Scheduling Overheads

Question

Consider three processes that require 10, 20, 30 units and arrive at time 0, 2, 4. The number of scheduling overheads needed using S.R.T.F. scheduling is 

I think answer is 1 at time t = 10

Comments

Even though SRTF is reduced to SJF, But since here the scheduling algorithm is SRTF, so every time STS has to decide whether running process is having minimum burst or the processes having in the ready queue.

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hello shubhanshu and shivam.

but in that strategy i have one doubt.

how will OS know that there is new process that also want to execute, it can only happen when OS checks ready queue , after every single time unit(1 time unit)and see if there is new process in ready queue or not..as you are saying at t=0 , no scheduling work , that can only happen when OS know that no process in ready queue , and then how does at t=2 OS knows that this time i have to execute some scheduling decision making algo to know which is best to execute...

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@Shubhanshu 

Suppose we have processes P1, P2, P3 currently in ready queue (at t=0) and we run scheduling algorithm to determine process with minimum next CPU burst (say, P1 is selected). Now suppose at t=t₁ a new process P4 arrives. WE NEED  MAXIMUM CPU UTILIZATION. COMMON SENSE APPLIES

If we apply scheduling algorithm on all four processes and determine next process and get among P1 or P4 to run.  Overhead

OR

Since We already know if P1 is running now then it must have minimum CPU burst among P1, P2, P3

so just check left CPU burst with P4 burst time and determine the next process.

Answer 1

Scheduling overhead means The time taken by dispatcher to move the process from ready state to running state.

So here it will be 3.