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Chapter 2.3 - Memory Management and Swap

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand how Linux manages memory
  • Explain virtual memory and paging
  • Understand swap and its role
  • Monitor memory usage
  • Optimize memory usage

What is Memory Management?​

Linux uses a sophisticated memory management system that combines physical memory (RAM) and virtual memory (swap) to optimize performance and allow more processes to run than the available RAM.

Physical Memory vs Virtual Memory​

Physical Memory (RAM)​

  • Fast: Direct access by the CPU
  • Limited: Fixed amount installed
  • Volatile: Lost on reboot
  • Costly: More expensive than disk storage

Virtual Memory​

  • Abstract: Each process sees a virtual memory space
  • Paging: Divided into pages (usually 4KB)
  • Mapping: Virtual pages mapped to RAM or swap
  • Protection: Isolation between processes

Paging and MMU​

The MMU (Memory Management Unit) handles address translation:

Swap​

Swap is disk space used as an extension of RAM:

Roles:

  • Extending available memory
  • Supporting more processes
  • Improving stability

Types:

  • Swap partition: Dedicated partition (recommended)
  • Swap file: Swap file (more flexible)

Configuration:

# View swap
free -h
swapon --show

# Create a swap file
sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

# Enable at boot
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Memory Monitoring​

Monitoring Commands​

# Overview
free -h

# Details per process
top
htop

# Detailed statistics
cat /proc/meminfo

# Usage by process
ps aux --sort=-%mem | head

Interpretation​

  • MemTotal: Total RAM
  • MemFree: Free RAM
  • MemAvailable: Available RAM (free + reclaimable cache)
  • SwapTotal: Total swap
  • SwapFree: Free swap

Memory Optimization​

Best Practices​

  1. Monitor regularly

    • Use free, top, htop
    • Identify resource-intensive processes

  2. Adjust swap

    • Recommended size: 1-2x RAM
    • More if server with a lot of RAM

  3. Limit processes

    • Use ulimit to limit memory
    • Configure OOM killer

  4. Clean cache

    # Free cache (if necessary)
    sync
    echo 3 | sudo tee /proc/sys/vm/drop_caches

Summary​

In this chapter, you learned:

Memory Management: RAM + Swap to optimize performance
Virtual Memory: Abstraction for each process
Paging: Division into pages, managed by MMU
Swap: Extension of RAM on disk
Monitoring: free, top, htop, /proc/meminfo
Optimization: Monitoring, swap adjustment, limits

Next Steps​

Chapter 2.4: Virtual File System (VFS)
Chapter 2.5: Process Management

Chapter created on: December 2024