Introduction to Process Scheduling

Process Scheduling 🔥

Basis of Multi-programming OS.
By switching the CPU among processes, the OS can make the computer more productive.
Many processes are kept in memory at a time, when a process must wait(I/O operations) or time quantum expires, the OS takes the CPU away from that process & gives the CPU to another process & this pattern continues.

CPU Scheduler

Whenever the CPU become ideal, OS must select one process from the ready queue to be executed.
Done by STS (short-term scheduler).

Non-Preemptive scheduling (not emptied pre (before) - ❌ time quantum)

Once CPU has been allocated to a process, the process keeps the CPU until it releases CPU either by terminating or by switching to wait-state (process is doing I/O).
Starvation, as a process with long burst time may starve less burst time process.
Low CPU utilization.

Preemptive scheduling (emptied pre (before) - ✅ time quantum)

CPU is taken away from a process after time quantum expires along with terminating or switching to wait-state.
Less Starvation
High CPU utilization

Terminologies associated with CPU scheduling

Throughput: No. of processes completed per unit time.
Arrival time (AT): Time when process is arrived at the ready queue.
Burst time (BT): The time required by the process for its execution.
Turn-around time (TAT): Time taken from first time process enters ready state till it terminates. (CT - AT)
Wait time (WT): Time process spends waiting for CPU. (WT = TAT – BT)
Response time: Time duration between process getting into ready queue and process getting CPU for the first time.
Completion Time (CT): Time taken till process gets terminated.