g++ "virtual memory exhausted: Cannot allocate memory"

The compiler tried to allocate more memory than was available and gave up with "virtual memory exhausted". Like an OOM kill, this is a resource ceiling - too much parallelism or a translation unit too heavy for the RAM.

What this error means

A compile aborts with "virtual memory exhausted: Cannot allocate memory". It tends to hit the largest file or the highest -j, and re-running on a bigger machine succeeds unchanged.

virtual memory exhausted: Cannot allocate memory
make[2]: *** [CMakeFiles/app.dir/codegen_all.cpp.o] Error 1

Common causes

Insufficient RAM / swap for the compile

On a small runner with little or no swap, a memory-hungry compile exceeds available memory and the allocation fails outright.

High parallelism multiplying memory use

Many concurrent cc1plus processes sum to more than the runner has, so one of them cannot allocate.

How to fix it

Lower parallelism

Fewer concurrent compilers means more memory each.

make -j2
# or
ninja -j2 -C build

Add memory or swap

Use a larger runner, or add swap as a stopgap. A self-healing CI platform can retry on a higher-memory runner automatically.

# stopgap: add swap on a Linux runner
fallocate -l 4G /swapfile && chmod 600 /swapfile
mkswap /swapfile && swapon /swapfile

How to prevent it

• Size runners by the build’s peak memory, not just CPU count.
• Cap -j to fit memory.
• Split or simplify translation units that dominate memory.

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