OS Interview Questions with Answers

Process: independent program with own memory space, heavier to create (fork). Thread: lightweight unit within a process, shares memory space. Processes communicate via IPC (pipes, sockets). Threads share heap but have separate stacks. Multi-threading is faster but riskier (race conditions).

Virtual memory lets programs use more memory than physically available. Pages (fixed-size blocks, typically 4KB) are mapped from virtual to physical addresses via page table. When a page isn't in RAM → page fault → OS loads it from disk (swap). Benefits: isolation, larger address space, memory sharing.

FCFS: first come, first served (convoy effect). SJF: shortest job first (optimal avg wait). Round Robin: time quantum rotation (good for interactive). Priority: highest priority first (starvation possible, solved with aging). MLFQ: multiple queues with feedback. Most modern OS use MLFQ/CFS variants.

Mutex: binary lock, only one thread can hold it. Owner must release it. Used for mutual exclusion. Semaphore: counting mechanism allowing N threads. sem_wait() decrements, sem_post() increments. Any thread can signal. Use mutex for exclusive access, semaphore for resource pools (e.g., connection pool of 10).

Thrashing occurs when the OS spends more time swapping pages than executing processes. Cause: too many processes competing for limited RAM → constant page faults. Solution: (1) Increase RAM, (2) Reduce degree of multiprogramming, (3) Use working set model, (4) Better page replacement algorithms (LRU).