Operating Systems interview questions and quick answers

It pretends to grant the request, then checks whether some order exists in which every process can still finish with the resources left. If such a safe sequence exists, the request is granted; otherwise it waits.

A condition variable lets a thread sleep until some condition becomes true. wait() atomically releases the lock and sleeps so other threads can change the state; on wakeup the thread reacquires the lock and rechecks.

Context switching is when the OS pauses the process or thread running on a CPU, saves its state (registers, program counter, stack pointer) in its control block, and loads the saved state of another one so it can continue. It lets many programs share one CPU, but each switch is pure overhead.

The convoy effect is when many short processes queue behind one long CPU-bound process, so the CPU and I/O devices sit idle in turns and average waiting time rises sharply.

A counting semaphore holds a non-negative integer that tracks how many units of a resource are free. wait() takes one unit (blocking at zero) and signal() returns one. It limits how many threads use a pool at once.

FCFS runs processes in arrival order, SJF picks the shortest job next and gives the lowest average waiting time, Round Robin gives each process a fixed time slice in turn for fair response, and Priority runs the most important process first but can starve low-priority ones unless aging is used.

Confusing unsafe state with deadlock. Unsafe only means deadlock is possible.

"Banker's algorithm tentatively allocates, then looks for a safe sequence: repeatedly find a process whose remaining need fits in the available resources, let it finish and reclaim its holdings. If all can finish, the request is granted. It avoids deadlock but needs maximum needs declared upfront."