📊 Performance Analysis

# Basic top
top

# Key interactive commands inside top:
# 1       -- show per-CPU stats
# M       -- sort by memory usage
# P       -- sort by CPU usage (default)
# T       -- sort by cumulative time
# k       -- kill a process (enter PID)
# r       -- renice a process
# f       -- field management (add/remove columns)
# W       -- save current config
# q       -- quit

# Run top non-interactively (1 iteration, useful in scripts)
top -bn1

# Watch a specific process
top -p 12345
top -p $(pgrep nginx | tr '\n' ',')

# Install htop
sudo dnf install -y htop    # RHEL
sudo apt install -y htop    # Debian

# Run htop
htop

# Key htop features over top:
# Visual CPU/memory bars at the top
# Color-coded process tree
# Mouse support
# F3 -- search for a process by name
# F4 -- filter processes
# F5 -- tree view (show parent/child)
# F6 -- sort by column
# F9 -- kill selected process
# Space -- tag a process (for bulk operations)

# htop showing only processes from one user
htop -u craig

# htop for a specific PID and its children
htop -p 12345

# vmstat with 2-second interval, 10 samples
vmstat 2 10

# vmstat with timestamps
vmstat -t 2 10

# Show disk statistics
vmstat -d 2 5

# Show slab memory info
vmstat -m

# Show memory in human-readable format
free -h

# Show in MB
free -m

# Watch memory every 2 seconds
watch -n2 free -h

# More detailed memory breakdown
cat /proc/meminfo
cat /proc/meminfo | grep -E "MemTotal|MemFree|MemAvailable|Buffers|Cached|SwapTotal|SwapFree"

# Install sysstat (provides iostat, sar, mpstat)
sudo dnf install -y sysstat    # RHEL
sudo apt install -y sysstat    # Debian

# Basic iostat -- 2 second intervals, 5 samples
iostat -x 2 5

# Show specific device
iostat -x sda 2 5

# Human readable sizes
iostat -xh 2 5

# Install iotop
sudo dnf install -y iotop
sudo apt install -y iotop

# Run iotop (requires root)
sudo iotop

# Show only processes doing I/O (cleaner view)
sudo iotop -o

# Non-interactive -- good for scripts
sudo iotop -b -n 3 -o    # 3 iterations, only active I/O

# Find top I/O consumers right now
sudo iotop -b -n 1 -o | head -20

# ss -- socket statistics (modern netstat replacement)
ss -tulpn                   # listening ports
ss -s                       # summary statistics
ss -tn state established    # established TCP connections

# Count connections by state
ss -tan | awk 'NR>1 {print $1}' | sort | uniq -c | sort -rn

# Count connections per remote IP
ss -tan state established | awk 'NR>1 {print $5}' | \
    cut -d: -f1 | sort | uniq -c | sort -rn | head -10

# iftop -- live bandwidth by connection
sudo dnf install -y iftop
sudo iftop             # all interfaces
sudo iftop -i ens3     # specific interface

# nethogs -- bandwidth per process
sudo dnf install -y nethogs
sudo nethogs
sudo nethogs ens3

When you get the call "the server is slow" — run this:

#!/bin/bash
# perf-triage.sh -- first 60 seconds on a slow server

echo "========================================"
echo " Performance Triage: $(hostname)"
echo " $(date)"
echo "========================================"

echo ""
echo "--- UPTIME AND LOAD ---"
uptime

echo ""
echo "--- CPU SUMMARY (5 seconds) ---"
vmstat 1 5

echo ""
echo "--- MEMORY ---"
free -h
echo ""
cat /proc/meminfo | grep -E "MemAvailable|SwapFree|SwapTotal"

echo ""
echo "--- TOP CPU CONSUMERS ---"
ps aux --sort=-%cpu | head -10

echo ""
echo "--- TOP MEMORY CONSUMERS ---"
ps aux --sort=-%mem | head -10

echo ""
echo "--- DISK I/O (5 seconds) ---"
iostat -x 1 5 | tail -20

echo ""
echo "--- DISK SPACE ---"
df -h | grep -v tmpfs

echo ""
echo "--- NETWORK CONNECTIONS ---"
ss -s
echo ""
ss -tan state established | wc -l | xargs echo "Established TCP connections:"

echo ""
echo "--- RECENT ERRORS ---"
journalctl -b -p err --since "1 hour ago" --no-pager | tail -15

echo "========================================"