CPU Scheduling

Question

If the arrival time of 1st process is 4,then will the time duration during 0-4 would be taken in case of CPU idle time or not ?

Comments

Yes. If no other process is executing or has lesser arrival time, 0-4 will be idle time.

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But the process arrived at 4 so the execution will start from 4 so why is 0-4 idle?

In that case if 0-4 is idle time then after execution of all the processes also CPU will be idle till infinity.

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Say the CPU is ready for execution. The CPU initiates 3 processes. Now, if all the 3 processes becomes busy with I/O operations, the CPU will stay idle for some time as it has temporarily no work to do.

Yes after execution of all processes a CPU is considered as idle.

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@ Purvi Agrawal 

we will not consider the cpu execution for this process only because AT at 4 but in multitasking system CPU will do some I/O or some other operation also

Answer 1

lets P₁ who's AT is 4 unit of time and P₂ AT is 5 unit of time then 0-4 unit of time CPU becomes idle, after the 4 unit of time only P₁ arrive so it schedule first. then comes P₂ (Fig A)

Another way If P₁ having AT of 4 unit of time, but P₂ having arrival time of 2 unit then first P₂ arrive then come to P₁ (fig B)

Comments

Is it like we always take starting of execution from 0 and if there is no process at 0 we consider it as CPU idle time ?

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not necessary, it's our convenient that process starts from o.