06-CPU_Scheduling_II_2spp

06-CPU_Scheduling_II_2spp - 1 CSE 421/521 - Operating...

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Unformatted text preview: 1 CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ar University at Buffalo September 15 th , 2011 Lecture - VI CPU Scheduling - II 2 Roadmap Multilevel Feedback Queues Estimating CPU bursts System Calls Virtual Machines 3 Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground RR background FCFS Scheduling must be done between the queues Fixed priority scheduling ; (i.e., serve all from foreground then from background). Possibility of starvation. Time slice each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR, 20% to background in FCFS 4 Multilevel Queue Scheduling 5 Multilevel Feedback Queue A process can move between the various queues; aging can be implemented this way Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when that process needs service 6 Example of Multilevel Feedback Queue Three queues: Q RR with time quantum 8 milliseconds Q 1 RR time quantum 16 milliseconds Q 2 FCFS Scheduling A new job enters queue Q which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1 . At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2 . 7 Multilevel Feedback Queues 8 Determining Length of Next CPU Burst Can only estimate the length Can be done by using the length of previous CPU bursts, using exponential averaging + 9...
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06-CPU_Scheduling_II_2spp - 1 CSE 421/521 - Operating...

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