CPU and I/O scheduling are critical components of operating systems, designed to optimize the performance and resource utilization of a computer system. They determine the order in which processes access the CPU and I/O devices, ensuring fairness and efficiency. This article provides an in-depth analysis of these scheduling techniques, supplemented by schematics and code examples.
What is CPU Scheduling?
CPU scheduling determines which process gets access to the CPU when multiple processes are in the ready queue. It maximizes CPU utilization by allocating CPU time to various processes based on specific algorithms.
CPU Scheduling Algorithms
1. First-Come, First-Served (FCFS):
Processes are scheduled in the order of arrival.
Simple but can lead to long waiting times for later processes.
2. Shortest Job Next (SJN):
Selects the process with the shortest burst time.
Can cause starvation for longer processes.
3. Round Robin (RR):
Processes are assigned a fixed time quantum.
Ensures fairness and responsiveness.
4. Priority Scheduling:
Processes are prioritized based on their importance or urgency.
Can lead to starvation, mitigated by aging techniques.
5. Multilevel Queue Scheduling:
Divides processes into multiple queues, each with a different priority level.
What is I/O Scheduling?
I/O scheduling manages the sequence in which I/O requests are processed. Efficient I/O scheduling minimizes disk seek time, latency, and overall system bottlenecks.
I/O Scheduling Algorithms
1. First-Come, First-Served (FCFS):
Processes I/O requests in the order of arrival.
Simple but inefficient for high workloads.
2. Shortest Seek Time First (SSTF):
Selects the request closest to the current disk head position.
Can lead to starvation for distant requests.
3. SCAN (Elevator Algorithm):
Moves the disk head back and forth, servicing requests in one direction at a time.
4. C-SCAN (Circular SCAN):
Like SCAN but only services requests in one direction, then resets.
5. LOOK/C-LOOK:
Similar to SCAN/C-SCAN but only goes as far as the last request in the current direction.
Schematic: CPU Scheduling Process
+———+ +———-+ +———+
| Ready | —-> | CPU Core | —-> | Waiting |
| Queue | +———-+ | Queue |
+———+ +———+
Code Example: Round Robin CPU Scheduling in Python
def round_robin(processes, burst_times, quantum):
n = len(processes)
remaining_time = burst_times[:]
time = 0
while any(remaining_time):
for i in range(n):
if remaining_time[i] > 0:
execution_time = min(remaining_time[i], quantum)
time += execution_time
remaining_time[i] -= execution_time
print(f”Process {processes[i]} executed for {execution_time} units, remaining time: {remaining_time[i]}”)
processes = [‘P1’, ‘P2’, ‘P3’]
burst_times = [10, 5, 8]
quantum = 3
round_robin(processes, burst_times, quantum)
Performance Metrics for Scheduling
1. CPU Utilization: Percentage of time the CPU is busy.
2. Throughput: Number of processes completed per unit time.
3. Turnaround Time: Total time taken by a process from submission to completion.
4. Waiting Time: Total time a process spends in the ready queue.
5. Response Time: Time from submission to the first response.
Conclusion
CPU and I/O scheduling play a pivotal role in optimizing system performance by efficiently allocating resources and reducing latency. Understanding and implementing the right scheduling algorithms are crucial for achieving a balance between responsiveness, fairness, and throughput. These mechanisms ensure that modern operating systems can handle complex multitasking and high workloads effectively.