YEAR 1 CO1 WEEK 25 SCHEDULING Flashcards
What is the OS of a multitasking computer expected to do and how is it made possible?
The OS of a multitasking computer is expected to run many programs in memory without any noticeable delay.
Made possible by scheduling which is the task of allocating CPU processing time amongst all running programs.
Describe Round Robin scheduling algorithm.
Each process given equal amount of time being processed.
Queue made holding all processes that are ready to be run. Each loaded into main memory in turn.
Each process given set amount of time to use.
If completed within time next process loaded immediately otherwise put back in queue to wait for another turn.
State advantages and disadvantages of Round Robin.
Advantages:
Easy to implement
Disadvantages:
Not take account of priority.
Not take account that some processes much longer than others.
Describe First Come First Server scheduling algorithm.
Lines up all processes ready to run in First In First Out queue.
Allows processes to run to completion.
State advantages and disadvantages of First Come First Serve.
Advantages:
Simple algorithm to implement.
Disadvantages:
Not take into account of priority.
Not consider some processes longer than others.
Once job started prevents other jobs from being processed until its complete.
Describe Shortest Job First Scheduling Algorithm.
Processes arranged in queue by how much time (FDE cycles) each process needs to run to completion.
If newer process comes along scheduler will work out processing time and slot in appropriate point in queue.
If shorter than current process running program interrupted and shorter one runs immediately.
State advantages and disadvantages of Shortest Job First Algorithm.
Advantages:
Ensures short jobs aren’t kept waiting for long processes.
Minimises average time process takes to complete.
Disadvantages:
Not take into account priority.
Longer jobs that are nearly done still interrupted by shorter jobs.
Describe Shortest Time Remaining Scheduling Algorithm.
For Shortest Job algorithm 1000 cycle job still be interrupted by 700 cycle as total length shorter than old one.
Fixed by ordering jobs by how much time they have remaining.
Jobs only interrupted if new process shorter than time remaining before completion.
State advantages and disadvantages of Shortest Time Remaining Algorithm.
Advantages:
Allows short processes to be handled very quickly.
Disadvantages:
Not take into priority.
Long jobs may never be completed if short jobs keep jumping the queue.
Describe Multi-Level Feedback Queues Scheduling Algorithms.
Maintains a number of queues with different priority levels.
Each job given priority by scheduler and then sorted into relevant queue.
High priority queue completed first then each of the lower queues in turn.
State advantages and disadvantages of Multi-Level Feedback Queues.
Advantages:
Takes in account of priority.
Disadvantages:
Complex to implement.
Low priority may take extremely long time to complete.
