In the intricate world of operating systems, scheduling algorithms are pivotal in managing how processes are allocated to the CPU for execution. These algorithms are designed to optimize various performance metrics such as CPU utilization, throughput, waiting time, and turnaround time. Let’s delve into the details of some common scheduling algorithms and understand their significance.
1. First-Come, First-Served (FCFS)
Overview
The First-Come, First-Served (FCFS) scheduling algorithm is one of the simplest types of CPU scheduling. As the name suggests, processes are executed in the order they arrive in the ready queue.
Characteristics
- Non-preemptive: Once a process starts its execution, it runs to completion.
- Implementation: Typically managed with a FIFO (First-In-First-Out) queue.
Pros
- Simple and easy to understand and implement.
- Fair in the sense that each process gets its turn.
Cons
- Can lead to the “convoy effect,” where shorter processes are delayed by longer ones.
- Poor performance for time-sharing systems due to high waiting times.
Example
Consider three processes arriving at different times with varying burst times:
- Process P1: Arrival Time = 0 ms, Burst Time = 5 ms
- Process P2: Arrival Time = 1 ms, Burst Time = 3 ms
- Process P3: Arrival Time = 2 ms, Burst Time = 8 ms
The order of execution will be P1 → P2 → P3, regardless of their burst times.
2. Shortest Job Next (SJN)
Overview
Shortest Job Next (SJN), also known as Shortest Job First (SJF), selects the process with the smallest burst time for execution next.
Characteristics
- Non-preemptive: Once a process starts, it runs to completion.
- Implementation: Requires knowledge of burst times beforehand, which is not always possible.
Pros
- Minimizes average waiting time.
- Efficient for batch processing systems.
Cons
- Difficult to predict burst times accurately.
- Can lead to “starvation” if shorter processes keep arriving.
Example
Using the same processes as above but sorted by burst time:
- Process P2: Burst Time = 3 ms
- Process P1: Burst Time = 5 ms
- Process P3: Burst Time = 8 ms
The order of execution will be P2 → P1 → P3.
3. Priority Scheduling
Overview
Priority Scheduling assigns a priority to each process, and the CPU is allocated to the process with the highest priority.
Characteristics
- Preemptive or Non-preemptive: Can be implemented in both ways.
- Implementation: Processes are placed in a priority queue.
Pros
- Ensures critical processes receive immediate CPU attention.
Cons
- Can lead to “starvation” of lower-priority processes.
- Requires careful assignment of priorities to avoid biases.
Example
Consider three processes with assigned priorities (higher number = higher priority):
- Process P1: Priority = 2
- Process P2: Priority = 3
- Process P3: Priority = 1
The order of execution will be P2 → P1 → P3.
4. Round Robin (RR)
Overview
Round Robin (RR) scheduling assigns a fixed time quantum to each process in the ready queue in a cyclic order.
Characteristics
- Preemptive: Each process is allowed to run for a maximum of one time quantum.
- Implementation: Processes are placed in a circular queue.
Pros
- Fair time allocation among processes.
- Good for time-sharing systems.
Cons
- Context switching overhead can be high if the time quantum is too small.
- Performance can degrade if the time quantum is too large.
Example
Consider three processes with a time quantum of 2 ms:
- Process P1: Burst Time = 5 ms
- Process P2: Burst Time = 3 ms
- Process P3: Burst Time = 8 ms
The order of execution will be P1 (2 ms) → P2 (2 ms) → P3 (2 ms) → P1 (3 ms) → P2 (1 ms) → P3 (6 ms).
5. Multilevel Queue Scheduling
Overview
Multilevel Queue Scheduling divides processes into different queues based on their priority or type. Each queue can have its own scheduling algorithm.
Characteristics
- Preemptive or Non-preemptive: Can be implemented in both ways.
- Implementation: Processes are assigned to different queues permanently.
Pros
- Flexibility in managing different types of processes.
- Efficient for systems with diverse workload types.
Cons
- Fixed assignment of processes to queues can be inflexible.
- Lower-priority queues may suffer from starvation.
Example
Consider two queues:
- System Processes (high priority, Round Robin)
- User Processes (low priority, FCFS)
System processes will always be executed before user processes.
6. Multilevel Feedback Queue (MLFQ)
Overview
Multilevel Feedback Queue (MLFQ) is a dynamic scheduling algorithm that allows processes to move between queues based on their behavior and execution history.
Characteristics
- Preemptive: Processes can be preempted based on dynamic criteria.
- Implementation: Multiple queues with different scheduling algorithms, allowing process migration.
Pros
- Adapts to varying process needs.
- Prevents starvation by aging processes.
Cons
- Complex to implement and manage.
- Requires fine-tuning of parameters for optimal performance.
Example
Consider three queues with different time quantums:
- Queue 1: Time Quantum = 2 ms
- Queue 2: Time Quantum = 4 ms
- Queue 3: FCFS
A process starts in Queue 1 and moves to lower-priority queues if it exceeds the time quantum.
Conclusion
Understanding scheduling algorithms is fundamental to optimizing CPU performance and ensuring efficient process management in operating systems. Each algorithm has its strengths and weaknesses, and the choice of algorithm depends on the specific requirements and constraints of the system. Whether it’s the simplicity of FCFS, the efficiency of SJN, or the flexibility of MLFQ, scheduling algorithms are the backbone of modern operating systems, ensuring that every process gets its fair share of CPU time.
