Process Scheduling / CPU Scheduling
- Process scheduling is the OS mechanism that decides which processes run and in what order.
Goal: Ensure that the CPU is not idle and is always doing useful work.
Role of CPU Scheduler
- Selects the next process to execute from the ready queue.
- Uses a scheduling algorithm to choose the next process.
Aims of CPU Scheduling
- Ensure Fairness: Allocate CPU time fairly among all processes.
- Maximize CPU Utilization: Minimize idle time to keep CPU busy.
- Maximize Throughput: Increase the number of completed processes in a given period.
- Minimize Waiting Time: Reduce time processes spend in the ready queue.
- Minimize Response Time: Shorten the time between process submission and response.
- Minimize Turnaround Time: Decrease total time from process submission to completion.
Types of CPU Scheduling
- Preemptive Scheduling:
- OS can interrupt and suspend a currently running process to start or resume another.
- Non-Preemptive Scheduling:
- Once a process starts executing, it cannot be interrupted until it finishes or voluntarily yields the CPU.
| Aspect | Preemptive Scheduling | Non-Preemptive Scheduling |
|---|---|---|
| Interruption | Yes, processes can be interrupted | No, once a process starts, it runs to completion |
| Context Switching | Frequent, leading to overhead | Less frequent, resulting in lower overhead |
| Complexity | More complex to implement | Simpler to implement |
| Responsiveness | High, suitable for real-time systems | Lower, not suitable for real-time systems |
| Starvation Risk | Lower but possible with improper priority management | Higher, especially for low priority processes |
| Example Algorithms | Round Robin, Priority Scheduling (Preemptive) | First-Come First-Served (FCFS), Priority Scheduling (Non-Preemptive) |
Important Keywords
- • Burst Time: The amount of time a process requires the CPU between I/O waits.
- Throughput: Number of processes completed per unit of time.
- Turnaround Time: Total time from submission to completion of a process.
- Waiting Time: Time a process spends waiting in the ready queue before it gets executed.
Pre-emptive Scheduling (Algorithms)
Round Robin Scheduling (RR)
- The oldest, simplest, fairest and most widely used algorithm
- Designed for time-sharing systems
- A time quantum / time slice is defined for each process
- The ready queue is treated as a circular queue
- To implement round robin, the ready queue is kept as a first in first out queue
- It tries to be fair by equally distributing the processing time among all the processes
- When the process uses up the time slice allocated to itself, it is put on the end of the ready list
Advantages:
- Fair distribution of processing time.
- Good response time for interactive processes.
Disadvantages:
- If time slice is too short: Too many context switches → Lower CPU efficiency.
- If time slice is too long: Poor response time for short tasks.