How to activate the Garbage Collection Algorithms and Invocation Triggers

In Java, garbage collection algorithms are automatically invoked by the Java Virtual Machine (JVM) when certain conditions are met, such as when memory usage reaches a threshold or when the JVM determines that memory needs to be reclaimed to prevent OutOfMemoryError. While developers don’t have direct control over when garbage collection occurs, they can influence it by tuning JVM parameters and configuring the garbage collector.

How Garbage Collection Algorithms Are Invoked

1. Triggered Automatically by the JVM

• Memory Pressure: The most common reason for the JVM to trigger garbage collection is when the heap memory starts filling up. For example:
  • When the Young Generation (Eden space) is full, a Minor GC occurs to free up space.
  • When the Old Generation gets full, a Full GC or Major GC is triggered to clean up long-lived objects.
• Thresholds: The JVM internally monitors memory usage, and when certain thresholds (such as heap occupancy) are reached, garbage collection is invoked.
• GC Algorithms: Different garbage collection algorithms are invoked based on the current heap usage, the number of surviving objects, or the specific garbage collector being used (e.g., Parallel GC, G1 GC, ZGC).

1. Explicit Invocations by the Developer

• System.gc(): The developer can explicitly request garbage collection by calling System.gc() or Runtime.getRuntime().gc(). However, it is only a “suggestion” to the JVM, and the garbage collector is not guaranteed to run immediately.
  java System.gc(); // Request GC
• Explicit Invocation Not Recommended: Generally, relying on System.gc() is not recommended because it can cause unpredictable performance and may lead to Stop-the-World (STW) events, pausing the application. The JVM usually handles garbage collection more efficiently on its own.

1. Triggered by Object Promotion

• When objects survive multiple Minor GC cycles in the Young Generation and are promoted to the Old Generation, the JVM may trigger a Full GC when the Old Generation becomes sufficiently filled.
• If many objects are moved to the Old Generation, the garbage collector may run more frequently in the Old Generation, invoking a Major GC.

1. Triggered by Heap Exhaustion

• If the heap becomes full and there’s no space left to allocate new objects, the JVM will trigger a garbage collection to reclaim memory.
• If after GC, the JVM still cannot allocate new objects, an OutOfMemoryError is thrown.

Garbage Collection Algorithms and Invocation Triggers

1. Serial Garbage Collector (Serial GC)

• Triggered By:
  • Full heap or Eden space becoming full.
  • JVM flag -XX:+UseSerialGC is used to enable Serial GC.
• Invocation:
  • Single-threaded; pauses all application threads (Stop-the-World) to run GC.
  • Suitable for small applications with small heap sizes.

2. Parallel Garbage Collector (Parallel GC)

• Triggered By:
  • Full heap, full Eden space, or Old Generation nearing its capacity.
  • Enabled with the JVM flag -XX:+UseParallelGC.
• Invocation:
  • Uses multiple threads to speed up garbage collection, but still involves STW events.
  • Often used in applications requiring high throughput.

3. G1 Garbage Collector (Garbage First GC)

• Triggered By:
  • Heap region becoming full (divided into regions for Young and Old Generations).
  • JVM flag -XX:+UseG1GC enables G1 GC.
• Invocation:
  • Region-based collection: G1 collects garbage from regions that contain the most garbage first (hence the name Garbage First).
  • Mixed GCs: Involves both Young and Old Generation collections, with a focus on reducing pause times.
  • G1 tries to meet a user-defined pause-time goal using -XX:MaxGCPauseMillis.

4. CMS Garbage Collector (Concurrent Mark-Sweep)

• Triggered By:
  • Old Generation reaching a configurable percentage of fullness (-XX:CMSInitiatingOccupancyFraction).
  • JVM flag -XX:+UseConcMarkSweepGC enables CMS GC.
• Invocation:
  • CMS performs the Mark phase concurrently with the application threads, but the Sweep phase still involves pauses.
  • Old Generation collection is triggered when it becomes full.

5. Z Garbage Collector (ZGC)

• Triggered By:
  • Heap usage reaching a certain threshold.
  • JVM flag -XX:+UseZGC enables ZGC.
• Invocation:
  • ZGC is designed for very low-latency garbage collection, with pause times in the millisecond range.
  • It performs most of its work concurrently with the application threads.

6. Shenandoah Garbage Collector

• Triggered By:
  • Heap nearing its capacity.
  • JVM flag -XX:+UseShenandoahGC enables Shenandoah GC.
• Invocation:
  • Similar to ZGC, Shenandoah minimizes pause times by performing garbage collection concurrently with the application threads.
  • Designed for low-latency applications.

Garbage Collection Metrics and Monitoring

• JVM Options: Developers can monitor or tune garbage collection behavior using JVM flags, for example:
• -verbose:gc: Enables GC logging to see when garbage collection events happen and how much memory is reclaimed.
• -Xms and -Xmx: Set the initial and maximum heap sizes to influence when GC occurs.
• -XX:+PrintGCDetails: Provides detailed information on GC events, memory usage, and pause times.
• JMX and Monitoring Tools: Developers can monitor garbage collection in real-time using tools like:
• JConsole or VisualVM: These tools provide a graphical interface to observe heap memory usage and GC events.
• Java Flight Recorder (JFR): Used to analyze JVM performance, including garbage collection.

How Developers Can Influence Garbage Collection

1. Tuning Heap Size:

• Using -Xms and -Xmx to adjust the initial and maximum heap sizes can reduce the frequency of garbage collection.
• Example:
  bash java -Xms512m -Xmx2g -jar MyApplication.jar

1. Choosing the Right GC Algorithm:

• For latency-sensitive applications, use G1 GC, ZGC, or Shenandoah.
• For high-throughput applications, use Parallel GC.

1. Tuning GC Parameters:

• G1 GC Tuning: Use -XX:MaxGCPauseMillis=<N> to target specific pause times for G1.
• CMS Tuning: Configure -XX:CMSInitiatingOccupancyFraction to trigger CMS when Old Generation reaches a specific fullness percentage.

1. Avoiding Excessive Object Creation:

• Reduce object churn by minimizing unnecessary object creation, especially in critical sections of code.
• Use object pooling or caching for reusable objects.

Conclusion

In Java, garbage collection is automatically managed by the JVM and is triggered based on memory conditions, such as when certain parts of the heap are full or nearing capacity. The specific garbage collector used (e.g., Serial, Parallel, G1, ZGC) determines how and when the GC process is invoked. While developers cannot explicitly control the timing of garbage collection, they can influence its behavior through JVM tuning, proper memory management, and choosing the appropriate garbage collection algorithm for their application’s needs.