Scheduling of processes/work is done to finish the work on time.
Below are different time with respect to a process.
Why do we need scheduling?
A typical process involves both I/O time and CPU time. In a uniprogramming system like MS-DOS, time spent waiting for I/O is wasted and CPU is free during this time. In multiprogramming systems, one process can use CPU while another is waiting for I/O. This is possible only with process scheduling.
Objectives of Process Scheduling Algorithm
Different Scheduling Algorithms
First Come First Serve (FCFS): Simplest scheduling algorithm that schedules according to arrival times of processes.
Shortest Job First(SJF): Process which have the shortest burst time are scheduled first.
Shortest Remaining Time First(SRTF): It is preemptive mode of SJF algorithm in which jobs are schedule according to shortest remaining time.
Round Robin Scheduling: Each process is assigned a fixed time in cyclic way.
Priority Based scheduling (Non Preemptive): In this scheduling, processes are scheduled according to their priorities, i.e., highest priority process is schedule first. If priorities of two processes match, then schedule according to arrival time.
Highest Response Ratio Next (HRRN) In this scheduling, processes with highest response ratio is scheduled. This algorithm avoids starvation.
Multilevel Queue Scheduling: According to the priority of process, processes are placed in the different queues. Generally high priority process are placed in the top level queue. Only after completion of processes from top level queue, lower level queued processes are scheduled.
Multi level Feedback Queue Scheduling: It allows the process to move in between queues. The idea is to separate processes according to the characteristics of their CPU bursts. If a process uses too much CPU time, it is moved to a lower-priority queue.
Some useful facts about Scheduling Algorithms:
1) FCFS can cause long waiting times, especially when the first job takes too much CPU time.
2) Both SJF and Shortest Remaining time first algorithms may cause starvation. Consider a situation when long process is there in ready queue and shorter processes keep coming.
3) If time quantum for Round Robin scheduling is very large, then it behaves same as FCFS scheduling.
4) SJF is optimal in terms of average waiting time for a given set of processes,i.e., average waiting time is minimum with this scheduling, but problems is, how to know/predict time of next job.