Ubuntu 18.04 LTS – PHP-FPM pool crashes under load with memory limit issues – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Web requests fail intermittently and backend returns 502/504 errors.

Environment & Reproduction

Ubuntu 20.04 running Nginx with PHP-FPM during traffic spikes.

Root Cause Analysis

Pool worker settings exceed available memory and trigger process churn.

Quick Triage

Check php-fpm logs, process manager stats, and host memory usage.

Step-by-Step Diagnosis

Calculate worker memory footprint and reconcile pm.* values with RAM.

Solution – Primary Fix

Tune pool concurrency and memory limits, then restart php-fpm.

Solution – Alternative Approaches

Split workloads into separate pools or scale app nodes horizontally.

Verification & Acceptance Criteria

Error rate drops and request latency remains stable at peak load.

Rollback Plan

Restore prior pool config if new limits reduce throughput excessively.

Prevention & Hardening

Track per-worker memory and tune automatically from observed baselines.

Related Errors & Cross-Refs

Kernel OOM events and swap thrash often accompany FPM instability.

Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.

View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

PHP-FPM process manager tuning for Ubuntu production systems.