Java 10 introduced Thread-Local Handshakes as a powerful feature in the JVM, designed to improve the performance and responsiveness of applications, especially in scenarios involving thread-based operations and garbage collection.

Here’s what you need to know about Thread-Local Handshakes and how to leverage them for performance tuning:

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1. What are Thread-Local Handshakes?

Thread-Local Handshakes allow a thread to execute a callback function locally without stopping all threads. This contrasts with traditional global safepoints in Java, where all threads must come to a safe state before any work can be done, such as garbage collection or code deoptimization.

In other words:

• A handshake is a mechanism to perform operations on a subset of threads (or even individual threads) without stopping the entire JVM.
• This is useful for operations that don’t require a global JVM safepoint, improving responsiveness and reducing latency.

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2. Benefits of Thread-Local Handshakes

• Avoids Global Safepoints: Operations can target some threads or a single thread, meaning other threads continue their work unaffected.
• Reduces Latency: No need to pause all threads, improving performance for multithreaded applications.
• Fine-Grained Control: Perform thread-specific tasks like flushing thread-specific memory buffers, deoptimizing code for just one thread, or collecting specific thread-local objects without interrupting the entire JVM.

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3. Use Cases

Here are some scenarios where Thread-Local Handshakes can be beneficial:

• Garbage Collection
  Garbage collectors rely on safepoints to pause threads while managing memory. Thread-Local Handshakes can isolate such operations to only the threads that need it, reducing pause times and improving application throughput.

• Code Deoptimization
  This happens during just-in-time (JIT) compilation when compiled code needs to revert to interpreted mode. Utilizing handshakes allows deoptimization to occur on specific threads, minimizing the impact on other threads.

• Thread-Specific Profiling and Debugging
  A developer or monitoring agent can perform profiling or diagnostic tasks on a single thread without disturbing other threads.

• Thread-Specific Resource Cleanup
  Thread-local data structures can be cleaned up or flushed for specific threads, optimally managing system resources.

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4. How Thread-Local Handshakes Work Internally

Thread-Local Handshakes introduce thread-specific “safepoints.” When a request is initiated:

1. The JVM signals specific threads to execute a callback function (like releasing resources or processing pending tasks).
2. Unlike global safepoints, only the targeted thread(s) pause and execute the operation.
3. Once the operation is complete, the thread resumes execution.

This makes operations more granular and non-blocking at the JVM level.

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5. Leveraging Thread-Local Handshakes in Performance Tuning

Although Thread-Local Handshakes are implemented at the JVM level, you can indirectly leverage them for performance tuning in the following ways:

1. Tuning for Garbage Collection
   If you’re using a garbage collector like G1GC or ZGC, you can reduce garbage collection pauses since these collectors take advantage of handshakes to avoid halting all threads during certain operations.
   • How to Monitor: Use tools like Java Mission Control (JMC), VisualVM, or JVM logging to monitor GC pause times and ensure thread-local synchronization is being effectively utilized.

   Relevant JVM Options:

   • -XX:+UseG1GC (or any GC of choice) to enable advanced garbage collection strategies.
   • Use -Xlog:gc to monitor GC logs and observe pauses.
2. Reducing Latency in Thread-Sensitive Applications
   If your application uses many threads (e.g., for handling requests or background tasks), Thread-Local Handshakes reduce overall latency by targeting specific threads instead of pausing all threads unnecessarily.

   Best practices:

   • Profile your application for thread contention and safepoints using tools like Async Profiler or JFR (Java Flight Recorder).
   • Optimize thread management through thread pools (using ForkJoinPool, ThreadPoolExecutor, etc.) to prevent thread starvation and maximize throughput.
3. Tuning Thread-Specific Tasks
   For tasks that manipulate thread-local data or thread-specific settings:
   • Optimize performance by ensuring the work is allocated to specific threads that need operations (e.g., specific callbacks).
   • Reduce contention by designing operations that leverage locality (thread-local memory, caches, etc.).

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6. Practical Tips for Developers

While Thread-Local Handshakes are managed by the JVM, the following tips help you align your code and architecture to take full advantage:

1. Choose Modern JVMs: Use JDK 10 or later for applications where fine-grained thread optimization matters. Newer garbage collectors like ZGC or Shenandoah optimize handshakes even further.

2. Monitor Safepoints and Utilization:

   • Safepoint statistics can be enabled using -XX:+PrintSafepointStatistics to understand how your threads interact with JVM-managed resources.
   • Use tools like JFR to detect safepoint delays or thread-local handshake activity.
3. Minimize Global Syncs in Application Code:
   • Avoid global thread synchronization where possible.
   • Use thread-local structures (e.g., ThreadLocal API) for thread-scoped data.
4. Benchmark Your Application:
   Profile how your code interacts with the JVM and threads. Use tools like JMH (Java Microbenchmark Harness) for thread and synchronization benchmarking.

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7. Example: Monitoring Thread Safepoints

java -XX:+PrintSafepointStatistics -XX:PrintSafepointStatisticsCount=1 -XX:+LogVMOutput -XX:LogFile=safepoints.log -jar YourApp.jar

This will output safepoint-related logs, showing where Thread-Local Handshakes may improve performance by reducing pauses.

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8. Conclusion

Thread-Local Handshakes represent an evolutionary step in how the JVM manages thread interactions, replacing costly global operations with thread-targeted approaches. While you may not directly invoke or control handshakes, you can optimize your application and JVM configuration to reap their benefits:

• Select JVM options and garbage collection strategies that leverage handshakes.
• Profile and diagnose thread safepoints to find opportunities for performance tuning.

These adjustments ensure better efficiency, reduced latency, and improved performance in multithreaded applications.