Questions
3–4 questions per semester exam
Difficulty
Medium
Importance
High yield for Operating Systems core papers
Overview
Process Management is a fundamental pillar of Operating Systems that governs how a CPU executes concurrent tasks efficiently. Understanding how the OS tracks state transitions, schedules jobs, and manages resource overhead through context switching is essential for high-scoring answers in university examinations.
Process States and PCB
A process transitions through various stages during its lifecycle, documented in a Process Control Block (PCB). The PCB is a data structure used by the kernel to store all necessary information to resume a process after an interrupt.
- Major States: New, Ready, Running, Waiting, Terminated
- PCB Contents: Process ID, Program Counter, CPU Registers, Memory Limits
- Transition: Scheduler dispatch moves process from Ready to Running
- Transition: I/O or event wait moves process from Running to Waiting
- PCB ensures OS can implement multiprogramming effectively
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Process Scheduling Algorithms
Scheduling algorithms determine the order of process execution to maximize CPU utilization and minimize response time. These are categorized into non-preemptive and preemptive types based on whether the CPU can be taken away from a process.
- First-Come, First-Served (FCFS): Non-preemptive, prone to Convoy effect
- Shortest Job First (SJF): Minimizes average waiting time
- Round Robin (RR): Preemptive, relies on a fixed Time Quantum
- Priority Scheduling: Based on urgency or importance level
- Multilevel Queue: Processes partitioned into separate queues
Context Switching
Context switching is the overhead process of saving the state of the currently running process and loading the state of a new process. While necessary for multitasking, frequent switching leads to performance degradation due to CPU idle time.
- Involves saving the PCB of the current process
- Reloading the PCB of the next scheduled process
- Performed by the CPU Dispatcher
- Context switch time is pure overhead and non-productive
- Hardware support can reduce switch latency
Formula Sheet
Turnaround Time = Completion Time - Arrival Time
Waiting Time = Turnaround Time - Burst Time
CPU Utilization = (Total Time - Idle Time) / Total Time * 100
Exam Tip
Always draw the Process State Transition Diagram and a clean Gantt chart for scheduling problems, as these carry the most marks in descriptive papers.
Common Mistakes
- Confusing the 'Ready' state with the 'Running' state during diagrammatic representations.
- Failing to account for arrival time in SJF or Round Robin calculations, leading to wrong Gantt charts.
- Neglecting to mention the overhead of context switching when discussing the efficiency of time-slicing algorithms.
More Revision Notes
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