JMeter OutOfMemoryError: Metaspace & GC overhead limit exceeded

OutOfMemoryError: Metaspace & GC overhead limit in JMeter

Symptom

During execution or extended soak tests, JMeter crashes with one of these JVM OutOfMemory errors in jmeter.log or console:

java.lang.OutOfMemoryError: Metaspace
  at java.lang.ClassLoader.defineClass1(Native Method)
  at groovy.lang.GroovyClassLoader.createSubClass(GroovyClassLoader.java:940)

Or when garbage collection takes up nearly 100% of CPU time with minimal memory recovered:

java.lang.OutOfMemoryError: GC overhead limit exceeded
  at org.apache.jmeter.threads.JMeterVariables.put(JMeterVariables.java:95)

Throughput drops to near zero, response times spike dramatically, and the load generator CPU spikes to 100% due to continuous full garbage collection cycles.

Quick diagnosis (TL;DR)

• OutOfMemoryError: Metaspace: The JVM class metadata area is exhausted. The most common cause is embedding \${var} variable interpolation inside JSR223 Groovy scripts, forcing the Groovy runtime to compile a brand-new Java class on every execution.
• GC overhead limit exceeded: Java spent more than 98% of its total CPU time performing garbage collection and recovered less than 2% of the heap.

Common causes

Cause	Error	Why it happens
In-script string interpolation (\${var})	Metaspace	Using vars.get("x") vs "Hello ${x}" avoids generating millions of temporary dynamic classes that fill Metaspace
Unchecked “Cache compiled script”	Metaspace / CPU	JSR223 elements re-parse and compile script source on every single sample execution
Small default Metaspace cap	Metaspace	JVM default -XX:MaxMetaspaceSize is too constrained for large test plans with many plugins
Heavy listener memory retention	GC Overhead	Listeners (View Results Tree, Graph Results) retaining millions of SampleResult objects in heap
Large in-memory file buffers	GC Overhead	Reading large CSV files or response bodies entirely into Groovy script memory variables

Fix (ordered)

1. Fix Groovy Variable Interpolation (The #1 Metaspace Culprit)

Never embed \${variable_name} directly inside JSR223 Groovy code. Groovy treats the script text as a template and generates a new class file every time the variable changes:

// ❌ WRONG: Leaks Metaspace and exhausts compilation cache!
String token = "${auth_token}";
log.info("Processing user ${user_id}");

// ✅ CORRECT: Reuses compiled class, zero Metaspace growth
String token = vars.get("auth_token");
log.info("Processing user " + vars.get("user_id"));

2. Enable “Cache compiled script if available”

In every JSR223 PreProcessor, JSR223 PostProcessor, JSR223 Sampler, and JSR223 Assertion:

1. Set Language to groovy (Groovy ... / Groovy Scripting Engine).
2. Ensure the checkbox Cache compiled script if available is checked.
3. Always supply a unique Compilation Key if using script files.

<!-- Correct JSR223 configuration in JMX -->
<JSR223Sampler guiclass="TestBeanGUI" testclass="JSR223Sampler" testname="Process Data">
  <stringProp name="scriptLanguage">groovy</stringProp>
  <stringProp name="parameters"></stringProp>
  <stringProp name="filename"></stringProp>
  <stringProp name="cacheKey">true</stringProp>
  <stringProp name="script">String id = vars.get(&quot;userId&quot;);
vars.put(&quot;processedId&quot;, id.toUpperCase());</stringProp>
</JSR223Sampler>

3. Increase JVM Metaspace and Heap Configuration

Set explicit Metaspace and Heap sizing in your launch environment:

# Linux / macOS environment variables
export JVM_ARGS="-Xms4g -Xmx4g -XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=512m -XX:+UseG1GC"
jmeter -n -t test.jmx -l results.jtl -j jmeter.log

@REM Windows bin/jmeter.bat or set in terminal
set JVM_ARGS=-Xms4g -Xmx4g -XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=512m -XX:+UseG1GC
jmeter.bat -n -t test.jmx -l results.jtl

4. Remove Memory-Heavy Listeners in CLI Mode

During load tests, disable all visual listeners:

• Delete or disable: View Results Tree, Summary Report, Aggregate Graph, View Results in Table.
• Use only command-line logging (-l results.jtl) and generate the HTML dashboard after the test completes (dashboard guide).

5. Tune Modern G1 Garbage Collector

Ensure modern G1GC settings are configured for low pause times:

# Recommended JVM arguments for high-throughput JMeter testing
-XX:+UseG1GC
-XX:MaxGCPauseMillis=100
-XX:G1ReservePercent=15
-XX:InitiatingHeapOccupancyPercent=45

Related tools and topics

Resource	Use when
OutOfMemoryError heap	Standard Java heap memory troubleshooting
JSR223 Groovy script errors	Resolving Groovy scripting exceptions
Heap Estimator	Sizing JVM heap and Metaspace memory
Best practices	Official performance tuning guidelines

Frequently asked questions

Why does Metaspace keep growing even when heap is healthy?

Metaspace lives in native memory outside the Java heap. If Groovy compiles new classes dynamically (due to \${var} string interpolation), these classes are loaded into Metaspace and are rarely garbage collected unless classloaders are completely unloaded.

What is the difference between Heap OOM and Metaspace OOM?

Heap OOM means Java objects (byte arrays, strings, collections) filled -Xmx. Metaspace OOM means loaded class metadata filled -XX:MaxMetaspaceSize.

Continue Learning

→

Next Practical Step

Audit all JSR223 elements to replace \${var} interpolation with vars.get("var") and verify that script caching is enabled.

📖

Related Reference

• JSR223 Groovy errors
• Heap Estimator
• Best practices