How to view slab/cache → slabtop

How to View Slab Cache with slabtop: A Complete System Administrator's Guide Table of Contents 1. [Introduction](#introduction) 2. [Prerequisites](#prerequisites) 3. [Understanding Slab Cache](#understanding-slab-cache) 4. [Installing slabtop](#installing-slabtop) 5. [Basic slabtop Usage](#basic-slabtop-usage) 6. [Command Options and Parameters](#command-options-and-parameters) 7. [Interpreting slabtop Output](#interpreting-slabtop-output) 8. [Practical Examples and Use Cases](#practical-examples-and-use-cases) 9. [Advanced Monitoring Techniques](#advanced-monitoring-techniques) 10. [Troubleshooting Common Issues](#troubleshooting-common-issues) 11. [Best Practices](#best-practices) 12. [Alternative Tools and Methods](#alternative-tools-and-methods) 13. [Conclusion](#conclusion) Introduction The Linux kernel's slab cache is a crucial memory management mechanism that optimizes the allocation and deallocation of frequently used kernel objects. Understanding and monitoring slab cache usage is essential for system administrators, developers, and performance analysts who need to diagnose memory-related issues, optimize system performance, and troubleshoot kernel memory leaks. The `slabtop` command provides a real-time, top-like interface for viewing kernel slab cache information, making it an invaluable tool for system monitoring and debugging. This comprehensive guide will walk you through everything you need to know about using `slabtop` effectively, from basic usage to advanced troubleshooting techniques. By the end of this article, you'll understand how to install, configure, and use `slabtop` to monitor your system's slab cache, interpret the output data, and identify potential memory management issues in your Linux environment. Prerequisites Before diving into `slabtop`, ensure you have the following: System Requirements - Linux operating system (kernel 2.6 or later) - Root or sudo privileges for installation and certain monitoring tasks - Basic understanding of Linux command line interface - Familiarity with system administration concepts Knowledge Prerequisites - Understanding of Linux memory management concepts - Basic knowledge of kernel operations - Familiarity with process monitoring tools like `top` and `htop` Hardware Requirements - Sufficient terminal access (local or SSH) - No specific hardware requirements beyond a functioning Linux system Understanding Slab Cache What is Slab Cache? The slab cache is a memory management mechanism in the Linux kernel that maintains pools of pre-allocated memory objects. Instead of repeatedly allocating and deallocating memory for commonly used kernel data structures, the slab allocator maintains caches of these objects, significantly improving performance and reducing memory fragmentation. How Slab Cache Works The slab allocator organizes memory into three main components: 1. Caches: Collections of one or more slabs containing objects of the same type 2. Slabs: Contiguous memory areas containing pre-allocated objects 3. Objects: Individual memory chunks of specific sizes for kernel data structures Common Slab Cache Objects Typical objects stored in slab caches include: - File system inodes - Directory entries (dentries) - Network socket structures - Process descriptors - Buffer heads - Memory mapping structures Installing slabtop Ubuntu/Debian Systems On Ubuntu and Debian-based distributions, `slabtop` is part of the `procps` package: ```bash Update package lists sudo apt update Install procps package (if not already installed) sudo apt install procps Verify installation which slabtop ``` Red Hat/CentOS/Fedora Systems For Red Hat-based distributions: ```bash For RHEL/CentOS 7 and earlier sudo yum install procps-ng For RHEL/CentOS 8+ and Fedora sudo dnf install procps-ng Verify installation slabtop --version ``` SUSE/openSUSE Systems ```bash Install procps package sudo zypper install procps Verify installation slabtop --help ``` Arch Linux ```bash Install procps-ng package sudo pacman -S procps-ng Verify installation slabtop --version ``` Building from Source If you need to compile from source: ```bash Download procps source wget https://sourceforge.net/projects/procps-ng/files/Production/procps-ng-3.3.17.tar.xz Extract and compile tar -xf procps-ng-3.3.17.tar.xz cd procps-ng-3.3.17 ./configure make sudo make install ``` Basic slabtop Usage Running slabtop The simplest way to run `slabtop` is without any arguments: ```bash slabtop ``` This command displays a real-time view of slab cache statistics, similar to the `top` command for processes. Basic Navigation Once `slabtop` is running, you can use these keyboard shortcuts: - q: Quit the program - Space: Refresh the display immediately - s: Change the refresh interval - c: Sort by cache size - l: Sort by number of slabs - o: Sort by number of objects - u: Sort by cache utilization Understanding the Default Display The default `slabtop` output shows: - Cache name - Number of objects - Object size - Cache size - Slab count - Objects per slab Command Options and Parameters Common Command Line Options Display Options ```bash Show help information slabtop --help Display version information slabtop --version Run once and exit (no continuous refresh) slabtop --once Set delay between updates (in seconds) slabtop --delay=2 Sort by specific column slabtop --sort=c # Sort by cache size slabtop --sort=l # Sort by number of slabs slabtop --sort=o # Sort by number of objects ``` Output Formatting ```bash Display only active caches slabtop --active Show specific number of lines slabtop --lines=20 Use alternative /proc/slabinfo path slabtop --slabinfo=/custom/path/slabinfo ``` Advanced Usage Examples Continuous Monitoring with Custom Interval ```bash Update every 5 seconds slabtop --delay=5 Monitor once per minute slabtop --delay=60 ``` Sorting and Filtering ```bash Sort by cache utilization and run once slabtop --sort=u --once Show only first 10 entries slabtop --sort=c --once | head -10 ``` Interpreting slabtop Output Column Meanings Understanding each column in the `slabtop` output is crucial for effective monitoring: OBJS (Objects) - Active: Number of objects currently in use - Total: Total number of objects allocated in the cache - Ratio: Active/Total ratio indicating cache efficiency SIZE (Object Size) - Size of individual objects in bytes - Helps identify memory usage patterns SLABS - Number of slabs allocated for this cache - Each slab contains multiple objects SOBJ (Objects per Slab) - Average number of objects that fit in each slab - Indicates packing efficiency CACHE SIZE - Total memory consumed by the cache - Calculated as: (number of slabs) × (slab size) NAME - Kernel cache identifier - Describes the type of objects stored Sample Output Analysis ``` Active / Total Objects (% used) : 1547325 / 1684742 (91.8%) Active / Total Slabs (% used) : 75234 / 75234 (100.0%) Active / Total Caches (% used) : 96 / 147 (65.3%) Active / Total Size (% used) : 284.44M / 301.23M (94.4%) Minimum / Average / Maximum Object : 0.01K / 0.19K / 8.00K OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME 524288 524288 100% 0.06K 8192 64 32768K anon_vma_chain 245760 245760 100% 0.19K 5856 42 46848K dentry 89600 89600 100% 0.55K 3200 28 49280K inode_cache ``` Key Metrics to Monitor Memory Utilization - High utilization (>90%) may indicate memory pressure - Low utilization (<50%) might suggest over-allocation Object Efficiency - Active/Total object ratio shows cache effectiveness - Consistently low ratios may indicate tuning opportunities Cache Growth Patterns - Rapidly growing caches may indicate memory leaks - Stable growth patterns are typically normal Practical Examples and Use Cases Example 1: Identifying Memory-Heavy Caches To find caches consuming the most memory: ```bash Sort by cache size and show top 10 slabtop --sort=c --once | head -20 ``` This helps identify which kernel subsystems are using the most memory. Example 2: Monitoring Inode Cache Usage File system performance often relates to inode cache efficiency: ```bash Monitor inode-related caches slabtop --once | grep -i inode ``` Look for: - `inode_cache`: File system inodes - `proc_inode_cache`: /proc filesystem inodes - `sock_inode_cache`: Socket inodes Example 3: Detecting Potential Memory Leaks Monitor cache growth over time: ```bash Create a monitoring script #!/bin/bash while true; do echo "=== $(date) ===" slabtop --sort=c --once | head -10 sleep 300 # Check every 5 minutes done ``` Example 4: Network-Related Cache Monitoring For network-intensive systems: ```bash Monitor network-related caches slabtop --once | grep -E "(tcp|udp|sock|skbuff)" ``` Common network caches include: - `tcp_bind_bucket` - `UDP-Lite` - `sock_inode_cache` - `skbuff_head_cache` Example 5: File System Cache Analysis Monitor file system caches: ```bash Monitor file system caches slabtop --once | grep -E "(dentry|buffer|ext4)" ``` Key file system caches: - `dentry`: Directory entries - `buffer_head`: File system buffers - `ext4_inode_cache`: ext4 filesystem inodes Advanced Monitoring Techniques Combining slabtop with Other Tools Using with vmstat ```bash Monitor overall memory statistics alongside slab info vmstat 5 & slabtop --delay=5 ``` Integration with sar ```bash Collect slab info with system activity reporter sar -r 5 10 & slabtop --delay=5 --once ``` Automated Monitoring Scripts Basic Monitoring Script ```bash #!/bin/bash slab_monitor.sh - Basic slab cache monitoring LOG_FILE="/var/log/slab_monitor.log" THRESHOLD_MB=100 while true; do TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S') # Get top 5 caches by size TOP_CACHES=$(slabtop --sort=c --once | head -10 | tail -5) echo "[$TIMESTAMP] Top slab caches:" >> $LOG_FILE echo "$TOP_CACHES" >> $LOG_FILE echo "" >> $LOG_FILE # Check for large caches slabtop --sort=c --once | awk -v threshold=$THRESHOLD_MB ' NR > 7 { if ($7 ~ /K$/) { size = substr($7, 1, length($7)-1) / 1024 } else if ($7 ~ /M$/) { size = substr($7, 1, length($7)-1) } if (size > threshold) { print "WARNING: Large cache detected - " $8 " (" $7 ")" } }' >> $LOG_FILE sleep 600 # Check every 10 minutes done ``` Advanced Analysis Script ```bash #!/bin/bash slab_analyzer.sh - Advanced slab cache analysis REPORT_FILE="/tmp/slab_analysis_$(date +%Y%m%d_%H%M%S).txt" echo "Slab Cache Analysis Report" > $REPORT_FILE echo "Generated: $(date)" >> $REPORT_FILE echo "=================================" >> $REPORT_FILE Overall statistics echo "" >> $REPORT_FILE echo "OVERALL STATISTICS:" >> $REPORT_FILE slabtop --once | head -6 >> $REPORT_FILE Top 10 caches by size echo "" >> $REPORT_FILE echo "TOP 10 CACHES BY SIZE:" >> $REPORT_FILE slabtop --sort=c --once | head -17 | tail -10 >> $REPORT_FILE Efficiency analysis echo "" >> $REPORT_FILE echo "CACHE EFFICIENCY ANALYSIS:" >> $REPORT_FILE slabtop --once | awk ' NR > 7 { if (NF >= 3 && $3 ~ /%/) { usage = substr($3, 1, length($3)-1) if (usage < 50) { print "Low efficiency: " $8 " (" $3 " utilization)" } } }' >> $REPORT_FILE echo "Report saved to: $REPORT_FILE" ``` Performance Tuning Based on slabtop Data Identifying Tuning Opportunities 1. Low Cache Utilization: Caches with consistently low usage might benefit from size reduction 2. High Fragmentation: Many small slabs might indicate fragmentation issues 3. Rapid Growth: Quickly growing caches might need size limits or investigation Kernel Parameter Tuning Based on slabtop observations, you might adjust: ```bash Adjust slab cache behavior echo 1 > /proc/sys/vm/slab_reclaim_ratio Modify memory reclaim behavior echo 60 > /proc/sys/vm/swappiness ``` Troubleshooting Common Issues Issue 1: slabtop Command Not Found Symptoms: - `bash: slabtop: command not found` - Command execution fails Solutions: ```bash Check if procps is installed dpkg -l | grep procps # Debian/Ubuntu rpm -qa | grep procps # Red Hat/CentOS Install missing package sudo apt install procps # Debian/Ubuntu sudo yum install procps-ng # Red Hat/CentOS 7 sudo dnf install procps-ng # Red Hat/CentOS 8+/Fedora ``` Issue 2: Permission Denied Errors Symptoms: - `slabtop: cannot open /proc/slabinfo` - Access denied messages Solutions: ```bash Check file permissions ls -la /proc/slabinfo Run with appropriate privileges sudo slabtop Check if /proc is mounted mount | grep proc ``` Issue 3: Incomplete or Garbled Output Symptoms: - Missing columns - Truncated display - Formatting issues Solutions: ```bash Increase terminal width export COLUMNS=120 slabtop Use alternative formatting slabtop --once | less Check terminal capabilities echo $TERM ``` Issue 4: High Memory Usage Without Clear Cause Symptoms: - Unexpectedly high slab cache usage - System performance degradation - Memory pressure warnings Investigation Steps: ```bash Identify largest caches slabtop --sort=c --once | head -20 Check for memory leaks echo 2 > /proc/sys/vm/drop_caches sleep 5 slabtop --sort=c --once | head -10 Monitor growth over time watch -n 30 'slabtop --sort=c --once | head -10' ``` Issue 5: Kernel Version Compatibility Symptoms: - Unexpected output format - Missing information - Feature unavailability Solutions: ```bash Check kernel version uname -r Verify /proc/slabinfo format cat /proc/slabinfo | head -5 Use kernel-appropriate options Older kernels may have different column layouts ``` Best Practices Monitoring Best Practices 1. Regular Monitoring Schedule ```bash Set up cron job for regular monitoring Add to crontab: 0 /6 /usr/local/bin/slab_monitor.sh ``` 2. Baseline Establishment Create baseline measurements during normal operation: ```bash Capture baseline during low activity slabtop --once > /tmp/slab_baseline.txt Compare current state to baseline diff /tmp/slab_baseline.txt <(slabtop --once) ``` 3. Alert Thresholds Establish meaningful alert thresholds: - Memory Usage: Alert when total slab usage exceeds 80% of available memory - Cache Growth: Alert on >50% cache size increase within an hour - Utilization: Alert on consistently low utilization (<30%) for large caches Performance Optimization 1. Cache-Specific Tuning ```bash For web servers, monitor dentry cache watch -n 10 'slabtop --once | grep dentry' For database servers, focus on buffer caches slabtop --once | grep -E "(buffer|page)" ``` 2. Memory Pressure Management ```bash Monitor memory pressure indicators cat /proc/meminfo | grep -E "(Slab|SReclaimable|SUnreclaim)" Force slab cache cleanup if needed (use carefully) echo 2 > /proc/sys/vm/drop_caches ``` Security Considerations 1. Access Control - Limit `slabtop` access to authorized personnel - Use sudo rules for controlled access: ```bash /etc/sudoers.d/slabtop %monitoring ALL=(root) NOPASSWD: /usr/bin/slabtop ``` 2. Log Management ```bash Secure log files chmod 640 /var/log/slab_monitor.log chown root:adm /var/log/slab_monitor.log Implement log rotation /etc/logrotate.d/slab_monitor /var/log/slab_monitor.log { daily missingok rotate 30 compress create 640 root adm } ``` Alternative Tools and Methods Direct /proc/slabinfo Access ```bash View raw slab information cat /proc/slabinfo Monitor specific cache watch -n 5 'grep dentry /proc/slabinfo' Parse with awk for custom analysis awk '/^[^#]/ {print $1, $3, $4}' /proc/slabinfo ``` Using /sys/kernel/slab ```bash Explore individual cache directories ls /sys/kernel/slab/ Get detailed cache information cat /sys/kernel/slab/dentry/aliases cat /sys/kernel/slab/dentry/cache_dma cat /sys/kernel/slab/dentry/cpu_slabs ``` Integration with Monitoring Systems Nagios Plugin Example ```bash #!/bin/bash check_slab.sh - Nagios plugin for slab monitoring WARNING_THRESHOLD=80 CRITICAL_THRESHOLD=90 TOTAL_SLAB=$(awk '/^Active.*Size/ {gsub(/[^0-9.]/, "", $6); print $6}' <(slabtop --once)) TOTAL_MEM=$(awk '/MemTotal/ {print $2/1024}' /proc/meminfo) USAGE_PERCENT=$(echo "scale=2; $TOTAL_SLAB / $TOTAL_MEM * 100" | bc) if (( $(echo "$USAGE_PERCENT > $CRITICAL_THRESHOLD" | bc -l) )); then echo "CRITICAL: Slab usage ${USAGE_PERCENT}% exceeds ${CRITICAL_THRESHOLD}%" exit 2 elif (( $(echo "$USAGE_PERCENT > $WARNING_THRESHOLD" | bc -l) )); then echo "WARNING: Slab usage ${USAGE_PERCENT}% exceeds ${WARNING_THRESHOLD}%" exit 1 else echo "OK: Slab usage ${USAGE_PERCENT}% is normal" exit 0 fi ``` Prometheus Metrics Collection ```bash #!/bin/bash slab_metrics.sh - Collect slab metrics for Prometheus METRICS_FILE="/var/lib/node_exporter/textfile_collector/slab_metrics.prom" { echo "# HELP slab_cache_size_bytes Size of slab cache in bytes" echo "# TYPE slab_cache_size_bytes gauge" slabtop --once | awk ' NR > 7 && NF >= 8 { cache_name = $8 size_str = $7 # Convert size to bytes if (size_str ~ /K$/) { size_bytes = substr(size_str, 1, length(size_str)-1) * 1024 } else if (size_str ~ /M$/) { size_bytes = substr(size_str, 1, length(size_str)-1) 1024 1024 } else { size_bytes = size_str } print "slab_cache_size_bytes{cache=\"" cache_name "\"} " size_bytes }' } > $METRICS_FILE ``` Conclusion The `slabtop` command is an essential tool for Linux system administrators and performance analysts who need to understand and monitor kernel memory usage. Through this comprehensive guide, we've explored everything from basic installation and usage to advanced monitoring techniques and troubleshooting strategies. Key Takeaways 1. Installation: `slabtop` is part of the procps package and is available on all major Linux distributions 2. Basic Usage: The command provides a real-time, top-like interface for viewing slab cache statistics 3. Interpretation: Understanding column meanings and metrics is crucial for effective monitoring 4. Practical Applications: Regular monitoring helps identify memory leaks, optimize performance, and troubleshoot issues 5. Advanced Techniques: Automation scripts and integration with monitoring systems enhance operational capabilities Next Steps To further enhance your system monitoring capabilities: 1. Implement Regular Monitoring: Set up automated monitoring scripts and establish baseline metrics 2. Integrate with Existing Tools: Combine `slabtop` with your current monitoring infrastructure 3. Develop Custom Solutions: Create tailored analysis scripts for your specific environment 4. Stay Updated: Keep your procps package updated to access the latest features and improvements Best Practices Summary - Monitor slab cache usage regularly as part of your system maintenance routine - Establish baselines and alert thresholds appropriate for your environment - Combine `slabtop` with other memory monitoring tools for comprehensive analysis - Document your findings and create runbooks for common issues - Train your team on proper interpretation and response procedures By mastering `slabtop` and implementing the techniques described in this guide, you'll be well-equipped to maintain optimal system performance and quickly identify memory-related issues in your Linux environment. Remember that effective system monitoring is an ongoing process that requires consistent attention and continuous improvement of your monitoring strategies. The knowledge gained from this guide provides a solid foundation for advanced Linux system administration and performance optimization. Continue to practice these techniques and adapt them to your specific use cases for maximum effectiveness.