The Out-Of-Memory (OOM) killer is part of the Linux kernel. When memory and swap are exhausted and the kernel cannot reclaim enough, it picks a process by a scoring heuristic and kills it with SIGKILL to keep the system alive.
Why it matters
From the outside, the symptom is a service that vanishes with no application error, because SIGKILL cannot be caught. The database or PHP worker pool is often the largest process, so it is the usual victim. In containers, a separate cgroup memory limit can trigger an OOM kill even when the host has memory to spare, and orchestrators show it as exit code 137.
Example
Confirm it from the kernel log:
dmesg -T | grep -i "out of memory"
journalctl -k | grep -i "killed process"A line such as Out of memory: Killed process 2314 (mysqld) is conclusive. Fixes: reduce memory per worker (for example fewer PHP-FPM children), add RAM or swap, set sensible container limits, and fix leaks. Monitoring should alert on available memory before the kill, not after.
Common mistakes
- Restarting the service repeatedly without finding why memory ran out.
- Configuring more worker processes than memory can hold at peak.
- Assuming swap solves it. Heavy swapping makes a site slow without crashing it.
Related
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