Kernel

/proc: processes and the kernel as files

/proc is a filesystem where the kernel shows processes (/proc/<pid>) and its own state: memory, load, parameters.

OOM killer: when memory runs out

A kernel mechanism: when memory runs out on the machine or in a cgroup with a limit, it kills a process with SIGKILL.

The kernel

What the Linux kernel is, what it does, and how to check its version and parameters.

Kernel modules

Kernel modules: loading and unloading with modprobe, kernel messages in dmesg, module parameters.

dmesg — kernel messages: OOM, disk and filesystem errors, segfaults

dmesg prints the kernel ring buffer: messages about disks, filesystems, OOM, crashed programs and hardware.

sysctl — kernel parameters at runtime

sysctl reads and changes kernel parameters; -w lasts until reboot, a file in /etc/sysctl.d/ makes it permanent.

Capabilities: root split into parts

Parts of root's rights: a process or a file gets only the one it needs, e.g. CAP_NET_BIND_SERVICE for a low port.

cgroups and namespaces — how containers work

Isolation (namespaces) and resource limits (cgroups) — the foundation of containers and systemd.