A Primer on Process Scheduling [Operating Systems]

Important Terms

Burst Time/Execution Time:
It is a time required by the process to complete execution. It is also called running time.
Arrival Time:
When a process enters in a ready state
Finish Time:
When process complete and exit from a system
Multiprogramming:
A number of programs which can be present in memory at the same time.
Jobs:
It is a type of program without any kind of user interaction.
User:
It is a kind of program having user interaction.
Process:
It is the reference that is used for both job and user.
CPU/IO burst cycle:
Characterizes process execution, which alternates between CPU and I/O activity. CPU times are usually shorter than the time of I/O.

Maximise
- Max CPU utilization
  Keep CPU as busy as possible (Fair allocation of CPU)
- Max throughput
  Number of processes that complete their execution per time unit
Minimise
- Min turnaround time
  Time taken by a process to finish execution
- Min waiting time
  Time a process waits in ready queue
- Min response time
  Time when a process produces first response

Job queue − This queue keeps all the processes in the system.
Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.
Device queues − The processes which are blocked due to unavailability of an I/O device constitute this queue.

Long-Term Scheduler	Short-Term Scheduler	Medium-Term Scheduler
It is a job scheduler	It is a CPU scheduler	It is a process swapping scheduler.
Speed is lesser than short term scheduler	Speed is fastest among other two	Speed is in between both short and long term scheduler.
It controls the degree of multiprogramming	It provides lesser control over degree of multiprogramming	It reduces the degree of multiprogramming.
It is almost absent or minimal in time sharing system	It is also minimal in time sharing system	It is a part of Time sharing systems.
It selects processes from pool and loads them into memory for execution	It selects those processes which are ready to execute	It can re-introduce the process into memory and execution can be continued.

Preemptive Scheduling	Non-preemptive Scheduling
A processor can be preempted to execute the different processes in the middle of any current process execution.	Once the processor starts its execution, it must finish it before executing the other. It can’t be paused in the middle.
CPU utilization is more efficient compared to Non-Preemptive Scheduling. Waiting and response time of preemptive scheduling is less.	CPU utilization is less efficient compared to preemptive Scheduling. Waiting and response time of the non-preemptive Scheduling method is higher.
Preemptive Scheduling is prioritized. The highest priority process is a process that is currently utilized.	When any process enters the state of running, the state of that process is never deleted from the scheduler until it finishes its job.
In this process, the CPU is allocated to the processes for a specific time period.	In this process, CPU is allocated to the process until it terminates or switches to the waiting state.
Examples: - Shortest Remaining Time First, Round Robin, etc.	Examples: First Come First Serve, Shortest Job First, Priority Scheduling, etc.