Java’s Generational ZGC: A Game-Changer for Application Performance

The objective of enhancing application performance is elevated to a new level by JEP 439 (Generational ZGC) in the dynamic world of Java. The Z Garbage Collector (ZGC) will now have access to the incredible advantages of generational memory management as a result of this significant upgrade from Targeted to Completed in JDK 21.

Basic Overview: Generational ZGC

Since JDK 15, the Z Garbage Collector (ZGC) has been available, most of the work is done while application threads are executing, pausing them only for a short period of time. ZGC’s pause times are measured in microseconds and are preferred for applications that need low latency but high scalability.

Generational ZGC recognizes that not every object in a Java application ages at the same time. The heap is divided into two logical generations by the new generational ZGC: the old generation consists of objects that have been allocated recently and the young generation of objects that have been around for a long time. In order to allow ZGC to concentrate on gathering the lucrative young objects, each generation is collected separately from the others.

In comparison to non-generational ZGC and other garbage collectors, generational ZGC has the following

• Optimized barriers
• Double-buffered remembered sets
• Huge objects
• Full garbage collections
• Dense heap regions
• Relocations without additional heap memory
• Dense heap regions.

This option may become the default option in subsequent releases, and non-generational ZGC will be gradually phased out.

java -XX:+UseZGC -XX:+ZGenerational

Since generational ZGC is still under development, its performance may vary depending on the application and hardware platform.

Different Garbage Collectors in the HotSpot JVM

There are numerous garbage collectors in the HotSpot JVM, and each has advantages and disadvantages of its own. The many garbage collectors available in the HotSpot JVM are compiled in the following table:

Garbage Collector	Characteristics	Suitable Scenarios
Serial GC	Simple and efficient for small memory footprints	Single-threaded applications with small memory footprints
Parallel GC	Multi-threaded and efficient for medium to large memory footprints	Multi-threaded applications with medium to large memory footprints
CMS GC	Concurrent and low-latency	Applications that require low latency and can tolerate some overhead during garbage collection
G1 GC	Generational and concurrent	Applications with mixed workloads and large memory footprints
ZGC	Concurrent and low-latency	Applications that require low-latency and high scalability

The Importance of Generational ZGC

Keeping your living space neat and tidy is similar to memory management, which is an essential part of programming in Java. By removing unnecessary data and making sure memory is used to its fullest potential, garbage collection serves as the digital housekeeper. Although Generational ZGC goes above and beyond, ZGC has faithfully carried out this function since Java 11.

The Advantages of ZGC for Generations

The following advantages of generational ZGC add up to a more productive environment for Java applications:

• Less Interruptions: Your program will run much more continuously thanks to these improved memory management strategies.
• Effective Memory Use: Generational ZGC reduces memory use, freeing up RAM for additional crucial tasks.
• Reduced CPU Burden: Generational ZGC lightens the load on your computer’s central processing unit (CPU).

Setting Up and Utilizing Generational ZGC

The command-line arguments listed below can be used to set up and operate Generational ZGC:

• -XX:+UseZGC – Enables Generational ZGC.
• -XX:+ZGenerational – Specifies that Generational ZGC should be used instead of non-generational ZGC.
• -XX:NewSize=<size> – Specifies the size of the new generation heap.
• -XX:MaxNewSize=<size> – Specifies the maximum size of the new generation heap.
• -XX:OldSize=<size> – Specifies the size of the old generation heap.
• -XX:MaxOldSize=<size> – Specifies the maximum size of the old generation heap.

You only need to add the -XX:+UseZGC command-line option to your JVM starting script in order to activate Generational ZGC. Take remove the -XX:+UseZGC command-line option from your JVM startup script in order to disable Generational ZGC. Add or remove the -XX:+ZGenerational command-line option from your JVM startup script to switch between generational and non-generational ZGC.

Advantages and Disadvantages of Generational ZGC

Benefits of generational ZGC include the following:

• Decreased latency: By gathering trash while application threads are operating, generational ZGC is intended to reduce the influence on application performance. Significant latency reductions may result from this, particularly for applications that are sensitive to brief pauses.
• Enhanced scalability: The architecture of Generational ZGC allows for huge memory and core counts. Because of this, it’s a fantastic option for apps that must operate on big servers or in the cloud.

However, there are certain disadvantages to Generational ZGC as well, such as:

• Added complexity: Compared to non-generational ZGC, generational ZGC is a more complicated garbage collector. This may make problem-solving and debugging more challenging.
• Impact on performance for specific workloads: Not every workload is a good fit for generational ZGC. Applications that often allocate and deallocate objects, for instance, could see a decrease in performance while using Generational ZGC.

Despite of generational ZGC has more complexity it is justified, because it tracks the age of objects in the heap and manage two garbage collectors at a time.

Generational ZGC in the OpenJDK brings in a system that manages operations of two garbage collectors and the utilization of barriers and colored pointers. Despite this increased complexity, the aim is to completely replace the non-generational ZGC with the generational version, and reducing maintenance overhead. While Generational ZGC is set to deliver significant advantages in most scenarios, some workloads that remain non-generational might encounter some performance dips. It is believed that drawbacks in these cases are outweighed by the benefits of less frequent object collection in the old generation.