Even though Garbage First (G1GC), ZGC, and Shenandoah divide the heap into regions instead of fixed generations, they still logically separate memory into Young and Old generations. This is because generational garbage collection is still the most efficient way to manage object lifecycles.
1️⃣ The Generational Hypothesis
Garbage collection is based on an observation known as the Generational Hypothesis:
- Most objects die young (they are short-lived).
- Few objects survive and become long-lived.
This means:
- Frequent minor collections in the Young Generation are fast because most objects can be quickly discarded.
- Less frequent major collections in the Old Generation reduce GC overhead.
Even in region-based collectors like G1GC, ZGC, and Shenandoah, this principle still holds. The difference is how memory is divided—instead of predefined heap partitions, regions dynamically serve different roles.
2️⃣ How Young and Old Generations Work in Region-Based GCs
| Traditional GC (e.g., Parallel, CMS) | Region-Based GC (e.g., G1GC, ZGC, Shenandoah) |
|---|---|
| Fixed Young and Old Generation | Heap divided into dynamic regions |
| Eden, Survivor, and Old are separate spaces | Some regions act as Eden, Survivor, or Old, but roles change dynamically. |
| Objects move between generations based on age | Objects still age and move to different regions based on survivability |
💡 Key difference: Instead of a predefined split (e.g., 25% for Young, 75% for Old), region-based collectors dynamically assign space to Young and Old generations based on the application’s needs.
3️⃣ Why Keep Young and Old Generations?
Even with region-based garbage collection, splitting memory into Young and Old still improves performance:
✅ Faster Allocation in Young Generation
- New objects are created in Eden Regions, which are fast to allocate (bump pointer allocation).
- The GC collects garbage in Young regions frequently but quickly (since most objects die young).
✅ Avoids Frequent Full GC
- Old objects are collected less frequently because they are less likely to be garbage.
- If all objects were in a single pool, GC would have to scan the entire heap every time, leading to longer GC pauses.
✅ Efficient Object Promotion
- Objects age through multiple GC cycles and only move to Old Generation when necessary.
- This prevents premature promotion of short-lived objects, reducing Old Generation fragmentation.
4️⃣ How G1GC Uses Regions for Generations
G1GC dynamically assigns regions to different roles:
| Eden | Eden | Survivor | Old | Eden | Survivor | Old | Humongous | Old |
- New objects go into Eden Regions.
- If they survive multiple GCs, they move to Survivor Regions.
- If they survive even longer, they are promoted to Old Regions.
💡 Difference from traditional GCs:
- Instead of fixed Young and Old spaces, G1GC adjusts the number of Young and Old regions dynamically based on application behavior.
5️⃣ What About ZGC and Shenandoah?
ZGC and Shenandoah do not explicitly separate Young and Old Generations.
- Instead, they use concurrent compaction and relocation to manage objects.
- However, short-lived objects are still prioritized for quick cleanup.
- These collectors rely on object lifetimes rather than explicit Young/Old separation.
6️⃣ Summary: Why Do We Still Need Generations?
Even with dynamic, region-based GC, Young and Old Generations are still necessary because: ✔ Young Gen is cheaper to collect (most objects die young).
✔ Avoids scanning the entire heap frequently (improves performance).
✔ Efficient promotion of long-lived objects prevents fragmentation.
✔ Reduces Full GC occurrences, making the application more responsive.
📌 In short:
Region-based GCs do not eliminate Young and Old Generations—they just manage them more flexibly! 🚀