A Deep Dive into Java’s Garbage Collection Algorithms (ZGC, Shenandoah, G1GC)

Memory management has always been one of Java’s strongest advantages—and one of its most misunderstood components. While Java developers rarely manage memory manually, the choice and tuning of the Garbage Collector (GC) can dramatically impact application latency, throughput, and scalability.

With modern JVMs offering multiple advanced collectors, understanding G1GC, ZGC, and Shenandoah is no longer optional for performance-critical systems.

This article provides a deep, practical comparison of these collectors, explaining how they work internally, when to use them, and what trade-offs they introduce.

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Why Garbage Collection Matters in Modern Java

As applications evolve toward:

• Microservices

• High-throughput APIs

• Low-latency trading systems

• Cloud-native deployments

GC behavior directly affects:

• Tail latency (P99 / P999)

• Throughput consistency

• CPU utilization

• Cost efficiency in cloud environments

A poorly chosen GC can introduce multi-second pauses, even in well-written code.

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The Evolution of Java Garbage Collectors

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G1GC (Garbage First Garbage Collector)

Design Philosophy

G1GC was designed to replace CMS with better predictability and less fragmentation.

Instead of fixed memory generations, G1GC divides the heap into equal-sized regions.

Key Concepts

• Region-based heap

• Concurrent marking

• Incremental compaction

• Predictable pause time goals

How G1GC Works

1. Heap divided into regions (1–32 MB)

2. Regions contain Eden, Survivor, or Old objects

3. Garbage is collected region-by-region

4. Collector prioritizes regions with most garbage (“Garbage First”)

Strengths

• Balanced throughput and latency

• Predictable pause targets (-XX:MaxGCPauseMillis)

• Default GC since Java 9

Limitations

• Still experiences stop-the-world pauses

• Not ideal for ultra-low latency systems

• Heap size tuning still required

Best Use Cases

• General backend services

• APIs with moderate latency sensitivity

• Applications with heap sizes up to ~100GB

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ZGC (Z Garbage Collector)

Design Philosophy

ZGC was built for consistently low latency, regardless of heap size.

Its core promise:

Pause times under 10 ms, even with multi-terabyte heaps.

Key Innovations

• Colored pointers

• Load barriers

• Fully concurrent compaction

• Region-based heap (called ZPages)

How ZGC Works

• Object relocation happens concurrently

• Threads are minimally paused

• Pointer coloring tracks object state

• No traditional stop-the-world compaction

Strengths

• Extremely low pause times

• Scales to multi-TB heaps

• Minimal tuning required

Limitations

• Slightly higher CPU overhead

• Requires newer JVMs (Java 11+ production-ready from Java 15)

• Less mature tooling compared to G1

Best Use Cases

• Low-latency systems (fintech, gaming, real-time analytics)

• Large heap applications

• Cloud workloads sensitive to tail latency

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Shenandoah GC

Design Philosophy

Shenandoah focuses on pause-time independence from heap size.

Unlike ZGC, Shenandoah uses brooks pointers instead of colored pointers.

Key Characteristics

• Concurrent compaction

• Region-based heap

• Minimal stop-the-world pauses

How Shenandoah Works

• Object relocation occurs concurrently

• Read barriers update references on the fly

• Heap fragmentation is actively prevented

Strengths

• Predictable low latency

• OpenJDK and Red Hat backing

• Good performance under allocation pressure

Limitations

• Slightly more CPU overhead

• Fewer JVM diagnostics compared to G1

• Smaller ecosystem adoption than ZGC

Best Use Cases

• Latency-sensitive services

• Kubernetes workloads

• JVMs provided by Red Hat distributions

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G1GC vs ZGC vs Shenandoah

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Choosing the Right GC

Choose G1GC if:

• You want stability and maturity

• Latency under 200 ms is acceptable

• You prefer default JVM behavior

Choose ZGC if:

• Tail latency is critical

• You operate large heaps

• You want minimal tuning

Choose Shenandoah if:

• You need low pauses on Red Hat JVMs

• You run containerized workloads

• You can tolerate slightly higher CPU usage

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Practical JVM Flags (Quick Reference)

G1GC

ZGC

Shenandoah

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Common GC Tuning Mistakes

• Over-tuning before understanding workload

• Ignoring allocation rate

• Monitoring only average pause times

• Forgetting container memory limits

⚠️ Always profile before tuning.

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Final Thoughts

Modern Java garbage collectors are engineering marvels. While G1GC remains a safe default, ZGC and Shenandoah redefine what low-latency Java looks like.

Understanding their internals empowers developers to:

• Make informed architectural decisions

• Reduce cloud costs

• Improve user experience

Mastering GC is no longer optional—it’s a competitive advantage.

References

• https://docs.oracle.com/en/java/javase/21/gctuning

• https://openjdk.org/groups/zgc

• https://openjdk.org/projects/shenandoah

• https://www.baeldung.com/java-garbage-collection

• https://inside.java/

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<> “Happy developing, one line at a time!” </>