Swap Usage Monitoring in Linux Systems

Table of Contents

Introduction to Swap

Swap space in Linux serves as virtual memory extension to physical RAM. When the system runs low on physical memory, the kernel moves less frequently used pages from RAM to swap space on disk, freeing up RAM for active processes. Understanding and monitoring swap usage is crucial for system performance optimization and troubleshooting memory-related issues.

Swap Types

| Swap Type | Description | Advantages | Disadvantages | |-----------|-------------|------------|---------------| | Swap Partition | Dedicated disk partition for swap | Better performance, fixed size | Requires repartitioning to resize | | Swap File | Regular file used as swap space | Flexible sizing, easy to create/remove | Slightly slower than partition | | Swap on ZRAM | Compressed swap in RAM | Very fast, reduces I/O | Uses RAM, limited by compression ratio | | Swap on Network | Network-based swap (rare) | Can use remote storage | Network latency issues |

Understanding Swap Usage

Memory Management Hierarchy

` ┌─────────────────┐ │ CPU Cache │ ← Fastest access ├─────────────────┤ │ Physical RAM │ ← Primary memory ├─────────────────┤ │ Swap Space │ ← Virtual memory extension ├─────────────────┤ │ Storage Disk │ ← Slowest access └─────────────────┘ `

Swap States and Metrics

| Metric | Description | Importance | |--------|-------------|------------| | Total Swap | Total configured swap space | Capacity planning | | Used Swap | Currently utilized swap space | Memory pressure indicator | | Free Swap | Available swap space | Remaining capacity | | Swap In | Pages moved from swap to RAM | Performance impact | | Swap Out | Pages moved from RAM to swap | Memory pressure | | Swappiness | Kernel parameter controlling swap tendency | Tuning parameter |

Commands for Monitoring Swap

Primary Swap Monitoring Commands

| Command | Purpose | Output Format | Update Frequency | |---------|---------|---------------|------------------| | free | Display memory and swap usage | Human-readable summary | Static snapshot | | swapon | Show swap devices and usage | Detailed swap information | Static snapshot | | vmstat | Virtual memory statistics | Continuous monitoring | User-defined interval | | top/htop | Process and system monitoring | Interactive display | Real-time updates | | cat /proc/swaps | Kernel swap information | Raw kernel data | Static snapshot | | cat /proc/meminfo | Detailed memory information | Comprehensive memory stats | Static snapshot |

Detailed Command Explanations

1. free Command

The free command provides a quick overview of memory and swap usage.

#### Basic Syntax `bash free [options] `

#### Common Options

| Option | Description | Example | |--------|-------------|---------| | -h | Human-readable format | free -h | | -m | Display in megabytes | free -m | | -g | Display in gigabytes | free -g | | -s N | Repeat every N seconds | free -s 5 | | -t | Show totals | free -t | | -w | Wide mode (separate buffers/cache) | free -w |

#### Example Output Analysis `bash $ free -h total used free shared buff/cache available Mem: 15Gi 8.2Gi 1.1Gi 324Mi 6.1Gi 6.8Gi Swap: 8.0Gi 1.2Gi 6.8Gi `

Field Explanations: - total: Total installed swap space - used: Currently used swap space - free: Unused swap space - Swap usage percentage: (used/total) × 100

2. swapon Command

The swapon command manages and displays swap space information.

#### Display Swap Information `bash swapon --show `

#### Example Output `bash NAME TYPE SIZE USED PRIO /dev/sda2 partition 8G 1.2G -2 /swapfile file 2G 0B -3 `

#### Field Descriptions

| Field | Description | |-------|-------------| | NAME | Device or file path | | TYPE | Swap type (partition/file) | | SIZE | Total swap space | | USED | Currently used space | | PRIO | Priority (-1 to 32767, higher = more preferred) |

#### Administrative Commands `bash

Enable specific swap

Enable all configured swap

Disable specific swap

Disable all swap

3. vmstat Command

The vmstat command provides detailed virtual memory statistics.

#### Basic Usage `bash vmstat [delay] [count] `

#### Example with Continuous Monitoring `bash $ vmstat 2 5 procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 2 0 1228800 1126528 89472 6234112 0 4 12 28 45 89 5 2 92 1 0 1 0 1228800 1124352 89472 6234240 0 0 0 64 156 298 3 1 96 0 0 `

#### vmstat Field Explanations

Memory Fields: | Field | Description | Units | |-------|-------------|--------| | swpd | Used swap space | KB | | free | Free physical memory | KB | | buff | Buffer memory | KB | | cache | Cache memory | KB |

Swap Fields: | Field | Description | Significance | |-------|-------------|--------------| | si | Swap in (pages/second) | High values indicate memory pressure | | so | Swap out (pages/second) | High values indicate memory pressure |

Performance Indicators: - si + so > 100: Significant swap activity - si + so > 1000: Severe memory pressure - Consistent si/so: Ongoing memory shortage

4. Monitoring with /proc/swaps

Direct kernel information about swap usage.

`bash $ cat /proc/swaps Filename Type Size Used Priority /dev/sda2 partition 8388604 1228800 -2 /swapfile file 2097148 0 -3 `

5. Comprehensive Memory Information

`bash $ cat /proc/meminfo | grep -i swap SwapCached: 45312 kB SwapTotal: 10485752 kB SwapFree: 9256952 kB `

#### Swap-related /proc/meminfo Fields

| Field | Description | |-------|-------------| | SwapCached | Memory that was swapped out but is now back in RAM | | SwapTotal | Total available swap space | | SwapFree | Currently unused swap space |

Swap Configuration

Creating Swap Space

#### Method 1: Swap File Creation `bash

Create 2GB swap file

Set proper permissions

Format as swap

Enable swap

#### Method 2: Swap Partition `bash

Create partition using fdisk/parted

Format partition as swap

Enable swap partition

Permanent Swap Configuration

Add to /etc/fstab: `bash

Swap file entry

Swap partition entry

Swap Priority Configuration

`bash

High priority swap (preferred)

Lower priority swap (fallback)

Performance Analysis

Swap Performance Metrics

| Metric | Good | Concerning | Critical | |--------|------|------------|----------| | Swap Usage | < 25% | 25-75% | > 75% | | Swap In/Out Rate | < 10 pages/sec | 10-100 pages/sec | > 100 pages/sec | | Swap Wait Time | < 1% | 1-5% | > 5% |

Monitoring Scripts

#### Simple Swap Monitor `bash #!/bin/bash

swap_monitor.sh

while true; do echo "$(date): $(free | grep Swap | awk '{printf "Swap: %s/%s (%.1f%%)\n", $3, $2, $3/$2*100}')" sleep 60 done `

#### Advanced Swap Analysis `bash #!/bin/bash

advanced_swap_monitor.sh

log_swap_stats() { local timestamp=$(date '+%Y-%m-%d %H:%M:%S') local swap_info=$(free | grep Swap) local swap_total=$(echo $swap_info | awk '{print $2}') local swap_used=$(echo $swap_info | awk '{print $3}') local swap_percent=$(awk "BEGIN {printf \"%.2f\", $swap_used/$swap_total*100}") local vmstat_info=$(vmstat 1 2 | tail -1) local swap_in=$(echo $vmstat_info | awk '{print $7}') local swap_out=$(echo $vmstat_info | awk '{print $8}') echo "$timestamp,SWAP_TOTAL:$swap_total,SWAP_USED:$swap_used,SWAP_PERCENT:$swap_percent,SWAP_IN:$swap_in,SWAP_OUT:$swap_out" }

Main monitoring loop

Swappiness Tuning

The swappiness parameter controls how aggressively the kernel swaps memory pages.

#### Current Swappiness Value `bash cat /proc/sys/vm/swappiness `

#### Swappiness Guidelines

| Value | Behavior | Use Case | |-------|----------|----------| | 0 | Avoid swapping except to prevent OOM | Servers with sufficient RAM | | 10 | Minimal swapping | Database servers | | 60 | Default balanced approach | General purpose systems | | 100 | Aggressive swapping | Systems with fast storage |

#### Temporary Swappiness Change `bash sudo sysctl vm.swappiness=10 `

#### Permanent Swappiness Configuration Add to /etc/sysctl.conf: `bash vm.swappiness=10 `

Troubleshooting Common Issues

High Swap Usage Diagnosis

#### Step 1: Identify Memory-Intensive Processes `bash

Top processes by memory usage

Processes using swap

#### Step 2: Analyze Swap Activity `bash

Monitor swap activity

Check for swap thrashing

Swap Performance Issues

#### Identifying Swap Bottlenecks `bash

I/O statistics for swap device

Detailed swap device performance

Memory Leak Detection

#### Process Memory Growth Monitoring `bash #!/bin/bash

memory_leak_detector.sh

monitor_process() { local pid=$1 local process_name=$(ps -p $pid -o comm=) while kill -0 $pid 2>/dev/null; do local mem_info=$(ps -p $pid -o pid,vsz,rss,pmem) echo "$(date): $mem_info" sleep 60 done }

Usage: ./memory_leak_detector.sh

Best Practices

Swap Sizing Guidelines

| System RAM | Recommended Swap Size | Use Case | |------------|----------------------|----------| | < 2GB | 2x RAM | Desktop systems | | 2-8GB | Equal to RAM | General servers | | 8-64GB | 0.5x RAM or 4GB minimum | High-performance servers | | > 64GB | 4GB or hibernation needs | Enterprise systems |

Optimization Strategies

#### 1. Swap Placement Optimization - Place swap on fastest available storage - Use separate physical drives for swap when possible - Consider SSD for swap if available - Avoid network storage for swap

#### 2. Multiple Swap Devices `bash

Configure multiple swap devices with equal priority

#### 3. Monitoring Automation `bash

Cron job for swap monitoring

Add to crontab: /5 * /usr/local/bin/swap_alert.sh

#!/bin/bash

swap_alert.sh

SWAP_THRESHOLD=80 CURRENT_SWAP=$(free | grep Swap | awk '{printf "%.0f", $3/$2*100}')

if [ "$CURRENT_SWAP" -gt "$SWAP_THRESHOLD" ]; then echo "WARNING: Swap usage at ${CURRENT_SWAP}%" | \ mail -s "High Swap Usage Alert" admin@example.com fi `

Security Considerations

#### Swap Encryption `bash

Encrypt swap partition

#### Secure Swap File Creation `bash

Create swap file securely

Performance Monitoring Dashboard

#### Creating Swap Metrics Collection `bash #!/bin/bash

swap_metrics.sh - Collect comprehensive swap metrics

collect_metrics() { local timestamp=$(date '+%s') # Basic swap info local swap_stats=$(awk '/^Swap/ {print $2,$3,$4}' /proc/meminfo) # Swap activity local vm_stats=$(vmstat 1 2 | tail -1 | awk '{print $7,$8}') # Per-device swap usage local device_stats=$(cat /proc/swaps | tail -n +2 | awk '{print $1":"$3":"$4}') echo "$timestamp|$swap_stats|$vm_stats|$device_stats" }

Continuous collection

This comprehensive guide provides the foundation for effective swap monitoring and management in Linux systems. Regular monitoring of swap usage helps maintain optimal system performance and prevents memory-related issues before they impact system stability.