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Key Concepts Glossary
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Key Concepts Glossary
A quick-reference glossary of core terms covered in Weeks 3 and 5 of the Operating Systems course. Each term links to its dedicated topic page where available. OS Course Notes — Weeks 3 & 5.md
Processes & Threads
| Term | One-line Definition |
|---|---|
| Program | A passive executable stored on disk; not yet running. |
| Process | An active entity loaded into memory with its own address space (code, data, heap, stack). |
| PCB (Process Control Block) | OS data structure tracking a process's state, program counter, registers, memory info, and open files. |
| Process States | The lifecycle a process moves through: *new → ready → running → waiting → terminated*. |
| Ready Queue | The queue of processes waiting to be assigned the CPU by the scheduler. |
| Context Switch | Saving the current process's state to its PCB and restoring the next process's state; considered pure overhead. |
| Thread | A unit of execution inside a process; threads share code, data, and heap but each has its own stack and registers. |
| Multithreading | Running multiple threads within a process to gain responsiveness, resource sharing, economy, and multicore utilisation. |
| Race Condition | A synchronisation problem that occurs when multiple threads share resources without proper coordination. |
CPU Scheduling
| Term | One-line Definition |
|---|---|
| CPU Scheduling | The OS mechanism that decides which process in the ready queue gets the CPU next. |
| CPU Utilisation | The percentage of time the CPU is kept busy; a key scheduling goal to maximise. |
| Throughput | Number of processes completed per unit of time; a key scheduling goal to maximise. |
| Turnaround Time | Total time from process submission to completion; a key scheduling goal to minimise. |
| Waiting Time | Total time a process spends sitting in the ready queue; a key scheduling goal to minimise. |
| Response Time | Time from submission until the first response is produced; important for interactive systems. |
| FCFS | *First-Come, First-Served* — schedules processes in arrival order; simple but susceptible to the convoy effect. |
| Convoy Effect | A long job at the front of the queue forces all shorter jobs to wait, raising average waiting time (FCFS weakness). |
| SJF | *Shortest-Job-First* — runs the shortest burst next; minimises average waiting time but risks starvation and requires burst prediction. |
| Starvation | A situation where a low-priority (or long) process is indefinitely delayed because higher-priority processes keep arriving. |
| Round Robin (RR) | Gives each process a fixed time quantum in turn; good responsiveness but small quanta increase context-switch overhead. |
| Time Quantum | The fixed time slice allocated to each process under Round Robin scheduling. |
| Priority Scheduling | Runs the highest-priority process first; starvation is mitigated by *aging*. |
| Aging | A technique that gradually increases a waiting process's priority to prevent starvation. |
| Preemptive Scheduling | The OS can forcibly reclaim the CPU from a running process (e.g., Round Robin, preemptive SJF). |
| Non-preemptive Scheduling | The CPU is only released when the running process voluntarily gives it up. |
| Gantt Chart | A bar chart used to visualise CPU scheduling order and calculate per-process waiting times. |
See Also
- Processes — deep dive into process concepts, states, and the PCB
- Threads — threads, multithreading benefits, and synchronisation issues
- CPU Scheduling — all scheduling algorithms explained with comparisons
- Exam Prep — Midterm — checklist and practice questions for the midterm
- Course Overview — full topic index and key dates