Which process in ready queue will get the processor?
Criterias:
- Processor Utilization
- Throughput
- Turn Around Time
- Waiting TIme
- Response Time
Scheduling Decisions take place under 5 possible scenarios:
- After process switches from Running to Blocked(Waiting) State [Process Request & I/O]
- After process switches from Running to Ready [Interrupt Occors]
- After process switches from Blocked to Ready State [Completion of I/O]
- Process Terminates [Completion of Running Processes]
- Newly created process arrives at ready state [New Job Accepted]
For scenario 1 and 4 => A new process must be selected for ready process queue
For scenario 2,3 and 5 => Should a previously running process be reselected? Or Would it be possible to select new process form ready queue?
| Pre-emptive | Non Pre-emptive |
|---|---|
| It acts on all above scenarios | It acts only on 1,4 scenarios (reselects previous running process in 2,3,5) |
| Highest Priority process among all ready process is allocated to CPU | Once CPU has been allocated to a process, processor keeps CPU until release of CPU |
| They allow real multi-programming | They do not allow real multiprogramming |
| They can lead the system to race condition | They do not lead the system to race condition |
| They are complex | They are simple |
1. First Come First Serve (FCFS)
2. Shortest-Job-First (SJF) Scheduling
3. Shortest Remaining Time
4. Priority Scheduling
5. Round Robin Scheduling
6. Multilevel Queue Scheduling
- Simplest
- Set of ready process is FIFO queue
- The process are serviced by CPU until completion of their order of their FIFO queue
- Once process is allocated, CPU keeps it until releasing CPU either by terminating or requesting I/O