Understanding Java 23 Z Garbage Collector (ZGC)

 Introduction to Garbage Collection in Java

Java’s memory management model is one of its key strengths, enabling automatic allocation and deallocation of memory. Garbage collection (GC) is an essential part of this system, ensuring that unused objects are removed to free up memory. Over the years, various garbage collectors have been developed to cater to different performance needs, such as throughput, latency, and memory footprint. The Z Garbage Collector (ZGC) is one such advanced GC introduced to handle modern-day application demands.

What is Z Garbage Collector?

The Z Garbage Collector, abbreviated as ZGC, is a low-latency GC designed to handle heaps ranging from small sizes to terabytes with minimal impact on application performance. Introduced in JDK 11, ZGC is a significant step forward in Java’s ability to manage memory for applications requiring both high responsiveness and scalability.

Understanding Java 23 Z Garbage Collector (ZGC)
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Key Features of ZGC:

1. Low Latency: Pause times do not exceed 10 milliseconds, regardless of heap size.

2. Scalability: Supports heap sizes from a few megabytes to multiple terabytes.

3. Concurrent Compaction: Performs compaction concurrently with application threads, reducing pauses.

4. NUMA Awareness: Optimized for Non-Uniform Memory Access architectures.

5. Unchanged Throughput: Provides competitive throughput compared to other GCs.

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Architecture of ZGC

ZGC employs a unique approach to memory management, relying on colored pointers and concurrent operations. Here’s a breakdown of its architecture:

1. Region-Based Memory Management

• The heap is divided into regions of fixed size.

• Regions can belong to one of the following categories:

  • Allocation regions for new objects.

  • Relocation regions during compaction.

  • Old regions for long-lived objects.

2. Colored Pointers

• ZGC uses colored pointers to embed metadata directly into object references.

• Each pointer has additional bits for identifying object state and region.

• This technique eliminates the need for traditional marking bitmaps.

3. Concurrent Phases

• Most GC operations, including marking and relocating, occur concurrently with application threads.

• The GC process includes:

  • Marking phase: Identifies live objects.

  • Relocation phase: Moves objects to new regions.

  • Reference update phase: Updates pointers to relocated objects.

4. Load Barriers

• ZGC uses load barriers to intercept object references and ensure correct metadata handling.

• These barriers ensure that application threads can safely access objects during concurrent phases.

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Configuring ZGC

ZGC can be enabled and tuned via JVM options. Below are steps to configure ZGC:

Enabling ZGC

To enable ZGC, use the following JVM option:

-XX:+UseZGC

For example:

java -XX:+UseZGC -Xmx10g -Xms10g MyApplication

Tuning ZGC

1. Heap Size: Define the minimum and maximum heap size using -Xms and -Xmx options. Example:

   -Xms2g -Xmx10g

2. GC Logging: Enable GC logs for better observability:

   -Xlog:gc*:file=gc.log:time,uptime,level,tags

3. Heap Dump: Use -XX:+HeapDumpOnOutOfMemoryError to generate heap dumps for analysis.

4. NUMA Awareness: For NUMA systems, enable NUMA optimizations:

   -XX:+UseNUMA

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How ZGC Works

The ZGC process is designed to minimize pauses by distributing GC tasks across application threads. Here’s an overview of its workflow:

1. Allocation

• New objects are allocated in allocation regions.

• When allocation regions are full, ZGC starts a new GC cycle.

2. Marking

• Live objects are identified using a concurrent marking process.

• ZGC employs multiple threads for marking to enhance efficiency.

3. Relocation

• Identified live objects are moved to new regions.

• The old regions are then marked as free and returned to the pool of available regions.

4. Pointer Updates

• Pointers to relocated objects are updated using load barriers.

5. Compaction

• Unlike traditional GCs, compaction in ZGC is performed concurrently, eliminating long pause times.

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Comparing ZGC with Other Garbage Collectors

1. ZGC vs G1 GC

Aspect	ZGC	G1 GC
Latency	< 10 ms	Aims for predictable latency
Scalability	Up to 16 TB	Limited to smaller heaps
Compaction	Concurrent	Stop-the-world phases

2. ZGC vs Shenandoah GC

Aspect	ZGC	Shenandoah GC
Heap Size	Up to 16 TB	Up to 2 TB
Latency	< 10 ms	Low latency, but higher than ZGC
Technology	Colored Pointers	Brooks Pointers

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Use Cases for ZGC

1. Low-Latency Applications

• Real-time systems such as trading platforms, gaming servers, and IoT systems benefit from ZGC’s low-pause characteristics.

2. Large-Scale Applications

• Applications with heaps in the terabyte range, such as data analytics and machine learning workloads.

3. Cloud-Native Applications

• Scalable microservices architectures requiring predictable performance under dynamic workloads.

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Monitoring and Debugging ZGC

1. GC Logs

• Use -Xlog:gc to analyze ZGC events and performance metrics.

2. JDK Tools

• Tools like jconsole, jvisualvm, and Java Mission Control provide visualization and insights into ZGC behavior.

3. Third-Party Tools

• Monitoring solutions like Grafana, Prometheus, and New Relic can integrate with GC logs to provide real-time metrics.

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Advantages of ZGC

1. Ultra-Low Latency: Consistently low pause times, independent of heap size.

2. High Scalability: Supports massive heaps up to 16 TB.

3. Concurrent Operations: Minimal disruption to application threads.

4. Ease of Configuration: Simple JVM options for enabling and tuning.

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Limitations of ZGC

1. CPU Overhead: Higher CPU usage due to concurrent operations.

2. Compatibility: Available only on specific platforms (Linux, Windows, macOS) and JDK versions.

3. Memory Overhead: Colored pointers require additional memory bits.

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Future of ZGC

ZGC is continuously evolving with improvements in each JDK release. Future enhancements aim to:

1. Extend platform support.

2. Reduce CPU overhead further.

3. Enhance integration with cloud-native and containerized environments.

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Conclusion

The Z Garbage Collector is a revolutionary addition to Java’s GC arsenal, offering unparalleled low latency and scalability. It’s particularly suited for modern applications demanding high performance and large heaps. While it has some trade-offs, such as higher CPU usage, its benefits far outweigh the limitations for many use cases. As Java continues to evolve, ZGC’s role in shaping the future of low-latency applications is set to grow