JVM.JIT.How can you monitor and debug JIT compilation behavior?

🔍 How to Monitor and Debug JIT Compilation Behavior in JVM

Since the JIT (Just-In-Time) compiler dynamically optimizes code, monitoring and debugging its behavior can help identify performance bottlenecks, method inlining decisions, and optimization failures. Below are the key tools and JVM options to track JIT activity.

1️⃣ Enable JIT Compilation Logs

The JVM provides flags to monitor JIT behavior, helping developers understand which methods are compiled, inlined, or deoptimized.

🔹 View JIT Compilation Activity

java -XX:+PrintCompilation -jar myapp.jar

Prints each method compiled by the JIT in real-time.Example output:

120   40 %  com.example.MyClass::compute (50 bytes)
145   41   !  com.example.MyClass::slowMethod (deoptimized)

% → The method was compiled.! → The method was deoptimized.

🔹 View Inlined Methods

java -XX:+PrintInlining -jar myapp.jar

Displays which methods were inlined and which were not.Example output:

@  com.example.MyClass::fastMethod (15 bytes) inline (hot)
@  com.example.MyClass::complexMethod (200 bytes) not inline (too big)

This helps identify why some methods were not inlined (e.g., too large).

2️⃣ Use JIT Watch Tools

JIT compilation behavior can be analyzed using specialized tools:

🔹 JITWatch (Advanced Visualization)

JITWatch is a GUI tool for analyzing HotSpot JIT logs.

Steps to Use JITWatch:

Run your Java application with JIT logging enabled:

java -XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:+PrintAssembly -jar myapp.jar

Open the log file in JITWatch.Analyze:

Which methods were compiled
Inlining decisions
Deoptimization reasons
Bytecode vs. machine code comparison

3️⃣ Use JVM Profilers

🔹 JVisualVM (Built-in JVM Profiler)

JVisualVM is a graphical tool that provides real-time JIT statistics, CPU profiling, and memory analysis.

How to Use JVisualVM:

Start JVisualVM:

jvisualvm

Select your Java process.Enable JIT Compilation monitoring under MBeans > HotSpot Diagnostic.

4️⃣ Identify Deoptimization Events

Deoptimization occurs when the JIT compiler’s speculative optimizations fail, forcing the JVM to revert to interpreted mode.

🔹 Enable Deoptimization Logs

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintDeoptimization -jar myapp.jar

Helps debug why the JIT reverted optimizations.Example output:

Deoptimizing com.example.MyClass::dynamicMethod - uncommon trap

Uncommon traps occur when speculative optimizations fail.

5️⃣ Analyze JIT-Generated Machine Code

🔹 Print JIT Assembly Code

To see the actual machine code generated by the JIT compiler:

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -jar myapp.jar

Requires hsdis (HotSpot Disassembly Plugin).Helps low-level performance tuning (useful for JVM engineers).

Control JIT Compilation Behavior

You can modify JIT settings to debug optimizations:

JVM Option	Purpose
`-XX:+TieredCompilation`	Enables tiered compilation (mixes C1 and C2 JIT)
`-XX:MaxInlineSize=50`	Adjusts method inlining threshold
`-XX:-Inline`	Disables inlining (for debugging)
`-XX:-TieredCompilation`	Disables tiered compilation (forces either C1 or C2)
`-Xcomp`	Forces full JIT compilation (disables interpretation)

📌 Summary

Method	Tool/Command	Use Case
Basic JIT Monitoring	`-XX:+PrintCompilation`	See which methods are compiled
Inlining Analysis	`-XX:+PrintInlining`	Debug inlining decisions
JIT Visualization	JITWatch	Analyze JIT optimizations
Real-time Profiling	JVisualVM	Track JIT and CPU usage
Deoptimization Logs	`-XX:+PrintDeoptimization`	Detect failed optimizations
JIT Assembly Output	`-XX:+PrintAssembly`	View generated machine code

🏆 Conclusion

To monitor and debug JIT compilation, you can use JVM flags (PrintCompilation, PrintInlining), profiling tools like JVisualVM, and advanced analysis with JITWatch. This helps optimize Java applications for better performance and reduced runtime overhead. 🚀