JMeter Log File Analysis

Understanding the JMeter Log File

The JMeter log file (jmeter.log) captured from each load engine provides detailed diagnostic output from the JMeter runtime. It includes INFO, WARN, and ERROR messages generated by JMeter core and plugins—crucial for troubleshooting configuration issues, plugin errors, or unexpected runtime behavior.

JMeter Log File Example

What’s in the JMeter Log

Each line in the log follows this pattern: <timestamp> [<thread-name>] <level> <logger> – <message>

  • timestamp Date and time when the log entry was written (e.g., 2025-04-23 07:57:24,894).

  • thread-name JMeter thread or component that generated the log (e.g., StandardJMeterEngine).

  • level Log level: INFO, WARN, ERROR, or DEBUG (if enabled).

  • logger The Java class or package emitting the message (e.g., org.apache.jorphan.reflect).

  • message The actual diagnostic text describing events, warnings, or errors.

How to Access and Use the JMeter Log

  1. Navigate to “Logs” In your test result dashboard, select the Logs tab and choose JMeter log file in the Test files panel.
  2. Scroll and Search The central panel streams the log content. Use your browser’s search to jump to keywords like ERROR or WARN.
  3. Download for Offline Analysis Click the download icon next to JMeter log file to save the complete jmeter.log for deeper inspection.

Common Log Entries

  • INFO StandardJMeterEngine – Running the test! Marks the start of test execution.
  • WARN ConfigTestElement – Missing property ‘foo’ Indicates a missing or malformed JMeter property.
  • ERROR HTTPSamplerProxy – Non HTTP response code: 500 Shows HTTP errors surfaced by samplers.
  • INFO JOrphanUtils – Will scan jar … Plugin and library loading messages (normal).
  • ERROR GC – OutOfMemoryError JVM or plugin memory issues—consider increasing heap or reducing threads.

Troubleshooting with the Log

  • Thread Startup Failures Look for ERROR entries immediately after the “Running the test!” line—often misconfigured thread groups or missing plugins.
  • Plugin Load ErrorsClassNotFoundException or NoClassDefFoundError messages point to JARs that failed to load on remote engines.
  • Resource Limits Out-of-memory or file-handle warnings signal the need to adjust JVM options (-Xmx, -XX:MaxFileDescriptor).
  • Latency & Timeout Warnings HTTP sampler timeouts and socket exceptions reveal network connectivity problems between LoadFocus agents and your target.

Best Practices

  • Enable Debug Logging Selectively Only set log_level.jmeter=DEBUG for narrow tests to avoid extremely large logs.
  • Synchronize Clocks Ensure all load agents use NTP so timestamps align across distributed logs.
  • Correlate with Other Views Match log timestamps with Engine Health and Errors charts to identify the root cause of spikes or failures.
  • Archive Logs Store your jmeter.log alongside JTL results for full forensic traceability of each test run.

Conclusion

The JMeter log file in LoadFocus is an essential troubleshooting tool—surface JVM errors, configuration warnings, and plugin issues that aren’t visible in high-level metrics. Regularly inspecting and archiving these logs helps you diagnose elusive test-execution problems and maintain reliable, repeatable load test campaigns.

Previous
Logs & JTL File Analysis
Next
Timeline View