What is Linux Kernel? | DevOps Dictionary

Understanding the Linux Kernel

What Is the Linux Kernel in Simple Terms

The kernel is the operating system's core. Everything else — bash, nginx, Python, Docker, even the desktop — runs on top of the kernel. The kernel manages physical hardware (CPU, memory, disk, network), schedules which processes run when, and provides controlled access to hardware through system calls.

As a DevOps engineer, you interact with the kernel constantly — through sysctl parameters, kernel modules, system calls, and the /proc and /sys virtual filesystems — even if you never touch the kernel source code.

How It Works

Bash

+------------------------------------------+
| User Space                               |
| nginx, postgres, node, bash, docker      |
+------------------------------------------+
         | system calls (read, write, open)
         v
+------------------------------------------+
| Kernel Space                             |
| Process scheduler, memory manager        |
| VFS (virtual filesystem layer)           |
| Network stack (TCP/IP)                   |
| Device drivers                           |
+------------------------------------------+
         | hardware access
         v
+------------------------------------------+
| Hardware                                 |
| CPU, RAM, Disk, Network interface card   |
+------------------------------------------+

Practical Commands

Bash

## Check kernel version
uname -r
## 5.15.0-1031-aws  <- kernel version, build, and platform
 
uname -a
## Linux mumbai-prod-node-1 5.15.0-1031-aws #35-Ubuntu SMP ...
 
## View kernel messages (hardware events, OOM kills, driver messages)
dmesg | tail -20
dmesg | grep -i 'error\|fail\|oom'
 
## View kernel messages with timestamps
dmesg -T | tail -20
 
## Kernel parameters at runtime
sysctl -a | grep net.ipv4
sysctl net.ipv4.tcp_syncookies   ## view one parameter
sudo sysctl -w net.ipv4.tcp_syncookies=1  ## set immediately
 
## Persist sysctl settings
sudo tee /etc/sysctl.d/99-custom.conf << 'EOF'
net.ipv4.tcp_syncookies = 1
vm.swappiness = 10
EOF
sudo sysctl -p /etc/sysctl.d/99-custom.conf
 
## List loaded kernel modules
lsmod
 
## Load a kernel module
sudo modprobe br_netfilter   ## needed for Kubernetes networking
 
## Check if module is loaded
lsmod | grep br_netfilter
 
## Make module load at boot
echo 'br_netfilter' | sudo tee /etc/modules-load.d/k8s.conf
 
## View kernel boot parameters
cat /proc/cmdline
 
## Available memory and swap
cat /proc/meminfo | grep -E 'MemTotal|MemAvailable|SwapTotal'

Troubleshooting

Symptom	Command	What to Check
OOM kills happening	`dmesg	grep -i oom`
Kernel module missing	`lsmod	grep modulename`
High system CPU (sy)	`perf top`	Kernel code consuming CPU
Network issues	`sysctl net.ipv4`	Kernel network parameters

PLACEMENT PRO TIP
**Tip:** `dmesg -T | grep -i oom` is your first command when applications are dying unexpectedly. The OOM (Out of Memory) killer logs every process it kills, including why — the process name, PID, and memory usage. This immediately tells you if memory pressure is causing your application crashes.

REMEMBER THIS
**Remember:** sysctl changes made with `sysctl -w` are immediate but temporary — lost on reboot. Always write them to `/etc/sysctl.d/99-custom.conf` and run `sysctl -p` to make them persistent. Kubernetes setup guides often require specific sysctl values (like `net.bridge.bridge-nf-call-iptables=1`) that must persist across reboots.