self doubt

Question

Suppose one process is running, and another high priority process arrives and lets assume CPU uses preemptive priority scheduling, how does short term scheduler come to know that a high priority process has arrived and it has to preempt the running process and schedule the new process since scheduler itself is a process, how does it run and everything gets managed??

please someone give a clear explanation. Thanx

Comments

@rude,thats a really practical question.if u could give some good link to study the actual implementation of pipeline or alteast a  slight idea of it,then that would be helpful.thanks:)

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I think it is his intuition

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what i think is

Short term Schduler continuously  check priority of a process by using comparator…..and then decide which process to schdule next…

correct if i am wrong

Answer 1

Well, you cannot use a process to invoke the scheduler process because who would invoke that process, right?

A timer is used to send an interrupt, now its corresponding interrupt handler runs in kernel mode and dispatches the scheduler.

It works just like Round Robin algorithm where the timer interrupts currently running process and the scheduler dispatches the next process from the Ready queue. In our case, the scheduler itself is dispatched by the interrupt handler.

Comments

Thanx a lot.

Answer 2

i don't remember much but in Os reference book by Andrew Tenunbum i have read somewhere that this is called priority inversion problem in sleep and wait.. Let H be the process with higher priority and L be the process with lower priority scheduling rules are such that H runs whenever its in ready state. At certain moment with L in its critical region H become ready to run but H begins busy waiting L is never schedules while H is running so L never gets the chance to leave CS & H keeps looping forever... 

This is what i have read some where in that book and i have made a note over it but i also have doubts regarding to this expatiation yet... 

Comments

Dude, she is not asking that. She is asking, how short term scheduler know that a high priority scheduler in the queue when scheduler itself a process.

Answer 3

I have not read it any where but I will answer what I have understood. Do not know its right or wrong.
As you said short term scheduler is a process. How can it know if another process with high priority comes....

In round robin we give time quantum...to switch...
Here I assume, at every time unit, it has to check for highest priority process. That is it's job.

Answer 4

From what I know, the priority of each process is mentioned in its PCB(Process Control Block). Whenever the CPU completes execution of a process (or, TimeQuantum expires), the short term scheduler (STS) is invoked by the CPU. The STS then checks for the process with highest priority in the ready Q. The dispatcher then assigns CPU to the highest priority process. This is how the process works. Please correct me if I am mistaken.