Scheduling Flashcards
What is Scheduling?
programs that ensures all sections of programs being run (jobs) recieve a fair amount of processing time
What is its purpose?
increases number of interactive users recieving reasonably fast reposne times with no apparent delay
increases number of jobs processed in least amount of time
obtains the most efficient use of processor time and utilises resources dependednt on priorities
prevents process starvation from applciations in deadlock failing to run
ensure all jobs are processes fairly by changing priorities wheere necessary so long jobs dont monopolise processor
Scheduling algortihms can be either?
Pre-emptive:jobs actively made to start and stop OS
Non pre-emptive:once job is started it is left alone until completed
What algorithms do schedulers use?
Round Robin
First come first serve
Mulitlevel Feedback queues
shortest job first and shortest remaining time
What is Round Robin? Give Advantages and Disadvantages
pre-emptive: each job gicen a time slice to execute.
if job has not finished executing in that time, it is moved to the back of the queue and the next job is processed, otherwise it leaves the queue
repeats until all jobs serviced
+ : all jopbs eventually attended to
-‘s : longer jobs will take much longer time to complete as execution is inefficiently split into multiple cyceles
doesn’t take into account job priority
What is First come first serve? Give Advantages and Disadvantages
non pre-emptive: jobs processed in chronological order by which they entered the queue
+ : straightforward to implement
- : doesn’t take into account job priorty
What is Multilevel Feedback queues? Give Advantages and Disadvantages
pre-emoptive: uses multiple queues, each ordered based on different priortiy
+ : takes job priority into consideration
- : difficult to implement
What is Shortest job first? Give Advantages and Disadvantages
non pre-emptive: queue stores jobs to be processed in order of time required for completion. jobs that take the longest are serviced at the end
batch systems-type of OS that allows multiple users at the same time
+ : suited to batch systems, as waiting is reduced
- : requires processor to calculate how long each job will take
processor starvation if short jobs are continously added to the queue
doesn’t take into account job priority
What is Shortest remaining time? Give Advantages and Disadvantages
pre-emptive: queue stores jobs to be processed in order of time left for completion. jobs with the least time are servicded first
+ : increased throughput as shorter processes can be quickly completed
- : doesn’t take into account job priortiy
processor starvation if short jobs are continously added to the queue
