Operating Systems Interview Questions 2026: Top 40 With Answers

Why OS Questions Appear in Every Interview

Operating system concepts test fundamental understanding of how computers work — and interviewers use them as a proxy for engineering depth. A developer who understands processes, memory, and scheduling is better at writing efficient code, debugging production issues, and understanding system bottlenecks.

Process and Thread (Q1-Q10)

Q1: What is the difference between a process and a thread?

A process is an independent program in execution with its own memory space (code, data, heap, stack). A thread is a lightweight unit of execution within a process — all threads in a process share the same memory space (code, data, heap) but each has its own stack and registers. Creating a thread is faster and cheaper than creating a process. Context switching between threads is faster than between processes.

Q2: What is a context switch?

Saving the state (registers, program counter, stack pointer) of the currently running process/thread and loading the state of the next one. Allows the OS to multitask on a single CPU core. Context switches are expensive — too many degrade performance.

Q3: What is the difference between concurrency and parallelism?

Concurrency: multiple tasks making progress by sharing a CPU (time-slicing). Parallelism: multiple tasks actually running simultaneously on multiple CPU cores. Concurrency is about structure, parallelism is about execution.

Synchronisation and Deadlock (Q11-Q20)

Q11: What is a race condition?

When two or more threads access shared data simultaneously and the result depends on the relative timing of their execution. Example: two threads both read a counter (value=5), both increment it, both write 6 — the increment is lost. Should be 7.

Q12: What are the four conditions for deadlock?

Mutual exclusion: resource can only be held by one process at a time. Hold and wait: process holds a resource while waiting for another. No preemption: resources cannot be forcibly taken away. Circular wait: process A waits for B, B waits for C, C waits for A.

Q13: How do you prevent deadlock?

Break one of the four conditions. Most common: prevent circular wait by imposing a total ordering on resource acquisition (always acquire Lock 1 before Lock 2). Or use tryLock with timeout to detect and break cycles.

Memory Management (Q21-Q30)

Q21: What is virtual memory?

An abstraction that gives each process the illusion of having a large, private address space. The OS maps virtual addresses to physical RAM using page tables. When physical RAM is full, infrequently used pages are swapped to disk (swap space). This allows running more processes than fit in physical RAM.

Q22: What is paging vs segmentation?

Paging: memory divided into fixed-size blocks (pages). Eliminates external fragmentation. Simple to implement. May have internal fragmentation (last page not fully used). Segmentation: memory divided into variable-size logical segments (code, data, stack). More natural for programs but causes external fragmentation.

Q23: What is thrashing?

When a system spends more time swapping pages to/from disk than executing actual code — because the working set of all processes exceeds physical RAM. Symptoms: CPU utilisation drops while disk activity spikes. Fix: reduce number of active processes or add RAM.

Scheduling (Q31-Q40)

Q31: What is CPU scheduling and why does it matter?

The OS decides which process gets CPU time when multiple are ready. Good scheduling maximises CPU utilisation, minimises response time, and ensures fairness.

Q32: Compare FCFS, SJF, and Round Robin.

FCFS (First Come First Served): simple, no starvation, but long jobs block short ones (convoy effect). SJF (Shortest Job First): optimal average waiting time but requires knowing burst time in advance; can starve long jobs. Round Robin: each process gets a fixed time quantum (e.g. 10ms), then CPU moves to next. Fair, good for interactive systems.

Q33: What is priority scheduling and the starvation problem?

Processes assigned priorities — highest priority runs first. Problem: low-priority processes may never run if high-priority processes keep arriving. Solution: aging — gradually increase priority of waiting processes over time.

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