Java "unable to create native thread" (OutOfMemoryError) in CI

By Daniel Zoghalchali·Latchkey

This OutOfMemoryError is not about heap; the JVM could not create another OS thread. The runner hit a process/thread cap or ran out of memory for thread stacks.

What this error means

A Java step fails with OutOfMemoryError: unable to create new native thread, usually when launching many threads or parallel tasks. Lowering parallelism or using a larger runner clears it.

java.lang.OutOfMemoryError: unable to create new native thread
	at java.base/java.lang.Thread.start0(Native Method)

Common causes

A process/thread limit (ulimit / pids) was hit

The runner caps the number of threads/processes; exceeding it blocks new native threads.

Memory exhausted by thread stacks

Each thread reserves stack memory; thousands of threads can exhaust RAM on a tight runner.

How to fix it

Lower thread/task parallelism

Reduce the thread pool or build parallelism so fewer native threads are needed.

./gradlew test --max-workers=2
# inspect limits
ulimit -u

Add headroom

1. Run on a runner with more RAM and higher process limits.
2. Shrink per-thread stack size (-Xss) if appropriate.
3. Cap thread pools in the application.

How to prevent it

• Bound thread pools and build parallelism.
• Right-size memory/process limits for thread-heavy jobs.
• Watch ulimit -u on constrained runners.

Frequently asked questions

Related guides

References

• GitHub Actions documentation