Introduction: Scheduling Algorithms
Operating systems play a critical role in managing computer resources efficiently and one of their key tasks is scheduling processes for execution on the CPU. Scheduling algorithms determine the order in which processes are executed, impacting factors such as CPU utilization, turnaround time and fairness.
In this article, we’ll delve into several common scheduling algorithms, illustrating their principles with examples.
First-Come, First-Served (FCFS)
- FCFS is one of the simplest scheduling algorithms.
- Processes are executed in the order they arrive in the ready queue.
- The potential downside is the “convoy effect“, where smaller processes get stuck behind longer processes.
Example: Consider three processes arriving in the following order.
Process Burst Time P1 10 P2 5 P3 8
The scheduling under FCFS would be:
Time 0: P1 Time 10: P2 Time 15: P3
Shortest Job Next (SJN) or Shortest Job First (SJF)
- SJF schedules the process with the shortest burst time next.
- Reduces turnaround time and waiting time.
- Requires knowledge of burst times, which is often not available in practice.
Example: Using the same processes as above, but with burst times.
Process Burst Time P1 10 P2 5 P3 8
The scheduling under SJF would be:
Time 0: P2 Time 5: P3 Time 13: P1
Round Robin (RR)
- RR assigns a fixed time quantum to each process.
- Processes are scheduled in a circular order.
- Aims for fairness but may result in higher turnaround time.
Example: Assuming a time quantum of 3 units.
Process Burst Time P1 10 P2 5 P3 8
The scheduling under RR would be:
Time 0: P1 Time 3: P2 Time 6: P3 Time 9: P1 Time 12: P3 Time 15: P1
Priority Scheduling
- Each process is assigned a priority, and the highest priority process is scheduled next.
- May suffer from priority inversion issues.
Example: Assigning priority values (lower number indicates higher priority).
Process Burst Time Priority P1 10 2 P2 5 1 P3 8 3
The scheduling under priority scheduling would be:
Time 0: P2 Time 5: P1 Time 15: P3
Multilevel Feedback Queue Scheduling:
- Processes are divided into multiple priority levels, and they move between queues based on their behavior.
- Helps in adapting to the dynamic nature of processes.
Example: Assuming three queues with different priorities and a time quantum.
Queue 1 (High Priority): Time Quantum = 8 Queue 2 (Medium Priority): Time Quantum = 16 Queue 3 (Low Priority): FCFS
Processes move between queues based on their behavior.
Lottery Scheduling
- Processes are assigned lottery tickets, and a random ticket is drawn to select the next process.
- Balances fairness and efficiency.
Example: Assigning lottery tickets to processes.
Process Tickets P1 5 P2 2 P3 3
A ticket is randomly selected to determine the next process.
Fair Share Scheduling
- Allocates CPU time based on the fair share each user or group is entitled to.
- Prevents a single user or group from monopolizing the CPU.
Example: Assuming two users, each entitled to 50% of CPU time.
User 1: P1, P2 User 2: P3, P4
The scheduler ensures each user gets a fair share of CPU time.
These examples provide a simplified understanding of how different scheduling algorithms work. In a real-world scenario, the calculations and decisions are more complex, and the actual scheduling might be influenced by factors such as process priority, quantum time, aging, etc.
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Conclusion
In conclusion, scheduling algorithms are a crucial aspect of operating systems, influencing system performance and user experience. The choice of a scheduling algorithm depends on the specific goals and requirements of the system.
Operating systems often employ a combination of these algorithms or variations tailored to meet the demands of diverse workloads. Understanding these scheduling algorithms is essential for system administrators and developers to make informed decisions regarding system performance and responsiveness.
