Ubuntu 14.04 LTS PHP-FPM workers exhausted causing 502 Bad Gateway

📖 ~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

Users receive 502 errors and application throughput drops during traffic peaks.

Environment & Reproduction

Seen on undersized PHP-FPM pools with long-running requests and slow dependencies.

Root Cause Analysis

All available workers become occupied, causing request queue overflows at upstream proxy.

Quick Triage

Check PHP-FPM status metrics and correlate with nginx/apache upstream failures.

Step-by-Step Diagnosis

Analyze request execution time, worker counts, and memory limits per pool.

Solution – Primary Fix

Tune pool settings, optimize slow requests, and increase capacity where needed.

Solution – Alternative Approaches

Add horizontal app nodes or offload expensive tasks to asynchronous workers.

Verification & Acceptance Criteria

502 rates drop to baseline and pool utilization remains below saturation thresholds.

Rollback Plan

Restore prior pool configuration if memory pressure rises beyond safe limits.

Prevention & Hardening

Monitor worker occupancy and enforce timeout safeguards for heavy endpoints.

Related Errors & Cross-Refs

upstream prematurely closed connection and server reached pm.max_children.

Related tutorial: View the step-by-step tutorial for Ubuntu 14.04 LTS.

View all Ubuntu 14.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

PHP-FPM process management and web stack capacity planning guides.