Feedback move bet queues multilevel feedback queue

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Unformatted text preview: es Multilevel-feedback-queue scheduler defined by the following 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 begins Example Example of Multilevel Feedback Queue Three queues: • Q0 – time quantum 8 milliseconds • Q1 – time quantum 16 milliseconds • Q2 – FCFS Priority rule: execute all in Q0, then Q1 , then Q2 preempt if necessary enter in Q0 Scheduling: Job requires 24 time milliseconds • A new job enters queue Q0. When it gains the CPU, it receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. MLFQ MLFQ What is the goal of MLFQ in this example? Lottery Lottery Paper Scheduling Scheduling Issues Context • multiple scarce resources: CPU, I/O bandwidth, memory, locks • concurrently executing tasks • resource requests of varying importance and characteristics Quality of Service • time-scales for responsiveness differ (think about an editor, browser, mpeg player, or simulation task …) • until now, what is the most responsive scheduling policy? – why doesn’t achieve this objective? Conventional Conventional Scheduling Priority Scheduling • absolute control (but crude) • does p=1 vs. p=2 mean p=1 always gets the CPU or simply 2/3? 2/3? Problems with priorities • often ad hoc • unable to control service rates • possibility of starvation, inversion, … Solution: Solution: Lottery Scheduling Type of proportional share scheduling • resource consumption rate proportional to share allocated • coupled with notion of scheduling quanta Flexible control over service rates • current schedulers are rigid Modular Abstraction • multiple resource management policies enabled by tickets + lottery mechanism Lottery Lottery Scheduling Basics Randomized mechanism with statistical guarantees Lottery tickets • encapsulate resource rights • issued in different amounts Lotteries • randomly select winning ticket • grant resource to client holding winning ticket Example Example Lottery 5 tasks or processes Lottery Lottery Scheduling Advantages Probabilistic guarantees • n lotteries, client...
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