Syuhada Rais's Blog

Scheduling – 19/1/10

21/01/2010 at 11:33 am (Computing A2 Level)

Questions:

1. Describe the objectives of scheduling. [4]
2. State & describe three methods of scheduling. [6]

Answers:

Scheduling is to allocate resources in a timely manner to each process that needs to run.

1.The objectives of the scheduling part of the operating system:

• Make maximum use of CPU time, so that, it can process as many jobs as possible.
• Be responsive to the user so they are unaware of a delay to their process.
• Make maximum use of resources such as input-output devices.
• Be fair to all jobs -  none left stranded for too long
• Be able to prioritize jobs
• Be able to alter priorities according to some rules built into the scheduler
• Avoid ‘deadlock’

The scheduler has many duties to fulfill. The ideal scheduler would be one that is making 100% use of the CPU, no jobs are left hanging around and the user is never aware of a time delay also no resources are left idle if a process is wanting to use it.

2.The three methods of scheduling are:

• Round Robin or ‘Time Slice’

This approach makes use of a special form of queue called a ‘FIFO’, meaning ‘First in First Out’. This is quite a fair approach – the process that has been waiting the longest gets the next turn. The process is allowed to run for a fixed amount of CPU time then it gets to the back of the queue once again. This is a good algorithm if every job is more or less equally important. The downside of the Round Robin approach is that it takes no account of job priority.

• Priority Scheduling

Unlike the Round Robin approach, priority scheduling is trying to take account of the relative priority of the jobs in the queue. This approach is very good for making sure that the most vital jobs get to run first, however, it must also deal with low-priority, otherwise they may never get a look-in. In this case the scheduler may start to bump up the priority of lower jobs to ensure they will eventually run.

• Shortest Burst

With this approach the idea is to get as many jobs through CPU as possible. As we have mentioned, many jobs (processes) spend considerable time waiting for some input-output event, then they run for a bit and once again they have to wait for another input-output event. This scheduler takes advantage of this behaviour. It schedules the jobs with the shortest burst time to run first. The good point about the shortest burst, it does ensure many jobs get processed, but the downside is if every job are of similar burst times then it is not as efficient.

Conclusion

There are many different scheduling algorithms and we have covered three of them here -  the Round Robin, the Priority and the Shortest Burst.

Each has its advantages and disadvantages.

http://www.teach-ict.com/as_as_computing/ocr/H447/F453/3_3_1/scheduling/theory_scheduling.html

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