Common OS Interview Questions
fork() + exec()pthread_create() / Thread()Thread: 2-5ΞΌs, Process: 10-20ΞΌsJava 21: Virtual Thread (Loom)This page aggregates the most important interview questions across all OS topics. Useful for FAANG-style system design and backend engineering interviews.
Processes & Threads
Q1. What is the difference between a process and a thread?
A process is an isolated execution environment with its own private virtual memory address space (code, heap, stack, data, file descriptors) managed by the OS MMU. A thread is a lightweight execution context within a process; threads in the same process share the heap, text, and data segments but maintain private stacks, registers, and program counters. Process communication requires kernel-managed IPC (pipes, sockets, shared memory), whereas threads communicate directly via shared heap memory.
Q2. What happens step-by-step when you call fork() in Linux?
- The kernel allocates a new
task_struct(PCB) for the child process.- Copies the parent's file descriptor table, signal handlers, and virtual memory mappings.
- Configures child page tables as Copy-on-Write (pages shared read-only).
- Assigns a new unique PID to the child.
- Returns
0to the child process and the child'sPIDto the parent process.- Both processes execute concurrently from the next instruction following
fork().- On the first write attempt to a shared page by either process, the hardware page fault handler triggers a private physical page allocation.
Q3. What is a Zombie Process and how do you prevent Zombie leaks?
A zombie process is a process that has completed execution via
exit()but retains its entry in the kernel's process table because its parent process has not yet executedwait()orwaitpid()to read its exit status. To prevent zombie leaks, parent processes must handleSIGCHLDsignals and reap child processes viawaitpid(-1, WNOHANG). If a parent process terminates, the kernel automatically reparents orphaned zombies toinit(PID 1) for automatic cleanup.
Q4. Explain context switching and its direct and indirect costs.
A context switch saves the executing thread/process's CPU register state into its PCB and loads another process/thread's saved register state. Direct costs include saving/restoring registers (1β5Β΅s). Indirect costs include CPU L1/L2 cache invalidation and TLB flushes across process boundaries, degrading memory access latency.
CPU Scheduling
Q5. Why is Shortest Job First (SJF) optimal in theory but impractical in production systems?
SJF minimizes average waiting time across a set of tasks. However, it requires exact advance knowledge of each process's next CPU burst length, which is impossible to predict accurately in non-deterministic operating environments. Production OS schedulers approximate SJF using exponential moving averages of past CPU bursts or dynamic MLFQ priority demotions.
Q6. What is the difference between SCHED_FIFO, SCHED_RR, and SCHED_OTHER in Linux?
SCHED_FIFOis a real-time policy where a thread runs until it voluntarily yields or blocks (no time quantum).SCHED_RRis a real-time policy with a fixed time quantum.SCHED_OTHER(SCHED_NORMAL) is the default non-real-time policy managed by the Completely Fair Scheduler (CFS) using virtual runtime (vruntime) and nice values. Real-time policies always preemptSCHED_OTHERtasks.
Memory Management
Q7. How does Virtual Memory eliminate external memory fragmentation?
Virtual Memory uses Paging to partition physical RAM into fixed-size frames (typically 4 KB) and virtual address spaces into matching fixed-size pages. The MMU maps any virtual page to any available physical frame, eliminating external fragmentation because any free frame can fulfill an allocation request regardless of contiguous physical alignment.
Q8. What is Memory Thrashing and how is it detected and prevented?
Thrashing occurs when a process's active working set exceeds available physical RAM frames, causing the system to spend more time swapping pages to disk than executing instructions. Diagnosis: high swap-in/swap-out activity (
vmstat) with low CPU utilization. Prevention: increase physical RAM, reduce concurrency limits, or usemlock()to pin critical memory pages in RAM.
Synchronization & Deadlocks
Q9. What are the four Coffman conditions for Deadlock and how can each be prevented?
- Mutual Exclusion: Make resources sharable (e.g., read-only files).
- Hold and Wait: Require processes to request all needed resources simultaneously.
- No Preemption: Allow the OS to preempt resources held by a waiting process.
- Circular Wait: Enforce a strict global ordering on resource acquisition across all threads.
Q10. What is Priority Inversion and how is it resolved?
Priority Inversion occurs when a low-priority thread holds a lock needed by a high-priority thread, and a medium-priority thread preempts the low-priority thread, indirectly delaying the high-priority thread. Solution: Priority Inheritance, where the low-priority thread temporarily inherits the high-priority thread's priority while holding the lock.
Linux Internals & I/O
Q11. How do Linux Namespaces and cgroups enable container isolation?
Namespaces isolate what a process can see (providing virtualized views of PIDs, Network interfaces, Mount points, Hostnames, and User IDs). Control Groups (cgroups) restrict how much a process can consume (enforcing strict limits on CPU quota, RAM RSS memory limits, and Block I/O bandwidth). Containers are standard Linux processes governed by Namespaces and cgroups.
Q12. What is the internal difference between write() and fsync()?
write()copies data from user space buffers into the OS kernel Page Cache in RAM and returns immediately (non-durable write).fsync(fd)forces the kernel to flush all dirty page cache bytes and metadata associated with the file descriptor directly to non-volatile physical disk storage, blocking until hardware confirmation is received.