JMeter Freezing, 100% CPU Usage & CLI Hanging at End

JMeter Freezing, 100% CPU Usage & CLI Hanging

Symptom

You encounter one of the following performance bottlenecks on the load generator machine:

1. JMeter GUI completely freezes / becomes unresponsive: The window grays out or locks up during a test run.
2. 100% CPU saturation on the load injector: JMeter consumes all CPU cores on the testing host, causing request times to distort and throughput to plateau.
3. CLI execution hangs at the end of a test: The test duration finishes, but the terminal command prompt never returns, and the process remains stuck indefinitely.

Quick diagnosis (TL;DR)

• GUI Freeze: Running tests with active listeners (e.g. View Results Tree) in GUI mode consumes memory and blocks the Swing event dispatch thread.
• 100% CPU on Injector: Caused by CPU-intensive JSR223 Groovy parsing (missing cache), catastrophic regex backtracking, full GC loops, or excessive disk logging.
• CLI Hang: Non-daemon background threads (unclosed DB connections, async listeners, or alive RMI engines) preventing the JVM from exiting.

Common causes

Symptom	Primary Cause	Why it happens
GUI Freeze	Listeners in GUI	View Results Tree stores full XML/JSON response payloads in RAM, blocking UI threads
100% CPU	Inefficient RegEx / Groovy	Complex regex patterns with catastrophic backtracking, or recompiling Groovy scripts
100% CPU	Continuous Garbage Collection	Java Heap nearly full, triggering back-to-back Full GC pauses
CLI Hang	Non-daemon active threads	Custom plugins, Backend Listeners (InfluxDB/Kafka), or open JDBC pools blocking process exit
CLI Hang	Remote RMI engines	Distributed testing controller waiting for unresponsive worker nodes

Fix (ordered)

1. Always Run Load Tests in Non-GUI (CLI) Mode

Never use the JMeter GUI for actual load generation. Use GUI strictly for test plan creation and debugging with 1–2 virtual users:

# Standard Non-GUI execution command
jmeter -n -t testplan.jmx -l results.jtl -j jmeter.log

2. Diagnose Active Threads with a JVM Thread Dump

When a CLI test hangs or freezes, take a thread dump to identify the exact blocking thread:

# 1. Find the JMeter process ID
jps -v | grep ApacheJMeter

# 2. Capture the stack trace of all running threads
jstack <PID> > thread_dump.txt

Look for threads in the RUNNABLE or BLOCKED state:

• BackendListener or InfluxDBRawData: Waiting for remote metrics flush.
• StandardJMeterEngine: Waiting for lingering worker threads to exit.
• HikariPool / JDBC: Open database connection pool connections.

3. Force CLI Process Exit on Test Completion

If a third-party plugin or listener leaves background non-daemon threads open, configure JMeter to force JVM exit when the test finishes:

Add to bin/user.properties or pass via CLI:

# Force JVM exit (System.exit(0)) when engine finishes test
jmeterengine.force.system.exit=true

# Maximum time (in milliseconds) to wait for threads to stop on shutdown
jmeterengine.threadstop.wait=5000

4. Optimize Regex and JSR223 CPU Overhead

• Replace complex Regular Expression Extractors with Boundary Extractor (significantly faster, zero CPU regex overhead).
• Ensure all JSR223 elements use Groovy with “Cache compiled script if available” checked.
• Never use BeanShell (BeanShell is 10–20x slower and CPU-heavy compared to Groovy).

5. Check Disk I/O Bottlenecks and Log Levels

Writing verbose debug logs during a 1,000 RPS test saturates disk I/O and locks CPU threads:

1. Ensure jmeter.log level is set to INFO or WARN in bin/log4j2.xml.
2. Save only required fields in results.jtl:

# Keep JTL output minimal for maximum throughput
jmeter.save.saveservice.output_format=csv
jmeter.save.saveservice.response_data=false
jmeter.save.saveservice.samplerData=false
jmeter.save.saveservice.requestHeaders=false
jmeter.save.saveservice.url=true
jmeter.save.saveservice.response_code=true

Related tools and topics

Resource	Use when
GUI works, CLI fails	Differences between GUI and CLI execution
OutOfMemoryError heap	Heap memory exhaustion troubleshooting
CLI Command Builder	Generate clean CLI commands with proper flags
Best practices	Official tuning guidelines

Frequently asked questions

Why does JMeter hang at the end when using InfluxDB Backend Listener?

The Backend Listener buffers metrics and attempts to flush remaining data to InfluxDB over HTTP. If the InfluxDB server is slow or unreachable, the listener blocks until timeout. Set summaryOnly=true or lower the client timeout.

How do I stop a hung JMeter CLI test safely?

Run ./bin/stoptest.sh (or stoptest.bat on Windows) from a separate terminal. This instructs JMeter to gracefully terminate thread groups. If it remains stuck, use ./bin/shutdown.sh.

Continue Learning

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Next Practical Step

Run your test in CLI mode with jmeter -n -t plan.jmx -l results.jtl and verify that jmeterengine.force.system.exit=true is set.

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Related Reference

• CLI Command Builder
• Best practices
• Troubleshooting overview