Apache Cannot Allocate Shared Memory

📖 ~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

Apache fails to start or reload, causing service interruption for hosted applications.

Environment & Reproduction

Ubuntu 20.04 Apache deployments with mod_ssl/cache modules on memory-constrained hosts.

Root Cause Analysis

Insufficient shared memory limits or oversized module caches prevent allocation at startup.

Quick Triage

Inspect Apache error log and system memory/shm mount usage immediately after failure.

Step-by-Step Diagnosis

Correlate module configuration with `shm` limits and process model memory footprint.

Solution – Primary Fix

Reduce cache segments, tune MPM workers, and adjust shared memory resources.

Solution – Alternative Approaches

Disable non-critical modules or migrate high-cache workloads to dedicated nodes.

Verification & Acceptance Criteria

Apache starts/reloads cleanly and no shared memory allocation errors recur.

Rollback Plan

Restore prior Apache config if new tuning degrades throughput or compatibility.

Prevention & Hardening

Capacity plan shared memory usage and track module growth during change reviews.

Related Errors & Cross-Refs

`Cannot allocate shared memory`, `AH00023`, and low-memory OOM side effects.

Related tutorial: View the step-by-step tutorial for Ubuntu 20.04 LTS.

View all Ubuntu 20.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Apache memory tuning references and Ubuntu service hardening guidance.