This phrase refers to how region-based garbage collectors (like G1GC, ZGC, and Shenandoah) optimize memory cleanup by prioritizing the collection of regions that contain the most garbage, instead of scanning and collecting the entire Young Generation at once (as done in traditional generational GCs).
1️⃣ How Traditional (Non-Region-Based) GC Works
In traditional garbage collectors like Serial, Parallel, and CMS:
- When a Minor GC occurs, the entire Young Generation (Eden + Survivor) is scanned, and all unreachable objects are removed.
- When a Full GC happens, the entire Old Generation is scanned.
- This means GC is forced to scan all allocated memory in a generation, even if some areas contain little to no garbage.
Problem with This Approach:
❌ Wasted computation: Scanning and collecting memory that does not have much garbage.
❌ Longer pause times: Even if only a small portion contains garbage, the whole generation is scanned.
❌ Less efficient for large heaps: The bigger the heap, the longer the pause.
2️⃣ How Region-Based GC Works (G1GC, ZGC, Shenandoah)
Instead of scanning all allocated memory in a generation, region-based GCs take a prioritized approach:
🟢 Step 1: Divide Heap into Regions
- The heap is split into many small, fixed-size regions.
- Each region can be dynamically classified as Eden, Survivor, Old, or Humongous.
🟢 Step 2: Track Garbage in Each Region
- Each region keeps track of how much garbage it contains.
- G1GC uses a priority queue to rank regions based on garbage percentage.
🟢 Step 3: Collect the Most Garbage-Heavy Regions First
- Instead of scanning and collecting all Young Generation regions, GC only collects the regions that have the most garbage first.
- This means:
- If some Eden regions are nearly empty, GC can skip them.
- If some Old regions contain high garbage, GC can clean them early (Mixed GC).
- GC always works on regions where cleanup gives the most memory back.
🟢 Step 4: Repeat as Needed
- The number of regions cleaned per cycle depends on system load.
- The collector dynamically adjusts how many regions to clean based on how fast the application is running.
3️⃣ Example: Traditional GC vs. Region-Based GC
🔴 Traditional GC (Parallel or CMS)
Heap layout:
| Eden (10% garbage) | Eden (90% garbage) | Eden (50% garbage) | Survivor | Old |
Minor GC scans and collects the entire Young Generation.Wasteful: GC must scan all Eden regions, even if some contain little garbage.
🟢 Region-Based GC (G1GC)
Heap layout (G1 regions):
| Eden (10% garbage) | Eden (90% garbage) | Eden (50% garbage) | Old (70% garbage) | Old (20% garbage) |
- G1GC first selects the Eden region with 90% garbage.
- It skips the Eden region with only 10% garbage (since it is not worth collecting).
- If some Old regions also have high garbage (70%), they can be collected together with Young regions (Mixed GC).
Results:
✔ Faster GC cycles because only garbage-heavy regions are collected.
✔ Lower pause times because GC does not scan unnecessary memory.
✔ Better efficiency for large heaps since it avoids scanning unused memory.
4️⃣ How Different Region-Based GCs Handle This
| Garbage Collector | How It Selects Regions for GC |
|---|---|
| G1GC | Uses a priority queue to select regions with the most garbage first. |
| ZGC | Performs concurrent garbage collection, relocating objects gradually. |
| Shenandoah | Uses thread-local heuristics to decide which regions to clean in parallel. |
5️⃣ Summary
✅ Traditional GC: Scans all Young or Old Generation, even if some areas have little garbage.
✅ Region-Based GC: Selects only the most garbage-heavy regions, skipping low-garbage areas.
✅ Result: Lower GC pause times and better heap utilization for large applications. 🚀
📌 In short: Instead of cleaning everything, region-based GC focuses on the dirtiest areas first! 💡