Add JVM Tuning and Profiling documentation

[Copilot]

Comprehensive technical documentation for JVM tuning and profiling, covering memory management, JVM flags, profiling tools, and performance optimization workflows.

Contents

• JVM Memory Management Basics — Heap/stack/metaspace architecture with ASCII diagrams showing G1 region layout
• JVM Flags Reference — Heap sizing (-Xmx, -Xms), GC selection (G1/ZGC/Shenandoah/Parallel), logging, heap dumps
• Profiling Tools — JFR (including custom events), VisualVM, JDK Mission Control with comparison table
• Tuning Workflow — Measure → Analyze → Tune → Verify cycle with sample JFR/VisualVM workflow
• Best Practices & Pitfalls — Common mistakes (heap sizing, GC log misinterpretation, ignoring app-level issues)
• Production Configurations — Ready-to-use examples for web apps, microservices, batch jobs

Example

// Custom JFR event for business metrics
@Name("com.example.OrderProcessed")
@Label("Order Processed")
@Category("Business Events")
class OrderProcessedEvent extends Event {
    @Label("Order ID")
    String orderId;
    
    @Label("Processing Time (ms)")
    long processingTimeMs;
}

void main() {
    var event = new OrderProcessedEvent();
    event.orderId = "ORD-123";
    long start = System.currentTimeMillis();
    // process order...
    event.processingTimeMs = System.currentTimeMillis() - start;
    event.commit();
}

Follows repository conventions: Java 25 compact source files, implicit imports, 80-char line limit.

Original prompt

On jvm-tuning.md, Create a detailed technical document explaining JVM Tuning and Profiling in Java, focusing on memory flags, advanced options, and profiling tools.
The document should include:

1. Introduction

   • Define JVM tuning and profiling.
   • Explain why tuning is critical for performance and stability.
   • Highlight the role of profiling in diagnosing bottlenecks.

2. JVM Memory Management Basics

   • Describe heap, stack, metaspace, and GC regions.
   • Explain how JVM allocates and manages memory.
   • Provide diagrams showing JVM memory layout.

3. Tuning with JVM Flags

   • Heap Size Flags
     • -Xmx (maximum heap size).
     • -Xms (initial heap size).
     • Best practices for setting values.
   • Garbage Collection Flags
     • -XX:+UseG1GC, -XX:+UseZGC, -XX:+UseShenandoahGC.
     • Explain tuning options for pause times and throughput.
   • Other Useful Flags
     • -XX:MaxMetaspaceSize, -XX:+PrintGCDetails, -XX:+HeapDumpOnOutOfMemoryError.
     • Provide examples of usage.

4. Profiling Tools

   • Java Flight Recorder (JFR)
     • Explain what JFR is and how it integrates with the JVM.
     • Show how to enable and analyze recordings.
   • VisualVM
     • Explain features: heap analysis, thread monitoring, GC visualization.
     • Provide examples of diagnosing memory leaks.
   • Mission Control
     • Explain integration with JFR.
     • Show how to analyze performance data and detect bottlenecks.

5. Performance Tuning Workflow

   • Step-by-step guide:
     • Identify performance issues.
     • Collect profiling data.
     • Adjust JVM flags.
     • Validate improvements.
   • Provide a sample workflow with VisualVM and JFR.

6. Best Practices

   • Avoid over-tuning; rely on defaults unless necessary.
   • Always test under realistic workloads.
   • Monitor GC logs and heap usage regularly.
   • Use profiling tools before changing flags.

7. Common Pitfalls

   • Setting heap sizes too small or too large.
   • Misinterpreting GC logs.
   • Ignoring application-level bottlenecks.
   • Over-reliance on tuning without profiling.

8. Examples

   • Show sample JVM configurations for:
     • Web applications.
     • Microservices.
     • High-throughput batch jobs.
   • Provide code snippets and flag combinations.

9. Conclusion

   • Summarize the importance of JVM tuning and profiling.
   • Emphasize balancing performance, stability, and maintainability.

Formatting Requirements:

• Use clear section headings.
• Include code snippets for JVM flag usage.
• Provide diagrams or tables comparing profiling tools and flags.
• Write in a professional, educational tone suitable for intermediate to advanced Java developers.
• Ensure the document is cohesive, well-organized, and easy to follow.

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