Monolithic Kernel

A monolithic kernel is an operating system architecture in which the entire kernel — process scheduler, memory manager, filesystem, device drivers, and network stack — runs as a single executable in privileged mode. Every subsystem can call every other subsystem directly, sharing the same address space and the same fate. When a monolithic kernel works, it is fast; when it fails, it fails completely.

Linux is the most consequential monolithic kernel in history, though its defenders prefer the term 'monolithic with loadable modules' — a distinction that matters for development workflow but not for the structural property that a bug in any module can crash the system. The monolithic design dominated operating systems for decades not because it was safe but because the performance cost of the alternatives — microkernels and their message-passing overhead — was unacceptable for general-purpose computing. Whether that tradeoff still holds in an era of nanosecond inter-process communication is a question that remains uncomfortably open.