Ubuntu 14.04 LTS PHP-FPM Pool Exhaustion Causes 502 Errors

📖 ~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 intermittent 502 responses during traffic peaks.

Environment & Reproduction

Seen with undersized pm.max_children and heavy long-running PHP requests.

Root Cause Analysis

Worker pool saturation leaves no capacity for new requests from web server.

Quick Triage

Check PHP-FPM status, process count, and upstream timeout logs.

Step-by-Step Diagnosis

Analyze slow logs, request concurrency, and memory per worker process.

Solution – Primary Fix

Tune pool limits, optimize slow endpoints, and adjust web upstream timeouts.

Solution – Alternative Approaches

Split workload into dedicated pools for critical and background endpoints.

Verification & Acceptance Criteria

502 rate drops to baseline and response times stabilize under load test.

Rollback Plan

Revert pool config if memory pressure causes broader instability.

Prevention & Hardening

Track pool utilization and enforce request timeout budgets.

Related Errors & Cross-Refs

Related with upstream timed out and max_children reached warnings.

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

Consult PHP-FPM pool management and Nginx/Apache proxy docs.