06_CPU_Scheduling_II_2spp

06_CPU_Scheduling_II_2spp - CSC 4103 Operating Systems Fall...

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1 CSC 4103 - Operating Systems Fall 2009 Tevfik Ko ! ar Louisiana State University September 14 th , 2009 Lecture - VII CPU Scheduling - II 2 Roadmap • Multilevel Feedback Queues • Estimating CPU bursts • Project Discussion • System Calls • Virtual Machines
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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
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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 0 – RR with time quantum 8 milliseconds Q 1 – RR time quantum 16 milliseconds Q 2 – FCFS • Scheduling A new job enters queue Q 0 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 .
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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 +
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06_CPU_Scheduling_II_2spp - CSC 4103 Operating Systems Fall...

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