Operating Systems {week 05a}

Rensselaer Polytechnic Institute CSCI-4210 – Operating Systems David Goldschmidt, Ph.D. Operating Systems{week 05a}

Short-term (CPU) scheduling (i) • The short-term scheduler decides which process the CPU executes next • The dispatcher gives control of the CPU to the process selected by the CPU scheduler: • Performs context switch • Switches to user mode • Jumps to the proper location in the user program to resume program execution

Short-term (CPU) scheduling (ii) 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

P1 P2 P3 0 24 27 30 First-Come-First-Served (FCFS) • FCFS dispatches processesin the order they enterthe ready queue • FCFS is non-preemptive time

P2 P3 P1 0 3 6 30 Shortest-Job-First (SJF) • SJF dispatches processes byselecting the process withthe lowest CPU burst time • SJF is non-preemptive (and predictive) time

SJF with varied arrival times • What happenswhen processesarrive at differentarrival times? • Calculate the turnaround times and wait times for each process • Calculate the average turnaround time and average wait time

P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Preemptive SJF • Same as SJF, but anew process maypreempt therunning process time

Limitations of SJF (i) • SJF is the optimal solution • The problem with SJF is the inability toknow the required CPU burst times • Apply a prediction algorithm that usesprevious CPU burst times • Algorithm uses exponential averaging: • tn = actual length of the nth CPU burst • τn+1 = predicted value for the next CPU burst • τn+1 = α tn + (1 – α) τn , where 0 < α < 1

Limitations of SJF (ii) α is ½

Limitations of SJF (iii) α is ¾ α is ½ α is ¼

Practice! • Apply the FCFS, SJF, and Preemptive SJF scheduling algorithms to this table: • Calculate the wait and turnaround times ofeach process, as well as overall averages recalculate using context switch time tcs = 20 μs

Operating Systems {week 05a}