are running Java 9 or later.
• Use CMS if your application is latency-sensitive and runs on older Java versions (though migration is recommended since CMS is deprecated).

Conclusion

We've explored the trade-offs between G1 GC and CMS. It's clear that there's no one-size-fits-all solution. CMS prioritizes latency, while G1 GC increasingly becoming the default choice, balances performance and pause times. In the end, the right choice depends on your application's specific requirements.