Operating System Schedupng algorithms

A Process Scheduler schedules different processes to be assigned to the CPU based on particular schedupng algorithms. There are six popular process schedupng algorithms which we are going to discuss in this chapter −

First-Come, First-Served (FCFS) Schedupng

Shortest-Job-Next (SJN) Schedupng

Priority Schedupng

Shortest Remaining Time

Round Robin(RR) Schedupng

Multiple-Level Queues Schedupng

These algorithms are either non-preemptive or preemptive. Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive schedupng is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state.

First Come First Serve (FCFS)

Jobs are executed on first come, first serve basis.

It is a non-preemptive, pre-emptive schedupng algorithm.

Easy to understand and implement.

Its implementation is based on FIFO queue.

Poor in performance as average wait time is high.

Wait time of each process is as follows −

Average Wait Time: (0+4+6+13) / 4 = 5.75

Shortest Job Next (SJN)

This is also known as shortest job first, or SJF

This is a non-preemptive, pre-emptive schedupng algorithm.

Best approach to minimize waiting time.

Easy to implement in Batch systems where required CPU time is known in advance.

Impossible to implement in interactive systems where required CPU time is not known.

The processer should know in advance how much time process will take.

Given: Table of processes, and their Arrival time, Execution time

Waiting time of each process is as follows −

Average Wait Time: (0 + 4 + 12 + 5)/4 = 21 / 4 = 5.25

Priority Based Schedupng

Priority schedupng is a non-preemptive algorithm and one of the most common schedupng algorithms in batch systems.

Each process is assigned a priority. Process with highest priority is to be executed first and so on.

Processes with same priority are executed on first come first served basis.

Priority can be decided based on memory requirements, time requirements or any other resource requirement.

Given: Table of processes, and their Arrival time, Execution time, and priority. Here we are considering 1 is the lowest priority.

Waiting time of each process is as follows −

Average Wait Time: (0 + 10 + 12 + 2)/4 = 24 / 4 = 6

Shortest Remaining Time

Shortest remaining time (SRT) is the preemptive version of the SJN algorithm.

The processor is allocated to the job closest to completion but it can be preempted by a newer ready job with shorter time to completion.

Impossible to implement in interactive systems where required CPU time is not known.

It is often used in batch environments where short jobs need to give preference.

Round Robin Schedupng

Round Robin is the preemptive process schedupng algorithm.

Each process is provided a fix time to execute, it is called a quantum.

Once a process is executed for a given time period, it is preempted and other process executes for a given time period.

Context switching is used to save states of preempted processes.

Wait time of each process is as follows −

Average Wait Time: (9+2+12+11) / 4 = 8.5

Multiple-Level Queues Schedupng

Multiple-level queues are not an independent schedupng algorithm. They make use of other existing algorithms to group and schedule jobs with common characteristics.

Multiple queues are maintained for processes with common characteristics.

Each queue can have its own schedupng algorithms.

Priorities are assigned to each queue.

For example, CPU-bound jobs can be scheduled in one queue and all I/O-bound jobs in another queue. The Process Scheduler then alternately selects jobs from each queue and assigns them to the CPU based on the algorithm assigned to the queue.