In the world of high-performance computing—where microseconds matter—Java has often been scrutinized for its garbage collection (GC) pauses. Traditional GCs like G1 and Parallel GC introduce unpredictable stop-the-world (STW) pauses, making them unsuitable for high-frequency trading (HFT), real-time analytics, and low-latency microservices.

However, with the introduction of Z Garbage Collector (ZGC) and Shenandoah GC, Java now competes with low-latency languages like C++ and Rust. These collectors offer sub-millisecond pause times, even with multi-terabyte heaps, making them ideal for latency-sensitive applications.

This article explores:

• How ZGC and Shenandoah minimize pauses
• Key differences between the two collectors
• Real-world use cases and benchmarks
• Configuration best practices

1. The Need for Low-Pause Garbage Collectors

In financial trading, gaming, or telecom systems, response times must be consistent and predictable. Traditional GCs struggle with this because they:

• Stop application threads during major collections (STW pauses).
• Scale poorly with large heaps, leading to longer pauses.
• Introduce latency spikes, making SLAs hard to meet.

ZGC and Shenandoah solve these problems by performing almost all GC work concurrently, drastically reducing pause times.

2. ZGC: The Ultra-Low-Latency Collector

ZGC (Z Garbage Collector), introduced in JDK 11 and made production-ready in JDK 15, is designed for predictable, ultra-low pause times—often under 1 millisecond, regardless of heap size.

How ZGC Works

• Concurrent Phases: Marking, relocation, and reference processing happen without stopping application threads.
• Colored Pointers: Uses metadata bits in object references to track object states efficiently.
• Load Barriers: Ensures application threads see consistent memory views without long pauses.

When to Use ZGC

• High-frequency trading (HFT) – Where even a 10ms pause can lose trades.
• Real-time analytics – For consistent query response times.
• Large heaps (TB+) – Scales efficiently without increasing pauses.

Example Configuration

java -XX:+UseZGC -Xmx32g -Xms32g -XX:+AlwaysPreTouch -jar app.jar 

(-XX:+AlwaysPreTouch reduces runtime memory allocation delays.)

3. Shenandoah GC: Balanced Latency and Throughput

Shenandoah, developed by Red Hat and integrated into OpenJDK, offers sub-10ms pauses while maintaining good throughput. Unlike ZGC, it performs even root scanning concurrently, further reducing pauses.

How Shenandoah Works

• Brooks Pointers: Each object has an additional forwarding pointer, enabling concurrent compaction.
• Concurrent Evacuation: Moves objects without stopping application threads.
• Better for Mixed Workloads: Balances latency and throughput better than ZGC in some cases.

When to Use Shenandoah

• Real-time data processing – Where both speed and consistency matter.
• Cloud-native microservices – For smoother scaling under load.
• Applications with moderate heap sizes (up to hundreds of GB).

Example Configuration

java -XX:+UseShenandoahGC -Xmx16g -Xms16g -XX:ShenandoahGCHeuristics=adaptive -jar app.jar 

(-XX:ShenandoahGCHeuristics=adaptive optimizes for varying workloads.)

4. Benchmarks and Real-World Performance

Latency Comparison

(Sources: Oracle ZGC Docs, Red Hat Shenandoah Blog)

Throughput Impact

• ZGC sacrifices some throughput for lower latency.
• Shenandoah offers better throughput in mixed workloads.
• G1 is still better for pure throughput if pauses are acceptable.

5. Choosing the Right GC for Your Application

Final Recommendations

• For strict sub-millisecond pauses: Use ZGC.
• For a balance of latency and throughput: Use Shenandoah.
• For legacy applications: G1 GC is still a safe default.

6. Further Reading

• ZGC Official Documentation
• Shenandoah GC Explained (Red Hat)
• Comparing Modern Java GCs (Baeldung)

7. Conclusion

With ZGC and Shenandoah, Java is now a strong contender for low-latency, high-performance applications. By choosing the right garbage collector and tuning it properly, developers can achieve near-real-time performance without switching to C++ or Rust.