The kernel is the core of an operating system. It sits between hardware and software, managing CPU, memory, devices, and communication so applications can run safely and efficiently.
What the kernel does
- Process management: Creates, schedules, and terminates processes/threads; enforces isolation.
- Memory management: Virtual memory, paging, and permissions; ensures processes don’t trample each other.
- Device management: Talks to hardware via drivers; presents consistent interfaces to user space.
- File systems & I/O: Handles reads/writes, caching, and permissions across file systems.
- Security & isolation: Syscalls, user/kernel mode separation, permissions, namespaces, cgroups.
Kernel space vs user space
- User space: Where applications run; limited privileges.
- Kernel space: Full hardware access; executes system calls on behalf of user programs.
- Syscalls: Controlled entry points from user space to kernel services (e.g., open, read, write, fork, exec).
Scheduling & processes
- Schedulers pick which process/thread runs next based on policies and priorities.
- Context switching swaps CPU state between tasks so multiple processes appear to run concurrently.
Memory basics
- Virtual memory maps per-process address space to physical memory.
- Paging and caching improve efficiency; permissions protect memory regions.
Drivers & hardware abstraction
- Device drivers let the kernel speak to hardware (disks, NICs, GPUs) and expose uniform APIs to user space.
Isolation and resource control
- Namespaces: Isolate views of processes, networking, mounts (used by containers).
- cgroups: Limit and account for CPU, memory, IO, and network per group of processes.
Boot flow (simplified)
- Firmware/bootloader loads the kernel into memory.
- Kernel initializes hardware, memory, and drivers.
- Kernel starts PID 1 (init/systemd), which launches user-space services and apps.
Why it matters for DevOps/engineers
- Performance: Understanding scheduling, memory, and IO helps tune workloads.
- Reliability: Knowing syscalls and kernel logs aids debugging.
- Containers: Namespaces and cgroups are kernel features that enable container isolation.
Think of the kernel as the traffic controller for all hardware and software activity. Knowing its basics helps you diagnose issues, tune systems, and design reliable services.