Scheduling: round robin, first come first served, multi-level feedback queues, shortest job first and shortest remaining time

• Makes the most efficient use of the processor/resources • Be fair to all users/applicants • Provide a reasonable response time • Prevent processes from failing to run/process starvation/deadlock

Jobs are actively made to start and stop by the operating system - Multilevel Feedback Queues, Shortest Remaining Time, Round Robin

Once a job is started, it is left alone until it is completed - First Come First Served, Shortest Job First

• Round robin • First come first served • Shortest job first • Shortest remaining time • Multilevel feedback queues

• Each job is given a section of processor time - known as a time slice (allowed to execute) • Once each job in the queue has used its first time slice, the operating system again grants each job an equal slice of processor time • This continues until a job has been completed, at which point it is removed from the queue

• Every process gets an equal share of the CPU - RR is cyclic in nature, so there is no starvation

• Longer jobs will take a much longer time for completion due to their execution being inefficiently split up into multiple cycles • Does not take into account job priority

Jobs are processed in chronological order by which they entered the queue

• Simple and easy to understand - Straightforward to implement

• FCFS again does not allocate processor time based on priority • The process with less execution time suffer i.e. waiting time is often quite long

This makes use of multiple queues, each which is ordered based on a different priority

• It is more flexible • It prevents starvation by moving a process that waits too long from the lower priority queue to the higher priority queue

• Difficult to implement due to deciding which job to prioritise based on a combination of priorities • Most complex algorithm

The queue storing jobs to be processed is ordered according to the time required for completion, with the longest jobs being serviced at the end

• It is suitable for the jobs running in batches, where run times are already known • SJF is better than the First come first serve (FCFS) algorithm as it reduces the average waiting time

• SJF may cause starvation if shorter processes keep coming, this problem is solved by ageing • Requires the processor to know or calculate how long each job will take and this is not always possible

The queue storing jobs to be processed is ordered according to the time left for completion, with the jobs with the least time till completion being serviced first

• This scheduling is optimal in that it always produces the lowest mean response time. • Processes with short CPU bursts are given priority and hence run quickly (are scheduled frequently)

• Long-burst (CPU-intensive) processes are hurt with a long mean waiting time (long-burst ones may never get scheduled), starvation • Effectiveness of meeting the scheduling criteria relies on the accuracy to estimate the length of the next CPU burst