Operating Systems { week 06.b }

Rensselaer Polytechnic Institute CSC 432 – Operating Systems David Goldschmidt, Ph.D. Operating Systems{week 06.b}

Short-term (CPU) scheduling the dispatcher operates here

CPU scheduling algorithms (i) • CPU scheduling requires an algorithm todetermine which process to dispatch next • Scheduling algorithms include: • First-Come, First-Served (FCFS) • Shortest-Job-First (SJF) • Round-Robin (RR) • Priority • Multilevel Queue (MQ)

process CPU scheduling algorithms (ii) • Preemptive scheduling preempts a running process before itstime slice expires • Or it preempts a processbecause its time slice has expired • Non-preemptive scheduling gives a process exclusive uninterrupted access to the CPU for the entirety of its execution process process process

CPU scheduling algorithms (iii) • Compare scheduling algorithms by measuring • CPU utilization – keep CPU as busy as possible • Throughput – maximize the number of processes that complete their execution per unit time • Turnaround time – minimize the elapsed time to fully execute a particular process • Waiting time – minimize the elapsed time a process waits in the ready queue

P3 P1 P2 P3 P1 P3 P1 0 2 4 6 8 10 12 13 Round-Robin (RR) • RR is a preemptive algorithmthat gives all ready processesa fair time slice of CPU time • Using a time slice of 2 ms.... time

Priority Scheduling (i) • Associate a priority number with each process • The dispatcher selects the processwith the highest priority • For multiple ready processes withidentical priority numbers, use FCFS (or ...) • Key problem is starvation • Overcome starvation by aging, increasingthe priority of a process as it ages

process Priority Scheduling (ii) • Is priority scheduling preemptiveor non-preemptive? • Non-preemptive priority scheduling places higher-priority processes at the head of the queue • Preemptive priority scheduling requires a running process to be interrupted and preempted upon the arrival of a higher-priority process (use this one for Project #1)

Multiclass systems • Operating systems that support priority schemes are often called multiclass systems use a separate schedulingalgorithm for each queue

Multilevel Queue (MQ) • Assign processes to multiple queues,each with its own scheduling algorithm

Multilevel Feedback Queue (MFQ) • Dynamically assign processes to multiple queues based on actual CPU burst times • i.e. feedback quantum is synonymouswith time slice

Practice! • Apply the FCFS, SJF, RR, and Preemptive Priority scheduling algorithms to this table: • For RR, use a time slice of 10 ms • Calculate the wait and turnaround times ofeach process, as well as overall averages lower number indicates a higher priority recalculate using context switch time tcs = 20 μs

Operating Systems { week 06.b }