The Shenandoah Saga: A Brief History

Before we jump into the nitty-gritty, let's take a quick stroll down memory lane. Shenandoah GC, named after the Shenandoah Valley (and not, disappointingly, after a magical forest of garbage-collecting elves), was introduced as an experimental feature in JDK 12. Fast forward to Java 21, and it's now a fully-supported, production-ready garbage collector.

What Sets Shenandoah Apart?

At its core, Shenandoah is designed to tackle the "large heap problem" - minimizing GC pause times regardless of heap size. But how does it differ from its cousins, G1 and ZGC?

  • Concurrent Compaction: While G1 performs compaction during stop-the-world pauses, Shenandoah does it concurrently.
  • Brooks Pointers: Unlike ZGC's colored pointers, Shenandoah uses Brooks pointers for object relocation.
  • Evacuation-style Compaction: Objects are moved to new memory locations, rather than sliding them within existing regions.

Diving into Shenandoah's Internals

The Brooks Pointer Technique

At the heart of Shenandoah's magic lies the Brooks pointer. Each object in the heap contains an additional word - the Brooks pointer - which initially points to the object itself. When an object is moved, only this pointer is updated, allowing other references to remain unchanged.


class ShenandoahObject {
    Object forwardingPointer; // Brooks pointer
    // Actual object data follows
}

This clever trick allows Shenandoah to move objects concurrently without stopping the world to update all references.

The Concurrent Compaction Dance

Shenandoah's compaction process is a beautifully choreographed dance of threads. Here's a simplified version of the steps:

  1. Marking: Identify live objects concurrently.
  2. Evacuation: Copy live objects to new locations, updating Brooks pointers.
  3. Update References: Scan the heap to update any references to moved objects.
  4. Cleanup: Reclaim memory from evacuated regions.

All of this happens while your application threads are still running. Mind-blowing, right?

Shenandoah vs. G1 vs. ZGC: The Showdown

Let's break down how Shenandoah stacks up against its competitors:

Feature Shenandoah G1 ZGC
Concurrent Compaction ✅ ❌ ✅
Pause Times Very Low Low Very Low
Memory Overhead Medium Low High
Large Heap Performance Excellent Good Excellent

Real-World Tuning: Minimizing Pause Times

Now, let's get our hands dirty with some practical tuning tips for Shenandoah:

1. Heap Size Matters (But Not As Much)

With Shenandoah, you can be more generous with heap sizes without fear of long GC pauses. Start with:


java -XX:+UseShenandoahGC -Xms16G -Xmx16G YourApp

2. Tune the Allocation Threshold

Adjust how aggressively Shenandoah triggers GC cycles:


-XX:ShenandoahAllocationThreshold=10

Lower values (like 10%) trigger more frequent but shorter GC cycles.

3. Embrace Adaptive Heuristics

Let Shenandoah adapt to your application's behavior:


-XX:ShenandoahGCHeuristics=adaptive

4. Monitor and Adjust

Use tools like JConsole or VisualVM to monitor GC behavior, and don't be afraid to experiment with settings.

The Catch (Because There's Always a Catch)

Before you rush off to rewrite all your JVM args, keep in mind:

  • Shenandoah trades CPU cycles for lower pause times. On CPU-bound applications, this could impact overall throughput.
  • The Brooks pointer technique increases memory usage slightly.
  • While rare, you might encounter the dreaded "allocation failure" if the GC can't keep up with allocation rates.

Wrapping Up: Is Shenandoah Right for You?

Shenandoah shines brightest in scenarios where consistent low latency is crucial, especially with large heaps. If your application falls into any of these categories, Shenandoah might be your new best friend:

  • Latency-sensitive services (e.g., financial trading, gaming servers)
  • Applications with large in-memory datasets
  • Systems that require predictable response times regardless of heap size

Food for Thought

"The best GC pause is the one that never happens." - Anonymous Java Developer (probably)

While Shenandoah is impressive, remember that the ultimate goal is writing efficient, memory-conscious code. No garbage collector, no matter how advanced, can fully compensate for poorly optimized applications.

What's Next?

As you embark on your Shenandoah journey, here are some resources to keep handy:

Remember, the world of GC is ever-evolving. Stay curious, keep experimenting, and may your pause times be ever in your favor!

Have you given Shenandoah a spin in your Java 21 projects? Drop a comment below with your experiences or any mind-bending GC puzzles you've encountered. Happy garbage collecting!