LSN 15 Processor Scheduling

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LSN 15 Processor Scheduling ECT362 Operating Systems Department of Engineering Technology

LSN 15 Processor Scheduling

LSN 15 FCFS/FIFO Scheduling Each process joins the Ready queue When the current process ceases to execute, the oldest process in the Ready queue is selected A short process may have to wait a very long time before it can execute Favors CPU-bound processes I/O processes have to wait until CPU-bound process completes

LSN 15 FCFS/FIFO Scheduling Process t(p i ) 0 350 1 125 2 475 3 250 4 75 0 350 475 950 1200 1275 p 0 p 1 p 2 p 3 p 4 T TRnd (p 0 ) = t(p 0 ) = 350 T TRnd (p 1 ) = (t(p 1 ) +T TRnd (p 0 )) = 125+350 = 475 T TRnd (p 2 ) = (t(p 2 ) +T TRnd (p 1 )) = 475+475 = 950 T TRnd (p 3 ) = (t(p 3 ) +T TRnd (p 2 )) = 250+950 = 1200 T TRnd (p 4 ) = (t(p 4 ) +T TRnd (p 3 )) = 75+1200 = 1275 T TRnd (AVG) = 850

LSN 15 Priority Based Scheduling Scheduler will always choose a process of higher priority over one of lower priority Have multiple ready queues to represent each level of priority

LSN 15 Priority Based Scheduling Process t(p i ) Pri 0 350 5 1 125 2 2 475 3 3 250 1 4 75 4 0 250 375 850 925 p 3 p 1 p 2 p 4 p 0 1275 T TRnd (p 0 ) = t(p 0 )+t(p 4 )+t(p 2 )+t(p 1 ) )+t(p 3 ) = 350+75+475+125+250 = 1275 T TRnd (p 1 ) = t(p 1 )+t(p 3 ) = 125+250 = 375 T TRnd (p 2 ) = t(p 2 )+t(p 1 )+t(p 3 ) = 475+125+250 = 850 T TRnd (p 3 ) = t(p 3 ) = 250 T TRnd (p 4 ) = t(p 4 )+ t(p 2 )+ t(p 1 )+t(p 3 ) = 75+475+125+250 = 925 T TRnd (AVG) = 735

LSN 15 Round Robin Scheduling Uses preemption based on a clock Clock interrupt is generated at periodic intervals When an interrupt occurs, the currently running process is placed in the read queue Next ready job is selected Known as time slicing

LSN 15 Round Robin Scheduling Process t(p i ) 0 350 1 125 2 475 3 250 4 75 0 p 0 100 200 300 400 475 550 650 p 1 p 2 p 3 p 4 p 0 p 1 p 2 p 3 p 4 p 0 p 1 p 2 p 3 650 750 850 950 1050 1150 1250 1275 p 0 p 2 p 3 p 0 p 2 p 3 p 0 p 2 p 0 p 2 p 2 p 2 p 2 T TRnd (p 0 ) = 1100 T TRnd (p 1 ) = 550 T TRnd (p 2 ) = 1275 T TRnd (p 3 ) = 950 T TRnd (p 4 ) = 475 T TRnd (AVG) = 870

LSN 15 Round Robin Scheduling Process t(p i ) 0 350 1 125 2 475 3 250 4 75 T TRnd (p 0 ) = 1320 T TRnd (p 1 ) = 660 T TRnd (p 2 ) = 1535 T TRnd (p 3 ) = 1140 T TRnd (p 4 ) = 565 T TRnd (AVG) = 1044 0 120 240 360 480 540 575 635 670 p 0 p 1 p 2 p 3 p 4 p 0 p 1 p 2 p 3 p 4 p 0 p 1 790 910 1030 1150 1270 p 2 p 0 p 2 p 3 p 0 p 2 p 0 p 2 p 3 p 0 p 3 1390 1510 1535 p 2 p 2 p 2 p 2

LSN 15 Shortest Process Next Scheduling Nonpreemptive policy Process with shortest expected processing time is selected next Short process jumps ahead of longer processes in ready queue Predictability of longer processes is reduced If estimated time for process not correct, the operating system may abort it

LSN 15 Shortest Process Next Scheduling Process t(p i ) 0 350 1 125 2 475 3 250 4 75 0 p 4 75 200 450 800 p 1 p 3 p 0 p 2 1275 T TRnd (p 0 ) = t(p 0 )+t(p 3 )+t(p 1 )+t(p 4 ) = 350+250+125+75 = 800 T TRnd (p 1 ) = t(p 1 )+t(p 4 ) = 125+75 = 200 T TRnd (p 2 ) = t(p 2 )+t(p 0 )+t(p 3 )+t(p 1 )+t(p 4 ) = 475+350+250+125+75 = 1275 T TRnd (p 3 ) = t(p 3 )+t(p 1 )+t(p 4 ) = 250+125+75 = 450 T TRnd (p 4 ) = t(p 4 ) = 75 T TRnd (AVG) = 560

LSN 15 Implementing The Scheduler From Other States Ready Process Process Descriptor Enqueuer Ready List Dispatcher Context Switcher CPU Running Process

LSN 15 Implementing The Scheduler Voluntary CPU sharing Each process/thread will explicitly invoke the scheduler periodically Some hardware uses a yield() instruction to allow a process to release the CPU Nonpreemptive scheduling Involuntary CPU sharing Timer interrupt Preemptive scheduling

LSN 15 Windows Thread Scheduling Multi-level feedback Tries to provide very high levels of service to threads that need very rapid responses Uses programmable interval times (20 200 ms) Supports 32 different scheduling levels 16 highest priority queues real-time level queues Next 15 priority queues variable level queues Lowest level queue system level queue Fully preemptive scheduling All threads that are ready to run are placed into their appropriate priority level queues

LSN 15 Linux Scheduling Mechanism Dispatcher is the kernel function schedule() Called by system functions after every system call and normal interrupt Inspects the set of tasks in TASK_RUNNING state Scheduling policy is a variant of round robin scheduling Uses a conventional time slicing mechanism Uses a computed dynamic priority Dispatcher selects process with maximum counter value

LSN 15 Homework Reading Chapter 9.1 9.2

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