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Description
| Question | Answer |
| What is Scheduling? | > Part of the OS that allows for multi-tasking of complex tasks > Queues up the next process required by another application to make the most efficient use of the CPU |
| What are the Scheduling Algorithms? | > Round robin > First come, first served > Multi-level feedback queues > Shortest job first > Shortest remaining time |
| What is Round Robin Scheduling? | > Processes are put in a (circular) queue > Each process is given an arbitrary amount of equal CPU time to be completed in > If the job is completed in this time, the next is loaded, otherwise, it goes to the end of the queue > This is done by the OS using an interval timer to interrupt the CPU at the correct time |
| What are the Pros and Cons of Round Robin Scheduling? | > Guarantees a reasonable response time to all users of the system > Simple and works if all jobs are similar in size and priority > Ignores the priority of any job > Inefficient if each job takes different amounts of time / priority |
| What is First Come, First Served Scheduling? | > Jobs are added to a queue > Jobs are completed in this order, disregarded time and priority (wow, so complex) |
| What are the Pros and Cons of First Come, First Served Scheduling? | > Easy to set up and manage > Can cause a long delay in getting jobs done > Disregards priority > Generates bad performance |
| What is Multi-Level Feedback Queues Scheduling? | > Maintains many queues of jobs, usually grouped by priority and similarity (majority of the time, there is a separate queue for slow I/O jobs) > CPU switches between queues to complete jobs > If a job is waiting too long in a low priority queue, it is moved to a high priority one to complete the job quicker > Each queue has its own scheduler |
| What are the Pros and Cons of Multi-Level Queues Scheduling? | > Prevents bottlenecks in slow jobs, e.g., I/O > Offers the best results by maximising processor use > CPU intensive |
| What is Shortest Job First Scheduling? | > Selects the shortest job in the queue to complete first > It's that simple |
| What are the Pros and Cons of Shortest Job First Scheduling? | > Shortest waiting time of all the schedulers > Reduces the number of small jobs waiting behind big jobs > Larger processes can face starvation and not get completed > The OS rarely knows the amount of time a job will take, thus can only estimate the time required |
| What is Shortest Remaining Time Scheduling? | > Selects the job that will take the least amount of time (as you would expect) > As a job arrives, it is compared to the currently running job, then added to the queue > Shortest job is run first each time a job completes (i.e., this comparison only happens when a job arrives of when a job completes) |
| What are the Pros and Cons of Shortest Time Remaining Scheduling? | > Little CPU usage > Starvation |
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