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Recently in an interview I was asked about the Scheduling algorithm used by Linux Operating system. What is the algorithm used any why?

Also, what algorithm is used in in real-time operating systems and why?

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For process or IO scheduling? Or even something else? – maxschlepzig Sep 5 '11 at 19:50

3 Answers

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The current Linux task scheduler is called Completely Fair Scheduler (CFS). You should have a look at http://people.redhat.com/mingo/cfs-scheduler/sched-design-CFS.txt for more details. The design is quite complex and in my view not suitable for RTOS.

A common technique in realtime systems is rate-monotonic scheduling, because it has strong guarantees if certain assumptions hold (e.g. static task priorities and fixed execution time and rate). There are a whole lot other algorithms and there has been a lot of research. So it's basically all about the properties you need and what you know about your task and what is fixed.

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I'm not quite sure, whether you are taking about the I/O scheduling of the Kernel. In case you are not: Ignore this answer.

Wikipedia states, that the CFG (completely Fair Queuing) is default since Kernel 2.6.18.

On my openSUSE (running Kernel 2.6.37) I can switch between the CFG, NOOP and Deadline.

Hope this helps.

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I'm curious,how can we switch to a different algorithm? Can you shed some light on this? thanks – rsjethani Nov 3 '12 at 9:22
@rsjethani Go to YaST -> System -> Kernel Settings -> 2nd Tab (Kernel Settings) -> Global IO Scheduler. (naming of the options might be different as I've translated from a German GUI) – Torbjoern Dec 18 '12 at 9:36
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The Round Robin algorithm is generally used in time sharing environments.

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