GET READY TO SCHEDULE YOUR PROCESSES!

What is a process scheduling?

Process scheduling is the act in which a process scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms.

Our 5 scheduling algorithms used are:

• Non-Preemptive First-Come, First-Served (FCFS) Scheduling
• Non-Premeptive Shortest-Job-Next (SJN) Scheduling
• Non-Preemptive Priority Scheduling
• Preemptive Round Robin(RR) Scheduling
• Preemptive Shortest Remaining Time First Scheduling

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 scheduling 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.

Non-Preemptive First-Come, First-Served (FCFS) Scheduling

Outline:

• Jobs are executed on first come, first serve basis.
• It is a non-preemptive, pre-emptive scheduling algorithm.
• Easy to understand and implement.
• Poor in performance as average wait time is high.

Non-Premeptive Shortest-Job-Next (SJN) Scheduling

Outline:

• This is also known as shortest job first, or SJF
• This is a non-preemptive, pre-emptive scheduling 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.

Non-Preemptive Priority Scheduling

Outline:

• Priority scheduling is a non-preemptive algorithm and one of the most common scheduling 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.

Preemptive Round Robin(RR) Scheduling

Outline:

• Round Robin is the preemptive process scheduling 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.

Preemptive Shortest Remaining Time First Scheduling

Outline:

• 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.