Java "unable to create new native thread" During Build in CI

The JVM could not create another OS thread. This is native-memory or limit exhaustion, not heap - the process hit the thread ulimit, the cgroup pids limit, or ran out of address space for thread stacks.

What this error means

A build with high parallelism fails with java.lang.OutOfMemoryError: unable to create new native thread. The heap may be far from full; the failure is in native thread creation.

java runtime

java.lang.OutOfMemoryError: unable to create new native thread
  at java.base/java.lang.Thread.start0(Native Method)
  at java.base/java.lang.Thread.start(Thread.java:809)

Common causes

Thread/pids limit reached

The OS ulimit -u or a container pids limit caps how many threads the process can spawn. High build/test parallelism hits that cap.

Native memory exhausted by thread stacks

Each thread reserves stack space outside the heap. A huge heap plus many threads leaves too little native memory/address space to create more.

How to fix it

Reduce concurrency

Lower worker and fork counts so fewer threads are created at once.

gradle.properties

# Gradle: gradle.properties
org.gradle.workers.max=2
# test { maxParallelForks = 1 }
# Maven surefire: <forkCount>1</forkCount>

Raise OS/container limits

Increase the thread/pids limit for the build process where you control the runner.

Terminal

ulimit -u 8192
# container: run with a higher pids limit (e.g. --pids-limit)

How to prevent it

Bound build/test parallelism, keep heap modest so native memory remains for thread stacks, and provision adequate thread/pids limits on runners.