HotSpot JVM Non-Generational Garbage Collectors - G1 GC (Garbage First Collector)

Modern garbage collectors like G1, Shenandoah, and ZGC are designed to address the challenges of large heaps, low-latency requirements, and scalability. Unlike traditional generational collectors, these collectors do not strictly separate the heap into young and old generations, but rather operate on region-based memory layouts for more flexibility and performance.

Note: While G1 is still technically generational, its region-based and incremental nature blurs the line. Shenandoah and ZGC, however, are truly non-generational.

Key Concepts

• Region-based memory layout: The Java heap is divided into many equally-sized regions.
• Each Region is the smallest unit of collection. GC reclaims memory in region multiples, not full-heap sweeps.
• Prioritizes regions with the highest garbage-to-live ratio to maximize collection efficiency.
• Remembered sets (RSet) are maintained per region to track cross-region references.
• Humongous objects (larger than 50% of a region) are stored across multiple contiguous regions, collectively referred to as a Humongous area.

G1 aims to balance throughput and low pause times, making it the default collector since JDK 9.

GC Phases in G1

1. Initial Mark
   • Marks objects directly reachable from GC Roots.
   • Requires Stop-The-World (STW) pause.
2. Concurrent Mark
   • Traverses the heap graph to identify live objects.
   • Runs concurrently with application threads.
3. Final Mark (Remark)
   • Fixes changes made during concurrent marking.
   • STW pause required.
4. Cleanup & Evacuation
   • Calculates region-wise live data and selects high-garbage regions for evacuation.
   • Moves live objects to empty regions and reclaims old ones.
   • Requires STW pause, but runs with multiple GC threads in parallel.

Comparison with CMS

G1 is more suitable for large heaps and modern hardware, while CMS was better for smaller heaps but is now deprecated.