Lec20-Scheduling III - Method used to select when demote...

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Operating System Operating System Spring 09 Spring 09 National University National University of Computer & Emerging Sciences of Computer & Emerging Sciences Lec20-Scheduling III Lec20-Scheduling III Prepared by Prepared by Uzma Maroof Uzma Maroof [email protected]
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2 Reference Material Reference Material Chapter 6 CPU Scheduling Operating system concepts Sixth edition Silberschatz
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3 Multilevel Feedback Queue Multilevel Feedback Queue Scheduling Scheduling In Multilevel Queue a process is permanently assigned to a queue The queue to which a process should belong is decided statically Multilevel Feedback Queue Scheduling: A Process may move between the Queues Aging can be implemented this way.
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4 Multilevel Feedback Queue Multilevel Feedback Queue Scheduling Scheduling Multilevel-feedback-queue scheduler defined by: Number of queues Scheduling algorithms for each queue Method used to select when upgrade process Method used to select when demote process
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Unformatted text preview: Method used to select when demote process Method used to determine which queue a process will enter when that process needs service 5 Multilevel Feedback Queue Multilevel Feedback Queue Scheduling Scheduling • Example If a process used too much CPU time, then move it to a lower-priority queue If a process waits too long in a lower priority queue, then move it to a higher priority queue 6 Multilevel Feedback Queue Multilevel Feedback Queue Scheduling Scheduling • Example: Three queues: Q – RR time quantum 8 milliseconds Q 1 – RR time quantum 16 milliseconds Q 2 – FCFS • Scheduling A new job enters queue Q served by RR. Then job receives 8 milliseconds. If not finished in 8 milliseconds, moved to Q 1 . At Q 1 job served by RR. Then receives 16 milliseconds. If not complete, preempted and moved to Q 2 ....
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