Junior Level

Young Generation — Heap area where all new objects are allocated.

Why a separate Young Gen: ~90-98% of objects die quickly. A copying collector is efficient for this pattern: copy only live objects (2%), dead ones are simply ignored.

Simple analogy: “Kindergarten” for objects. Most objects “die” here very quickly.

Structure:

• Eden — where new objects are created
• Survivor 0 and Survivor 1 — where survivors go after GC

Minor GC (young generation collection):

1. Eden is full → GC is triggered
2. Dead objects are removed
3. Live objects are copied to Survivor
4. Eden is cleared

Why it’s fast: 98% of objects are dead → only 2% are copied

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Middle Level

Allocation: TLAB

Each thread → its own TLAB in Eden
  TLAB = 64 KB of exclusive memory

Allocation:
  pointer += object_size  // Bump-the-pointer
  → Faster than malloc() in C++!

Lifecycle

new Object() → Eden
  ↓ Minor GC
Survivor (age +1)
  ↓ Minor GC (age +1)
Survivor (age +2)
  ↓ ... (until age = 15)
Old Generation

Premature Promotion

Problem: Survivor overflow
  → Objects pushed to Old Gen
  → Old Gen fills with garbage
  → Full GC!

Solution: increase SurvivorRatio or Young Gen

Tuning

-Xmn2g                     # Fixed Young Gen
-XX:SurvivorRatio=8        # Eden:Survivor = 8:1
-XX:MaxTenuringThreshold=15  # Age before Old Gen

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Senior Level

PLAB (Promotion Local Allocation Buffer)

During Minor GC: survivors copied to Survivor/Old
  → PLAB = TLAB for copying
  → Each GC thread has its own PLAB
  → Avoids contention during evacuation

PLAB (Promotion Local Allocation Buffer) — analog of TLAB for Old Gen.
Thread copies objects from Survivor to Old Gen via its private buffer,
without synchronization.

G1 Humongous Regions

Object > 50% of region → Humongous
  → Directly to Old Gen (not Eden!)
  → Not copied in Minor GC
  → Saves resources on useless copying

Humongous Regions — G1 regions for objects > 50% of region size.
Allocated directly in Old Gen.

PrintTenuringDistribution

-XX:+PrintTenuringDistribution

# Output:
 Desired survivor size: 10 MB
 Age 1: 5000 objects, 40000 bytes
 Age 2: 2000 objects, 16000 bytes
 ...
 Age 15: 100 objects, 800 bytes

→ Helps tune MaxTenuringThreshold

Best Practices

1. TLAB — allocation is almost free
2. Avoid Premature Promotion
3. G1 Humongous → increase G1HeapRegionSize
4. For G1: do not fix -Xmn — G1 adaptively changes Young Gen size. For Serial/Parallel GC, fixing -Xmn may be useful.
5. Monitor tenuring distribution

Senior Summary

• Young Gen = copying collector, O(live objects)
• TLAB/PLAB = lock-free allocation/evacuation
• Premature Promotion = main enemy of performance
• Humongous Objects = directly to Old Gen
• G1 adaptively changes Young Gen size

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Interview Cheat Sheet

Must know:

• Young Gen = Eden (80%) + Survivor S0 (10%) + Survivor S1 (10%)
• All new objects created in Eden; Minor GC copies live to Survivor, Eden cleared
• TLAB (Thread Local Allocation Buffer) — thread-private buffer in Eden, allocation = pointer bumping (0 synchronization)
• PLAB (Promotion Local Allocation Buffer) — analog of TLAB for Old Gen during survivor evacuation
• MaxTenuringThreshold = 15 (default); object goes to Old Gen after 15 collections
• Premature Promotion: Survivor overflows → objects pushed to Old Gen → Full GC
• Humongous Objects (G1): object > 50% of region → directly to Old Gen, not copied in Minor GC

Common follow-up questions:

• Why is Minor GC so fast? — 98% of objects in Eden are dead; only 2% live are copied (O(live), not O(all))
• What does -XX:+PrintTenuringDistribution do? — Shows object distribution by age; helps tune MaxTenuringThreshold
• Why shouldn’t -Xmn be fixed for G1? — G1 adaptively changes Young Gen to achieve MaxGCPauseMillis; fixing breaks adaptivity
• What is SurvivorRatio? — Eden:Survivor ratio (default 8:1); too small → Premature Promotion

Red flags (DO NOT say):

• “All objects live long, Young Gen is useless” — Weak Generational Hypothesis holds for most applications
• “I fix -Xmn for G1 to control memory” — G1 loses adaptivity, pauses become unpredictable
• “Minor GC scans Old Gen” — Card Table optimization: only Dirty cards are scanned

Related topics:

• [[8. What are generations in GC]]
• [[10. What is Old Generation (Tenured)]]
• [[13. What is G1 GC]]
• [[2. What is stored in Heap]]
• [[12. What GC algorithms exist]]