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OOM killer — glossary

TL;DR

Linux kernel mechanism that terminates processes when memory is critically low.

Key facts

Term
OOM killer

TL;DR

The OOM killer is a Linux kernel safeguard that terminates processes when the system runs out of RAM — often taking down PHP-FPM, MySQL, or Redis without a graceful shutdown.

Symptoms

  • Out of memory: Killed process in dmesg or /var/log/kern.log
  • Sudden 502/504 from nginx when all FPM workers vanish
  • MySQL gone away errors after mysqld is chosen as victim

Manual response

  1. Identify the victim process and its RSS from kernel logs
  2. Reduce pm.max_children, queue concurrency, or runaway cron
  3. Add swap only as a buffer — fix allocation, do not mask leaks

Use the OOM analyser and PHP-FPM calculator.

Automation

Reflex playbooks react to memory pressure and OOM signatures with dry-run gates and rollback-safe actions.

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