CPU SCHEDULING ALGORITMS
11:16 PM Posted In OS-6 Edit This 0 Comments »1.First-come, first-served (FCFS) scheduling
2.Shortest-job first (SJF) scheduling
3.Priority scheduling
4.Round-robin scheduling
5.Multilevel queue scheduling
6.Multilevel feedback queue scheduling
First-come, First-served(FCFS) scheduling
-is the simplest scheduling algorithm, but it can cause short processes to wait for very long processes.
Shortest-job-first (SJF) scheduling
-is provably optimal, providing the shortest average waiting time. Implementing SJF scheduling is difficult because predicting the length of the next CPU burst is difficult. The SJF algorithm is a special case of the general
Comments: SJF is proven optimal only when all jobs are available simultaneously.
Problem: SJF minimizes the average wait time because it services small processes before it services large ones. While it minimizes average wiat time, it may penalize processes with high service time requests. If the ready list is saturated, then processes with large service times tend to be left in the ready list while small processes receive service. In extreme case, where the system has little idle time, processes with large service times will never be served. This total starvation of large processes may be a serious liability of this algorithm.
Solution: Multi-Level Feedback Queques
Multi-Level Feedback Queue
Several queues arranged in some priority order.
Each queue could have a different scheduling discipline/ time quantum.
Lower quanta for higher priorities generally.
Defined by:
- # of queues
- scheduling algo for each queue
- when to upgrade a priority
- when to demote
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