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name: os-kernel-architecture description: | Operating-system and kernel architecture decisions: monolithic vs microkernel vs hybrid vs unikernel/exokernel, scheduler design, virtual memory & paging, IPC mechanisms, syscall/ABI boundaries, and interrupt handling. Architect-level trade-offs, not driver implementation.

USE WHEN: designing or evaluating an OS/kernel, RTOS-vs-GPOS choice, kernel structure, scheduler/memory/IPC subsystem design, syscall/ABI surface, "monolithic", "microkernel", "unikernel", "exokernel", "scheduler", "virtual memory", "IPC".

DO NOT USE FOR: Windows driver implementation (use windows driver skills); app-level concurrency (use language skills); container internals (use virtualization). allowed-tools: Read, Grep, Glob

OS / Kernel Architecture

Architect-level decisions for operating systems and kernels.

Kernel structure — the core decision

Structure	Idea	Pros	Cons	Fits
Monolithic	All services in kernel space (Linux)	Fast (no IPC for services), mature	Large TCB, a fault can panic the system	General-purpose, performance-first
Microkernel	Minimal kernel; drivers/FS/net as user servers (seL4, QNX)	Isolation, verifiability, restartable servers	IPC cost on hot paths	Safety/security-critical, high-assurance
Hybrid	Monolithic core + some servers (XNU, NT)	Pragmatic balance	Ambiguous boundaries	Commercial desktop/mobile OS
Unikernel	App + minimal libOS into one address space (MirageOS)	Tiny attack surface, fast boot	Single app, weak isolation within	Single-purpose cloud/edge appliances
Exokernel	Kernel only multiplexes hardware; libOS in app	Max app control	Complexity pushed to apps	Research / specialized perf

Decision drivers: isolation/assurance vs IPC overhead, TCB size, fault containment, restartability, verification goals (seL4 = formally verified).

Subsystems the architecture must pin down

• Scheduler: fairness (CFS) vs real-time (RMS/EDF, priority + inheritance to avoid priority inversion) vs throughput (batch). Preemptible vs cooperative. Tickless vs periodic tick. SMP load balancing, CPU affinity, NUMA awareness.
• Memory management: virtual memory + paging, page table levels, TLB pressure, huge pages, demand paging vs pinned, copy-on-write, NUMA placement, OOM policy. MMU-less (embedded) changes everything.
• IPC: synchronous rendezvous (seL4/L4) vs async message queues vs shared memory + doorbells. IPC latency is the microkernel make-or-break metric.
• Syscall/ABI: trap vs syscall instruction, vDSO for hot read-only calls, capability-based vs ambient-authority, ABI stability contract.
• Interrupts: top-half/bottom-half split, threaded IRQs, interrupt latency and determinism (hard real-time needs bounded latency), MSI/MSI-X.

When to recommend what

• Need provable isolation / restartable drivers → microkernel (seL4/QNX).
• Need max throughput, rich ecosystem → monolithic (Linux).
• Single cloud/edge appliance → unikernel.
• Hard real-time → RTOS or PREEMPT_RT, EDF/RMS scheduling, bounded IRQ latency (see embedded-rtos).

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