week5_friday - CSCC69H3 OperatingSystems Spring2010 UofT...

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CSCC 69H3 Operating Systems Spring 2010 Professor Bianca Schroeder U of T
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Short announcement… Midterm will be on Wed March 3 rd  2-3pm (i.e.  the tutorial slot). Material includes: Everything covered in lectures, tutorials,  exercises & assignments.
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Today … Finishing processor scheduling Starting memory management
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Recap Last time we looked at a number of possible  scheduling policies First-Come-First-Serve (FCFS) Shortest-Job-First (SJF) Round-robin (RR) Priority scheduling
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What do real systems do? Combination of Multi-level queue scheduling Typically with RR and priorities Feedback scheduling
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Multi-Level Queue Scheduling Have multiple ready queues Each runnable process is on only one queue Processes are assigned to a queue Criteria include job class, priority, etc. Each queue has its own scheduling algorithm Another level of scheduling decides which queue to  choose next Usually priority-based
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Multi-Level Queue Scheduling Have two scheduling policies First decides which queue to serve E.g. based on queue priorities Seconds decides which job within a queue to choose E.g. FCFS Job 1 Job 2 Job 3 Job 4 Job 5 Queue 1 Queue 2
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Feedback Scheduling Motivation: Want to give priority to shorter jobs Want to give priority to IO bound jobs Want to give priority to interactive jobs Want to … But don’t know beforehand whether a job is short or  long and whether it’s IO bound or CPU bound …
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Feedback Scheduling Adjust criteria for choosing a particular  process based on past history Can prefer processes that do not use full quantum Can change priority of processes based on age Can change priority of processes based on CPU  consumed so far Can boost priority following a user-input event Can adjust expected next-CPU-burst Combine with MLQ to move processes  between queues
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Linux 2.6 CPU scheduling Combination of  Multilevel queues With priorities and RR Feedback scheduling Distinguishes 3 classes Realtime FIFO processes Realtime RR processes Timesharing processes Our discussion focuses on timesharing processes
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Linux 2.6 CPU scheduling Active array High est priority Low est priority Always run the task in active array with highest priority If there are procs with same priority, do RR between them Switch after  granularity  time units RR How do you avoid starvation of low priority processes?
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Linux 2.6 CPU scheduling Active array Expired array  (timeslice expired) High est priority Low est priority Always run the task in active array with highest priority
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