G1 GC and ZGC are two modern garbage collectors in Java, and the choice depends on application requirements. 1. G1 is suitable for scenarios where the heap size is between 4GB and about 1TB, can accept pauses within 200ms, priority is given to throughput, and runs in JDK 7; 2. ZGC is suitable for low-latency systems that require stable pauses below 10ms, heaps exceed 32GB or even up to 16TB, running in JDK 15 and able to withstand higher CPU overhead; ultimately, benchmarking should be carried out through GC log analysis tools for real workloads (such as GCViewer or ZGC statistics) to determine the best choice.

When it comes to high-performance Java applications, garbage collection (GC) can make or break your system's responsiveness and scalability. Over the years, the JVM has evolved from simple stop-the-world collectors to advanced, low-latency solutions. Two of the most discussed modern GCs today are G1 (Garbage-First) and ZGC (Z Garbage Collector) . Let's take a deep dive into how they work, where they excel, and which one might be right for your use case.

What Is G1 GC?

G1 (Garbage-First) was introduced in Java 7 as a replacement for the older CMS (Concurrent Mark-Sweep) collector. It's designed to provide predictable pause times while maintaining good throughput, especially for applications with large heaps (typically 4GB to hundreds of GB).

Key Features of G1:

• Heap Partitioning : G1 divides the heap into fixed-size regions (1–32MB), allowing it to collect garbage region by region instead of the entire heap at once.
• Concurrent Marking : Like CMS, G1 performs much of its work (marking live objects) concurrently with the application threads.
• Evacuation (Compaction) : During cleanup, G1 compacts live objects into fewer regions, reducing fragmentation.
• Pause Time Goals : You can set a target max pause time (eg, -XX:MaxGCPauseMillis=200 ), and G1 tries to meet it by adjusting how many regions it collects per cycle.

When G1 Works Well:

• Applications needing sub-second GC pauses .
• Medium to large heaps where CMS is too fragment or prone to fragmentation.
• Workloads with modern allocation rates and object lives that allow for efficient region-based collection.

G1 Limitations:

• Pause times are not guaranteed —they're best-effort. Under memory pressure, pauses can spike.
• Throughput can drop under high allocation rates due to concurrent cycle overhead.
• Not truly scalable to multi-terabyte heaps or ultra-low-lateency requirements.

What Is ZGC?

ZGC (Z Garbage Collector) was introduced in JDK 11 (as experimental) and became production-ready in JDK 15. It's designed for extremely low pause times even with massive heaps—think sub-10ms pauses on heaps up to 16TB .

Key Features of ZGC:

• Pause Time Independence : GC pause times are not proportional to heap size . Whether you have 10GB or 10TB, pauses stay under 10ms.
• Load-Barrier-Based Coloring : ZGC uses colored points and load barriers to track object references during concurrent phases, enabling most work to happen without stopping the world.
• Concurrent Everything : Marking, relocating (compacting), and reference processing happens concurrently with the app.
• Scalability : Designed for modern hardware with many cores and huge memory capacity.

How ZGC Achieves Low Pauses:

• Uses colored points (metadata stored in unused bits of object points) to track object state.
• Employs load barriers —tiny checks on every object access—to keep the GC informed without pausing.
• Performs relocation (compaction) concurrently, eliminating fragmentation without long pauses.

When ZGC Shines:

• Low-latency applications (eg, financial trading, real-time analytics, gaming).
• Systems with multi-terabyte heaps .
• Environments where predictable response times are more important than raw throughput.

ZGC Trade-offs:

• Higher CPU overhead due to load barriers and concurrent threads.
• Slightly lower throughput compared to G1 under ideal conditions.
• Requires newer JVMs (JDK 15 for production use).
• Not available on 32-bit platforms or some older architectures.

G1 vs ZGC: Head-to-Head Comparison

Choosing Between G1 and ZGC

Here's a practical guide to help you decide:

• Stick with G1 if :

  
  • You're on an older JDK (pre-15).
  • Your heap is under 32GB and GC pauses under 200ms are acceptable.
  • You prioritize throughput over ultra-low latency.
  • You want a well-understood, battle-tested collector.

• Switch to ZGC if :

  • You need predictable, sub-10ms pauses .
  • You're running on JDK 15 and can afford the CPU overhead.
  • Your heap is larger than 32GB , especially approaching terabytes.
  • You're building real-time or near-real-time systems .

? Pro Tip : Always benchmark with realistic workloads. Enable GC logging ( -Xlog:gc* ) and analyze with tools like GCViewer or ZGC's built-in stats ( -Xlog:gc,zgc ) to see real-world behavior.

Final Thoughts

G1 is still a solid choice for many applications—especially those that don't need extreme low latency. But ZGC represents the future of Java GC , especially as memory sizes grow and latency requirements tighten.

If you're building modern, scalable, low-latency services and can run on a recent JDK, ZGC is worth the investment . It removes the fear of GC pauses scaling with heap size, letting you focus on your application logic instead of GC tuning.

Basically, if low latency and big heaps matter, ZGC is the way to go. Otherwise, G1 remains a reliable, well-balanced option.

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