Minor vs Major GC: Young Generation and Old Generation Explained

Java’s automatic memory management relies on Garbage Collection (GC), which is optimized around the observation that most objects in Java applications are short-lived. To take advantage of this, the JVM divides the heap into two main regions: the Young Generation and the Old Generation. The GC behavior in each region is different, leading to Minor GC and Major GC (Full GC).

In this tutorial, we’ll explore the differences between Minor and Major GC, how they work, and their impact on performance in production applications.

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Why Understanding Minor vs Major GC Matters

• Helps optimize application performance and latency.
• Prevents unexpected long GC pauses.
• Crucial for tuning JVM flags in high-throughput and low-latency systems.
• Explains why certain workloads trigger frequent GCs.

Analogy: Think of cleaning your house. Minor GC is like tidying up your desk daily (quick, frequent). Major GC is like deep-cleaning the whole house (slow, less frequent).

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JVM Memory Regions Recap

The Java heap is split into regions:

1. Young Generation

   • Eden Space → Where new objects are allocated.
   • Survivor Spaces (S0, S1) → Objects that survive multiple GCs move here.

2. Old Generation (Tenured)

   • Stores long-lived objects promoted from Young Gen.

3. Metaspace (Java 8+)

   • Stores class metadata (not part of heap).

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What is Minor GC?

Minor GC collects garbage in the Young Generation. Since most objects are short-lived, Minor GC is frequent but usually fast.

Process

1. Allocate new objects in Eden.
2. When Eden fills, Minor GC is triggered.
3. Live objects move to Survivor spaces.
4. Objects surviving multiple Minor GCs get promoted to Old Gen.

Characteristics

• Quick and frequent.
• Stop-the-world (STW) → Pauses all threads briefly.
• Optimized for throughput.

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What is Major GC (Full GC)?

Major GC (or Full GC) collects garbage in the Old Generation and sometimes the entire heap (Young + Old + Metaspace).

Process

1. Triggered when Old Gen is full.
2. Identifies unreachable objects in Old Gen.
3. Compacts memory to avoid fragmentation.

Characteristics

• Slow and infrequent.
• Much longer pause times compared to Minor GC.
• Can cause application latency spikes if frequent.

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Minor vs Major GC: Key Differences

Feature	Minor GC (Young Gen)	Major GC (Old Gen / Full GC)
Memory Area	Young Generation	Old Generation (+ sometimes whole heap)
Frequency	Frequent	Infrequent
Pause Time	Short	Long
Performance Impact	Low (optimized)	High (can stall app)
Triggered By	Eden space full	Old Gen full

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Example Code Demonstrating GC Behavior

public class GCTest {
    public static void main(String[] args) {
        for (int i = 0; i < 100000; i++) {
            String data = new String("Object-" + i); // short-lived objects
        }
        System.gc(); // Suggests Full GC
    }
}

• The loop generates many short-lived objects → Minor GCs occur frequently.
• System.gc() suggests a Major GC (though not guaranteed).

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GC Algorithms and Generations

• Parallel GC → Optimized for throughput, uses Minor + Full GC.
• CMS (Concurrent Mark-Sweep) → Low-pause Old Gen GC, removed in Java 14.
• G1 GC → Region-based, collects both generations concurrently. Default since Java 9.
• ZGC & Shenandoah → Ultra-low pause collectors, reduce distinction between Minor/Major GCs by concurrent collection.

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GC Tuning for Minor and Major GC

• -Xms<size> / -Xmx<size> → Control heap size.
• -XX:NewRatio=<n> → Ratio of Young to Old Gen size.
• -XX:SurvivorRatio=<n> → Ratio of Eden to Survivor spaces.
• -XX:+UseG1GC → Enables G1 GC.
• -XX:+PrintGCDetails → Logs GC events.

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Pitfalls and Troubleshooting

• Frequent Major GCs → Indicates memory leaks or insufficient heap size.
• Long GC Pauses → Cause application latency spikes.
• Promotion failures → Objects cannot move to Old Gen → triggers Full GC.
• Stop-the-world events → Both Minor and Major GCs cause pauses, but Major is worse.

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Real-World Case Study

A payment processing system experienced long latencies due to frequent Full GCs under load. By tuning Survivor space ratios and switching to G1 GC, pause times were reduced by 70%, stabilizing transaction processing.

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Monitoring GC Activity

• Java Flight Recorder (JFR) → Low-overhead GC profiling.
• Java Mission Control (JMC) → Detailed GC visualization.
• VisualVM → Heap and GC monitoring.
• jstat → Command-line GC statistics.

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JVM Version Tracker

• Java 8 → Parallel GC default, G1 optional.
• Java 9 → G1 became default.
• Java 11 → ZGC added (experimental).
• Java 17 → ZGC and Shenandoah stable.
• Java 21+ → Project Lilliput reduces object header size, improving GC efficiency.

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Best Practices

• Tune Young vs Old Gen size based on workload.
• Use G1, ZGC, or Shenandoah for latency-sensitive apps.
• Monitor promotion rates to avoid Old Gen bloat.
• Profile before tuning—avoid guesswork.
• For microservices, prefer low-latency collectors.

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Conclusion & Key Takeaways

• Minor GC cleans the Young Generation, fast but frequent.
• Major GC cleans the Old Generation, slower with longer pauses.
• Modern collectors like G1, ZGC, and Shenandoah minimize pause times.
• Tuning heap sizes and ratios is key for performance optimization.
• Always monitor GC in production with proper tools.

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FAQs

1. What is the JVM memory model and why does it matter?
It ensures thread-safe interactions with memory, preventing concurrency issues.

2. How does G1 GC differ from CMS?
G1 is region-based and compacting, while CMS was prone to fragmentation.

3. When should I use ZGC or Shenandoah?
For large heaps and ultra-low latency requirements.

4. What are JVM safepoints?
Points where threads pause for GC or JIT optimizations.

5. How do I solve frequent Full GCs?
Increase heap, tune Survivor/Old Gen ratios, fix memory leaks.

6. What are the trade-offs of throughput vs latency?
Throughput favors maximizing work, latency minimizes pause impact.

7. How do I read and interpret GC logs?
Enable -XX:+PrintGCDetails and analyze with GCViewer or JMC.

8. How does JIT affect GC behavior?
JIT reduces allocations via escape analysis, lowering GC pressure.

9. What’s new in Java 21 for GC?
NUMA-aware GC and Project Lilliput reduce memory footprint and pauses.

10. How does GC differ in microservices vs monoliths?
Microservices optimize for startup and low latency; monoliths tune for throughput.

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