Demystifying G1 GC: How Java’s Modern Collector Manages Heap Regions

G1 Heap Structure

The G1 heap is divided into many fixed‑size blocks called regions. At JVM startup the region size is chosen (typically resulting in about 2,000 regions, each ranging from 1 MB to 32 MB).

Memory Allocation and Region Types

Each region is logically labeled as Eden, Survivor, or Old, similar to traditional collectors, but these labels are not physical partitions. G1 also defines a fourth type, Humongous, for objects larger than about 50 % of a region; such objects occupy a contiguous set of regions and are not fully optimized, so they should be avoided when possible.

Young Generation in G1

The heap’s ~2,000 regions are split between young and old generations. Young‑generation regions are typically colored green, old‑generation regions blue. Unlike older collectors, G1 does not require these regions to be contiguous, allowing flexible resizing.

A Young GC Cycle

During a young‑generation GC, surviving objects are copied to one or more Survivor regions; objects that have reached a tenure threshold are promoted to the old generation. This pause (a stop‑the‑world event) also measures the sizes of Eden and Survivor spaces for the next cycle, using the collected statistics to tune pause times.

After Young GC – Summary

The heap is a single memory space divided into many regions.

Young‑generation memory consists of a set of non‑contiguous regions, making capacity adjustments easy.

Young‑generation GC (young GCs) triggers a stop‑the‑world pause during which all application threads are paused.

Young‑generation GC runs in parallel across multiple threads.

Surviving objects are copied to new Survivor regions or promoted to the old generation.

Old‑Generation Collection in G1

Like the CMS collector, G1 aims for low‑pause old‑generation collection. The process consists of several phases, illustrated in the diagram below.

1. Initial Mark

The initial marking of live objects occurs during a young‑generation GC pause and is logged as GC pause (young)(initial-mark).

2. Concurrent Marking Phase

While the application runs, G1 computes region liveness in parallel. Empty regions (marked with a red “X”) are identified for immediate reclamation in the subsequent Remark phase.

3. Remark Phase

Empty regions are reclaimed, and the liveness of all regions is calculated (Region liveness).

4. Copying / Cleanup Phase

G1 selects regions with the lowest liveness for collection. These regions are reclaimed together with a concurrent young‑generation GC, resulting in a mixed pause logged as [GC pause (mixed)].

5. After Cleanup

The reclaimed regions are compacted into deep‑blue (old) and deep‑green (young) areas, as shown in the final diagram.

Old‑Generation GC Summary

Concurrent Marking Phase : Liveness is computed in parallel during program execution; no separate sweep phase unlike CMS.

Remark Phase : Uses the Snapshot‑at‑the‑Beginning (SATB) algorithm, which is faster than CMS; completely empty regions are reclaimed immediately.

Copying / Cleanup Phase : Young and old generations are collected together; whether an old‑generation region is selected depends on its liveness.