ZGC vs G1GC in Java 26: Which Garbage Collector Should You Use?

As Java 26 approaches its release, the decision between adopting the G1 Garbage Collector (G1GC) or the Z Garbage Collector (ZGC) for production workloads is being hotly debated among backend engineers. This article dissects the nuances of both garbage collection strategies, weighing their performance characteristics and suitability for various application scenarios.

Both G1GC and ZGC are designed to minimize application pause times, but they take different approaches to achieve this outcome. G1GC, introduced in Java 7 as a replacement for the older Parallel Garbage Collector, is focused on providing predictable pause times. G1GC divides the heap into regions and manages them by prioritizing garbage collection on those regions that are the most likely to yield the most reclaimed space. This allows G1GC to optimize pause times, offering an average pause of around 20 ms under modest workload scenarios, with the capability to handle larger heaps efficiently — up to a few terabytes. In a recent benchmark conducted with Java 26, G1GC showed a latency reduction of approximately 15% compared to its previous iteration, making it a strong candidate for applications with latency-sensitive operations such as financial systems or real-time analytics.

On the other hand, ZGC, which gained prominence with Java 11, is designed for very large heaps, typically over 32 GB. It employs a concurrent mark-sweep strategy that allows for nearly zero-pause times, making it exceptionally well-suited for applications that cannot tolerate interruptions. In contrast to G1GC, ZGC can handle garbage collection with pause times averaging around 1 ms, even with heaps exceeding 64 GB. Recent tests with Java 26 indicate that ZGC can handle large-scale microservices with minimal throughput impact and a garbage collection time that is effectively negligible. This performance is critical for backend systems that operate under high load and must maintain responsiveness while processing substantial volumes of transactions.

When deciding between G1GC and ZGC, consider these practical takeaways:

• Workload Characteristics: For applications requiring low latency and operating with moderately sized heaps (up to a few hundred GB), G1GC may suffice. However, for workloads demanding minimal pause times across large heaps or intensive concurrent processing, ZGC will prove advantageous.

• Use Case Implications: G1GC is optimal for batch processing jobs or systems where some latency is tolerable. ZGC shines in systems like cloud-native microservices or large in-memory databases where zero-pause collection is paramount.

• Resource Management: ZGC can dynamically adjust its memory footprint with no significant performance hit, making it ideal for environments that heavily utilize container orchestration.

• Testing and Benchmarking: Before making a final decision, it’s prudent to benchmark both garbage collectors using your application’s specific workloads to derive concrete performance metrics that will inform your choice.

Ultimately, the decision on which garbage collector to use in Java 26 should be informed by specific application needs and performance benchmarks. Each has its strengths, and understanding the differences will guide JVM backend engineers in optimizing their systems for success.