CI "Cannot allocate memory" on fork()

By Daniel Zoghalchali·Latchkey

A fork() failed with ENOMEM: the kernel could not reserve memory for a new process. On a memory-tight runner this hits when overall usage (or a process/thread cap) is already near the limit.

What this error means

A command fails with fork: Cannot allocate memory or Resource temporarily unavailable, often while a build spawns many subprocesses. Freeing memory or using a larger runner clears it.

bash: fork: Cannot allocate memory
make: *** [build] Error 1

Common causes

Memory is nearly exhausted

With little free RAM (and no swap headroom), the kernel cannot satisfy the fork allocation.

Too many concurrent processes

High build parallelism spawns more processes than the runner can back with memory.

How to fix it

Lower concurrency and free memory

Reduce parallel processes and reclaim RAM before the heavy step.

free -h
make -j2          # instead of -j$(nproc)
# drop caches is host-only; prefer fewer workers in CI

Add memory headroom

1. Run on a runner with more RAM.
2. Raise a tight container --memory limit.
3. Cap parallel jobs/tests to the runner size.

How to prevent it

• Match build parallelism to runner cores and RAM.
• Right-size memory for fork-heavy build systems.
• Watch free -h in CI before heavy steps.

Frequently asked questions

Related guides

References

• GitHub Actions documentation