Java’s Highly Scalable Low-Latency Garbage Collector : ZGC

Summary

This video explores the Z Garbage Collector (ZGC), a scalable, low-latency garbage collector for Java that handles heaps from 8MB to 16TB with sub-millisecond pause times. It delves into ZGC's history, design goals, and key architectural features like concurrent processing and colored pointers. The video also discusses the importance of choosing the right garbage collector for specific workloads and highlights the evolution of ZGC from its experimental phase in JDK 11 to a production-ready feature in JDK 15, with improvements leading to even lower pause times.

Takeaways

• 🗑️ Garbage collection is essential for reclaiming memory in Java applications, and the JDK offers four garbage collectors: Serial, Parallel, G1, and ZGC.
• 🔄 ZGC (Z Garbage Collector) is designed for applications with tight latency requirements and the ability to handle large heaps, maintaining sub-millisecond pause times.
• 📈 ZGC's history dates back to 2015, with its development aimed at achieving low latency and scalability for large heaps, serving as a framework for future GC development.
• 📊 The choice of garbage collector involves trade-offs between memory footprint, throughput, and pause time, often represented in a performance triangle or tetrahedron.
• 🔑 Key architectural features of ZGC include concurrent processing, colored pointers, load barriers, and regionalized memory management.
• 🔄 Concurrent processing in ZGC allows for most garbage collection activity to be performed while the application is running, reducing pause times.
• 🖍️ Colored pointers in ZGC use metadata bits to track object marking and relocation, ensuring safe object movement during garbage collection.
• 🚧 Load barriers are segments of code that interpret colored pointers, ensuring objects are valid and correctly relocated during garbage collection.
• 🏗️ ZGC divides the heap into regions of different sizes, allowing for efficient memory management and garbage collection of objects that may leave scope together.
• 📈 ZGC was merged into the OpenJDK mainline as part of JDK 11 and promoted to a production feature in JDK 15, with ongoing improvements including concurrent thread scanning.
• 🔮 Future developments for ZGC include a move towards a multi-generational garbage collector, aiming to make it more practical for a wider range of workloads.

Q & A

What is the primary purpose of garbage collection in Java applications?

The primary purpose of garbage collection is to automatically reclaim memory by deleting objects that are no longer in use, thus allowing the memory to be reused.

How many garbage collector options does the JDK provide?

The JDK provides four garbage collector options: Serial GC, Parallel GC, G1 GC, and ZGC.

What is ZGC designed to achieve that sets it apart from other garbage collectors?

ZGC is designed to be a scalable, low-latency garbage collector capable of handling heaps from 8 megabytes to 16 terabytes in size while maintaining sub-millisecond pause times.

What does the term 'concurrent' mean in the context of garbage collectors?

In the context of garbage collectors, 'concurrent' means that the garbage collection activity is performed alongside the application's execution, rather than pausing the application.

What are the four key architectural features of ZGC that enable it to meet its design goals?

The four key architectural features of ZGC are concurrent processing, colored pointers, load barriers, and regionalized memory management.

What is the significance of colored pointers in ZGC?

Colored pointers in ZGC are used to carry metadata about the pointer, which helps in marking objects and managing relocation without causing application errors during garbage collection.

What is the role of load barriers in ZGC?

Load barriers in ZGC are segments of code that interpret colored pointers, ensuring that objects being loaded from the heap have valid references and are correctly marked during garbage collection.

How does ZGC handle the heap space differently from G1 GC?

ZGC, like G1 GC, divides the heap into regions, but ZGC goes further by dynamically growing and shrinking the number of active regions and having multiple region sizes, which allows for more efficient memory management.

What was the most significant change to ZGC since its release as a production feature?

The most significant change was the implementation of concurrent thread scanning processing (JEP 376), which reduced maximum pause times and ensured that pause times do not grow with the size of the live set.

What does the future hold for ZGC in terms of development?

The future development of ZGC includes moving towards a multi-generational garbage collector, which aims to make it a more practical choice for more workloads and to take advantage of concepts like the weak generational hypothesis.

How can developers get started with using ZGC in their applications?

Developers can get started with ZGC by using the JVM argument `-XX:+UseZGC` and turning on GC logging for detailed insights into the garbage collection process within the JVM.