CPU Scheduling, involves many different scheduling algorithms which have their Advantages and Disadvantages.
- It is simple and easy to understand.
- The process with less execution time suffer i.e. waiting time is often quite long.
- Favors CPU Bound process then I/O bound process.
- Here, first process will get the CPU first, other processes can get CPU only after the current process has finished it’s execution. Now, suppose the first process has large burst time, and other processes have less burst time, then the processes will have to wait more unnecessarily, this will result in more average waiting time, i.e., Convey effect.
- This effect results in lower CPU and device utilization.
- FCFS algorithm is particularly troublesome for time-sharing systems, where it is important that each user get a share of the CPU at regular intervals.
- Shortest jobs are favored.
- It is provably optimal, in that it gives the minimum average waiting time for a given set of processes.
- SJF may cause starvation, if shorter processes keep coming. This problem is solved by aging.
- It cannot be implemented at the level of short term CPU scheduling.
3. Round Robin (RR):
- Every process gets an equal share of the CPU.
- RR is cyclic in nature, so there is no starvation.
- Setting the quantum too short, increases the overhead and lowers the CPU efficiency, but setting it too long may cause poor response to short processes.
- Average waiting time under the RR policy is often long.
- This provides a good mechanism where the relative importance of each process maybe precisely defined.
- If high priority processes use up a lot of CPU time, lower priority processes may starve and be postponed indefinitely.The situation where a process never gets scheduled to run is called starvation.
- Another problem is deciding which process gets which priority level assigned to it.
Application of separate scheduling for various kind of processes is possible.
- System Process – FCFS
- Interactive Process – SJF
- Batch Process – RR
- Student Process – PB
- Application of separate scheduling for various kind of processes is possible.
- The lowest level process faces starvation problem.
- Low scheduling overhead.
- Allows aging, thus no starvation.
- It’s not flexible.
- It also requires some means of selecting values for all the parameters to define the best scheduler, thus it is also the most complex.
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