CODEWITHSUNDEEP
schedulingcpu
Vanwaril
Vanwaril

Reputation: 7518

How does a scheduler end a running process?

I just realized that after learning a lot about various scheduling algorithms, how a context switch is done, etc. one thing still isn't clear to me.

Take a uniprocessor system:

If process A is running and it's time slot should end in 5 seconds, how does the scheduler or the operating system know how to end it after 5 seconds? No part of the operating system can run while A is running. The scheduler is supposed to be monitoring it, but how can it if it cannot run? Does the operating system's scheduler write an ISR and have an interrupt generate every 5 seconds? Is this possible? Even if it is, it doesn't seem a good way to implement it.

How exactly does a scheduler do this?

Upvotes: 1

Views: 317

Answers (2)

rlibby
rlibby

Reputation: 6021

Does the operating system's scheduler write an ISR and have an interrupt generate every 5 seconds? Is this possible? Even if it is, it doesn't seem a good way to implement it.

Yes, this is exactly how it works on a preemptive multitasking system (although on desktop systems the interval is usually more like 10 milliseconds).

Yes, there are other schemes, such as cooperative multitasking, where each process decides for itself when to yield.

Upvotes: 1

Carl Norum
Carl Norum

Reputation: 224844

Yes, normally there is some kind of timer interrupt that fires. The kernel can then run for a bit and switch process context if it needs to - normally that interrupt would fire an awful lot more often than just once every 5 seconds though. Why doesn't it seem like a good way to implement it?

Upvotes: 1

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