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Operating Systems - Quick Revision

📌 Last-minute revision before interviews. Scan these points quickly.

Process & Thread

  • Process: Instance of program, separate memory space, heavyweight
  • Thread: Lightweight unit within process, shared memory, cheap context switch
  • Context switch: Save/restore PCB, expensive (TLB flush, cache invalidation)
  • fork(): Creates child process (copy-on-write), exec(): Replaces process image
  • Zombie: Terminated, not waited on. Orphan: Parent terminated, adopted by init

Scheduling

  • FCFS: Simple, convoy effect
  • SJF: Optimal avg wait, starvation possible
  • Round Robin: Fair, time quantum matters (too small = overhead, too large = FCFS)
  • Priority: Starvation solution = aging
  • MLFQ: Processes move between queues, most general

Synchronization

  • Mutex: Binary lock, owner must unlock
  • Semaphore: Counter (binary or counting), any thread can signal
  • Spinlock: Busy-wait lock, good for short critical sections
  • Critical Section: Code accessing shared resource, needs mutual exclusion

Deadlock

  • Four conditions: Mutual exclusion, Hold & wait, No preemption, Circular wait
  • Prevention: Break one condition
  • Avoidance: Banker’s algorithm (safe state check)
  • Detection: Resource allocation graph, cycle = deadlock

Memory Management

  • Paging: Fixed-size pages (4KB), no external fragmentation, page table maps VPN→PFN
  • Segmentation: Variable-size, matches programmer’s view, external fragmentation
  • TLB: Cache of page table entries, TLB miss = walk page table
  • Page replacement: FIFO (Belady’s anomaly), LRU (expensive), Clock (practical)
  • Thrashing: Working set > frames, solution: reduce multiprogramming

Virtual Memory

  • Demand paging: Load pages on demand, page fault if not in memory
  • Page fault: Expensive (~10ms), load from disk
  • Working set: Pages actively used in time window Δ

File System

  • Inode: Metadata (permissions, size, pointers to data blocks)
  • Hard link: Same inode, different name. Soft link: Different inode, stores path
  • Journaling: WAL for crash recovery

IPC

  • Pipes: Unidirectional, parent-child
  • Shared memory: Fastest, needs synchronization
  • Message queues: Structured messages
  • Sockets: Network-capable
  • Signals: Async notifications (SIGTERM, SIGKILL)

Key Concepts

  • User mode vs Kernel mode: Restricted vs full access, system calls transition
  • DMA: Direct Memory Access, bypasses CPU for I/O
  • Race condition: Non-deterministic from concurrent access
  • Priority inversion: High-priority waits on low-priority, solution: priority inheritance
  • Copy-on-write: fork() shares pages, copies on modification
  • Real-time OS: Hard (deadline = failure) vs Soft (degraded performance)

🔗 Cross-References