Garbage Collection (GC) is central to Java’s memory management model. But not all garbage collectors are created equal—different collectors target different workloads, from low-latency trading systems to high-throughput batch jobs.
Choosing the right garbage collector can make the difference between smooth production performance and unpredictable latency spikes. This tutorial explains the major JVM collectors, their strengths, weaknesses, and how to choose the right one for your application.
The Role of the Garbage Collector
- Memory Management: Frees unused objects automatically.
- Application Impact: GC affects throughput, latency, and scalability.
- No One-Size-Fits-All: Choice depends on workload characteristics.
Analogy: Picking a GC is like choosing between a delivery truck (throughput) or a sports car (latency). Both move things, but with different performance priorities.
Overview of Major Garbage Collectors
1. Serial GC
- Target: Small applications or single-threaded environments.
- Strengths: Simple, low overhead.
- Weaknesses: Stop-the-world pauses, not scalable.
- Enable with:
-XX:+UseSerialGC
2. Parallel GC (Throughput Collector)
- Target: Batch jobs, compute-intensive workloads.
- Strengths: High throughput, multi-threaded collection.
- Weaknesses: Long pauses under large heaps.
- Enable with:
-XX:+UseParallelGC
3. CMS (Concurrent Mark-Sweep) [Deprecated]
- Target: Low-latency workloads (pre-G1 era).
- Strengths: Concurrent collection, reduced pause times.
- Weaknesses: Fragmentation, removed in Java 14.
- Enable with:
-XX:+UseConcMarkSweepGC
4. G1 GC (Garbage-First)
- Target: General-purpose, low-pause workloads.
- Strengths: Region-based, compacts heap, predictable latency.
- Weaknesses: Higher CPU overhead than Parallel.
- Enable with (default in Java 11+):
-XX:+UseG1GC
5. ZGC
- Target: Ultra-low latency, large heaps (TB scale).
- Strengths: Pause times < 10ms, concurrent compaction.
- Weaknesses: Higher memory overhead, newer tech.
- Enable with:
-XX:+UseZGC
6. Shenandoah GC
- Target: Low-latency workloads.
- Strengths: Concurrent compaction, pause times independent of heap size.
- Weaknesses: Higher CPU usage, tuning complexity.
- Enable with:
-XX:+UseShenandoahGC
Choosing Based on Application Needs
Throughput-Oriented (Batch Processing, ETL)
- Prefer Parallel GC or G1 GC.
- Example: Nightly big-data jobs.
Latency-Sensitive (Trading, Microservices)
- Prefer ZGC or Shenandoah.
- Example: Low-latency trading engine.
Balanced Workloads (Web Applications)
- G1 GC provides predictable performance.
- Example: E-commerce platform with mixed traffic.
Small Heaps or Embedded Systems
- Serial GC works fine.
- Example: CLI tools, IoT devices.
Case Studies
Case 1: E-commerce Platform
- Problem: Latency spikes with CMS.
- Solution: Migrated to G1.
- Result: Reduced GC pauses, stable SLAs.
Case 2: High-Frequency Trading App
- Problem: Microsecond-level latency requirements.
- Solution: Adopted ZGC.
- Result: Consistent <10ms GC pauses.
Case 3: Batch Analytics Pipeline
- Problem: Long-running jobs with massive allocations.
- Solution: Parallel GC.
- Result: Improved throughput, tolerable pauses.
Pitfalls and Troubleshooting
- Wrong Collector Choice: Throughput collector in latency-sensitive app → spikes.
- Over-Tuning: Excessive JVM flags can worsen performance.
- Ignoring Logs: GC logs provide insights into tuning effectiveness.
- Migration Issues: Moving from CMS → G1 requires testing.
Best Practices
- Profile before choosing GC.
- Start with G1 (default in Java 11+).
- Use ZGC/Shenandoah for ultra-low latency.
- Monitor with JFR, Mission Control, and GC logs.
- Avoid CMS in new deployments.
JVM Version Tracker
- Java 8: Parallel/CMS common.
- Java 11: G1 default.
- Java 17: ZGC and Shenandoah production-ready.
- Java 21+: NUMA-aware GC, Project Lilliput improves object headers.
Conclusion & Key Takeaways
- GC choice depends on workload type: throughput vs latency.
- G1 is the default safe option for most workloads.
- ZGC/Shenandoah excel in low-latency environments.
- Parallel GC shines in batch processing.
- Always profile before deploying to production.
FAQ
1. What is the JVM memory model and why does it matter?
It defines thread interactions and memory visibility, critical during GC.
2. How does G1 GC differ from CMS?
G1 compacts regions; CMS left fragmented memory.
3. When should I use ZGC or Shenandoah?
When predictable low latency is more important than throughput.
4. What are JVM safepoints and why do they matter?
GC requires safepoints to pause threads consistently.
5. How do I solve OutOfMemoryError in production?
Check GC logs, tune heap, fix leaks, and consider a different GC.
6. What are the trade-offs of throughput vs latency tuning?
Throughput favors fewer but longer pauses; latency tuning favors shorter pauses.
7. How do I read and interpret GC logs?
Look at pause duration, frequency, and heap usage trends.
8. How does JIT compilation optimize performance?
By inlining hot methods and enabling efficient memory allocation.
9. What’s the future of GC in Java (Project Lilliput)?
Smaller object headers and NUMA optimizations improve GC scalability.
10. How does GC differ in microservices vs monoliths?
Microservices require predictable latency; monoliths prioritize throughput.