The Shenandoah Garbage Collector is a low-pause, concurrent GC available in OpenJDK. It’s designed to keep garbage collection (GC) pauses short, typically under 10ms, regardless of heap size. Unlike older collectors, Shenandoah runs most of its work concurrently with the application threads, making it ideal for latency-sensitive workloads.
In this tutorial, we’ll dive into Shenandoah’s design, tuning, and real-world use cases.
Why Shenandoah GC Matters
- Targets low-latency systems (microservices, financial apps, cloud-native workloads).
- Reduces stop-the-world (STW) pauses by doing concurrent compaction.
- Works efficiently with medium to large heaps.
- Competes with ZGC for sub-10ms latency goals.
Analogy: While traditional GC waits for you to stop working before cleaning, Shenandoah cleans as you work, minimizing disruptions.
How Shenandoah Works
Shenandoah is a region-based, concurrent, compacting collector. Its key innovation is concurrent evacuation, meaning it moves objects while application threads are still running.
Key Concepts
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Region-Based Heap
- Like G1 and ZGC, the heap is divided into fixed-size regions.
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Concurrent Marking
- Identifies live objects while the application runs.
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Concurrent Evacuation
- Relocates live objects concurrently, unlike CMS or G1.
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Brooks Pointers
- Each object carries an indirection pointer, allowing references to be updated seamlessly during concurrent moves.
Shenandoah GC Phases
- Initial Mark (STW) → Marks root objects (short pause).
- Concurrent Mark → Finds live objects across the heap.
- Final Mark (STW) → Completes marking.
- Concurrent Evacuation → Moves objects concurrently.
- Concurrent Update References → Fixes references across the heap.
- Cleanup → Reclaims memory from dead objects.
Example: Enabling Shenandoah GC
java -XX:+UseShenandoahGC -Xms2g -Xmx2g MyApplication
Tuning Shenandoah GC
-XX:+UseShenandoahGC→ Enable Shenandoah.-XX:ShenandoahUncommitDelay=300→ Release unused memory after 300s.-XX:+AlwaysPreTouch→ Improves NUMA efficiency.- Minimal tuning needed; it’s designed to be self-tuning.
Advantages of Shenandoah
- Very low GC pauses (<10ms).
- Concurrent compaction eliminates fragmentation.
- Works well for cloud-native and microservices architectures.
- Available in OpenJDK distributions (Red Hat, Amazon Corretto).
Limitations of Shenandoah
- Slightly higher throughput cost compared to G1.
- Initially not included in Oracle JDK (now available since Java 17).
- Less effective for tiny heaps (<1GB).
Real-World Case Study
A telecom analytics platform running on OpenJDK adopted Shenandoah GC to replace G1. It reduced average GC pause times from 50ms to <10ms, improving call-routing latency and customer experience.
JVM Version Tracker
- Java 8 (Red Hat builds) → Shenandoah introduced.
- Java 11 → Shenandoah available in Red Hat and Amazon Corretto builds.
- Java 17 → Shenandoah included in mainline OpenJDK.
- Java 21+ → Enhanced concurrent reference updating and NUMA awareness.
Best Practices with Shenandoah
- Use Shenandoah for low-latency, cloud-native workloads.
- Start with defaults; tune only when necessary.
- Monitor GC logs with
-Xlog:gc*. - For very large heaps (multi-terabyte), prefer ZGC.
Conclusion & Key Takeaways
- Shenandoah GC is a low-pause, concurrent GC ideal for latency-sensitive apps.
- It keeps pauses under 10ms, regardless of heap size.
- Best suited for microservices, cloud workloads, and medium to large heaps.
- ZGC is a stronger candidate for extremely large heaps.
FAQs
1. What is the JVM memory model and why does it matter?
It defines how threads access shared memory safely.
2. How does Shenandoah differ from G1?
Shenandoah evacuates objects concurrently, while G1 compacts mostly in STW phases.
3. When should I use Shenandoah GC?
For latency-sensitive apps needing predictable pause times below 10ms.
4. Does Shenandoah handle fragmentation?
Yes, concurrent compaction removes fragmentation issues.
5. How do I enable Shenandoah GC?
Use -XX:+UseShenandoahGC along with heap size options.
6. What’s the difference between Shenandoah and ZGC?
Both are low-latency; Shenandoah is better for cloud/microservices, ZGC for huge heaps.
7. What GC tuning options are useful?ShenandoahUncommitDelay, AlwaysPreTouch, and defaults are usually enough.
8. Is Shenandoah available in Oracle JDK?
Yes, starting with Java 17.
9. How does Shenandoah impact throughput?
Throughput is slightly lower than G1 but acceptable for latency-critical apps.
10. What’s next for Shenandoah?
Further NUMA optimizations and integration with Project Lilliput.