Symptoms & Diagnosis
Upgrading to Java 21 is a major milestone for many development teams, yet some users report unexpected latency spikes or reduced throughput. Identifying the root cause is the first step toward a resolution.
Common symptoms include increased CPU usage during garbage collection cycles and high “time-to-safepoint” metrics. In many cases, applications utilizing Virtual Threads experience “pinning,” where a virtual thread gets stuck to a carrier thread, negating the performance benefits of lightweight concurrency.
Diagnosis begins with monitoring JVM metrics. Look for high memory allocation rates or long GC pauses. If you are using the new ZGC (Generational ZGC), ensure your heap size is correctly configured, as insufficient memory can lead to aggressive collection cycles that mimic a performance bottleneck.
Troubleshooting Guide
To resolve Java 21 performance issues, you must first isolate whether the bottleneck is related to the Garbage Collector, the JIT compiler, or thread scheduling. Use the following table to identify common culprits:
| Issue | Potential Cause | Recommended Fix |
|---|---|---|
| High Latency | Generational ZGC overhead | Increase -Xmx or tune -XX:MaxGCPauseMillis |
| Thread Starvation | Virtual Thread Pinning | Replace ‘synchronized’ blocks with ReentrantLock |
| Slow Startup | Tiered Compilation latency | Use -XX:+TieredCompilation and tune JIT levels |
If you suspect Virtual Thread pinning is the cause of your performance degradation, you can enable specific Java Flight Recorder (JFR) events to track when a thread becomes pinned to its carrier.
# Run your application with JFR to detect pinned threads
java -XX:StartFlightRecording=filename=recording.jfr,settings=default -jar your-app.jar
# Or use the system property to print to console
java -Djdk.tracePinnedThreads=full -jar your-app.jarFor Garbage Collection issues, many developers find that Java 21’s Generational ZGC requires a different tuning mindset compared to G1GC. If you are experiencing slow performance on high-core machines, ensure Generational ZGC is actually enabled:
# Enable Generational ZGC in Java 21
java -XX:+UseZGC -XX:+ZGenerational -Xmx8g -jar your-app.jarOptimizing Memory Management
Java 21 introduces significant improvements to how memory is handled, but default settings may not fit every workload. Monitor the MetaSpace and CodeCache sizes, as filling these can trigger frequent full GCs or JIT de-optimizations.
Prevention
Preventing Java 21 performance regressions requires a proactive approach to coding and configuration. Moving away from legacy patterns is essential for the modern JVM environment.
Avoid using synchronized blocks within Virtual Threads if they wrap long-running I/O operations. This prevents the underlying OS thread from being released. Instead, use java.util.concurrent.locks.ReentrantLock to ensure the scheduler can unmount the virtual thread during blocking calls.
Keep your dependencies updated. Many libraries have released specific patches for Java 21 to better handle the nuances of the new concurrency model. Using an outdated database driver or networking library can often be the hidden cause of “slow Java 21 performance.”
Lastly, establish a performance baseline using JMH (Java Microbenchmark Harness) before and after the upgrade. This allows you to catch regressions in hot code paths before they reach production environments.