18 Silberschatz Galvin and Gagne 2013 Operating System Concepts 9 th Edition

# 18 silberschatz galvin and gagne 2013 operating

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Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionExample of Shortest-remaining-time-firstNow we add the concepts of varying arrival times and preemption to the analysisProcessAarri Arrival TimeTBurst TimeP108P2 14P329P435Preemptive SJF Gantt ChartAverage waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msecP40126P1P210P3P1517
6.19Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionPriority SchedulingA priority number (integer) is associated with each processThe CPU is allocated to the process with the highest priority (smallest integer highest priority)PreemptiveNonpreemptiveSJF is priority scheduling where priority is the inverse of predicted next CPU burst timeProblem Starvation – low priority processes may never executeSolution Aging – as time progresses increase the priority of the process
6.20Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionExample of Priority SchedulingProcessAarri Burst TimeTPriorityP1103P2 11P324P415P552Priority scheduling Gantt ChartAverage waiting time = 8.2 msec
6.21Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionRound Robin (RR)Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.If there are nprocesses in the ready queue and the time quantum is q, then each process gets 1/nof the CPU time in chunks of at most qtime units at once. No process waits more than (n-1)q time units.Timer interrupts every quantum to schedule next processPerformanceqlarge FIFOq small q must be large with respect to context switch, otherwise overhead is too high
6.22Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionExample of RR with Time Quantum = 4ProcessBurst TimeP124P23P33The Gantt chart is: Typically, higher average turnaround than SJF, but better responseq should be large compared to context switch timeq usually 10ms to 100ms, context switch < 10 usecPPP111018302614471022P2P3P1P1P1
6.23Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionTime Quantum and Context Switch Time
6.24Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionTurnaround Time Varies With The Time Quantum80% of CPU bursts should be shorter than q
6.25Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9thEditionMultilevel QueueReady queue is partitioned into separate queues, eg:foreground(interactive)background(batch)Process permanently in a given queueEach queue has its own scheduling algorithm:foreground – RRbackground – FCFSScheduling must be done between the queues:Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.