Problem

Operating system internals directly determine the cost of I/O operations, memory allocations, and scheduling decisions in backend services.

Why It Matters (Latency, Throughput, Cost)

System calls, page faults, memory allocations — each has a measurable cost that shows up in application latency when done in hot paths.

Mental Model

The OS provides abstractions (files, sockets, processes) backed by hardware resources. Understanding the cost of each abstraction is the key to avoiding unnecessary overhead.

Underlying Theory

References: processes-and-threads.md (01), virtual-memory.md (05), and network programming (05 module).

Complexity Analysis

Operating system operations range from O(1) (hash table lookups in VFS) to O(N) (scanning process lists). Hot-path operations are engineered to be O(1) or O(log N).

Benchmark

Syscall overhead: ~100–500ns per syscall (on modern Linux with Spectre/Meltdown mitigations).

Page fault: ~1–10μs for minor fault, ~100μs+ for major fault (disk I/O).

Key Takeaways

1. System calls cross the user/kernel boundary — ~100–500ns each.
2. I/O is expensive; buffering amortizes the cost.
3. Page faults are unavoidable on first access; prefetching reduces their impact.
4. The OS scheduler determines thread fairness and response time.

Related Modules

• ./01-processes-and-threads.md
• ../../05-network-programming/02-multiplexing-epoll-kqueue.md