Understand JVM GC: G1 Regions, Remembered Sets, SATB Barriers

G1 Garbage Collector

G1 (Garbage-First) divides the heap into equal-sized regions and collects the ones with the most garbage first. It targets predictable pause times on large heaps, unlike single-heap stop-the-world collectors.

Regions, Remembered Sets, SATB

Key mechanisms:

• Regions: typically 1-32 MiB; eden, survivor, and old data can live in different regions over time
• Remembered sets: per-region logs of cross-region references so GC need not scan the entire heap
• SATB barriers: snapshot-at-the-beginning logging keeps concurrent marking sound while mutators run

Young collections evacuate live objects from eden regions; mixed collections fold in old regions with high reclaim potential.

For example, a 8 GiB heap split into 2048 regions lets G1 target a 200 ms pause by collecting just 20 regions worth of garbage.

Design Notes

Pause time goals are hints, not guarantees. Under allocation pressure G1 may still collect more regions than planned; pair `-XX:MaxGCPauseMillis` with GC logs to see real behavior.

For This Exercise

This exercise asks you to document G1's region model, remembered sets, and SATB write barriers. You will explain how G1 bounds pause time compared to whole-heap collectors.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.