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Uniprocessor Scheduling

Uniprocessor Scheduling. Chapter 9. Aim of Scheduling. To improve: Response time : time it takes a system to react to a given input Turnaround Time (TAT) = Total time spent in the system = waiting time + service time

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Uniprocessor Scheduling

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  1. Uniprocessor Scheduling Chapter 9

  2. Aim of Scheduling To improve: • Response time: time it takes a system to react to a given input • Turnaround Time (TAT) = Total time spent in the system = waiting time + service time • Throughput: jobs per minute (inverse of TAT)

  3. Types of Scheduling

  4. Long-Term Scheduling • Determines which programs are admitted to the system for processing • Controls the degree of multiprogramming • More processes, smaller percentage of time each process is executed • Which job to admit? FCFS, Priority, expected exec time, I/O req.s • Part of the swapping function • Swapping-in decision is based on the need to manage the degree of multiprogramming Medium-Term Scheduling Short-Term Scheduling • Known as the dispatcher • Executes most frequently • Invoked when an event occurs: Clock interrupts, I/O, OS calls, Signals, etc.

  5. Evaluation Criteria for Short-Term Scheduling Policies: • User-oriented, performance related • Turnaround time:time between submission and completion • Response Time:time between submission and the first output. • User-oriented, other • Predictability: For users, it is important that a job runs the same way (Time & cost) regardless of the system load • System-oriented, Performance related • Maximize Throughput: rate at which processes are completed • Processor utilization: percentage of time that the processor is busy • System-oriented, other • Fairness • Enforcing priorities? • Balancing resources? – Is it favoring processes that do not request stressed resources?

  6. Priorities • Scheduler chooses a process of higher priority over one of lower priority • Use multiple ready queues to represent each level of priority • Lower-priority may suffer starvation • allow a process to change its priority based on its age or execution history Decision Mode • Nonpreemptive • Once a process is in the running state, it will continue until it terminates or blocks itself • Preemptive • Currently running process may be interrupted and moved to the Ready state due to an external event. • No single process can monopolize the processor for very long  Better service

  7. Process Scheduling Example

  8. 5 0 10 15 20 First-Come-First-Served (FCFS) • Each process joins the Ready queue • Nonpreemptive • The oldest process in the ready queue is selected to run next • Disadvantage: A short process may have to wait a very long time before it can execute • Advantage: Favors CPU-bound processes • I/O processes have to wait until CPU-bound process completes A B C D E

  9. 5 0 10 15 20 Round-Robin (RR) • Preemption based on a clock (time slicing) • Clock interrupt is generated at periodic intervals (time slice is determined a priori) • When an interrupt occurs, the currently running process is placed in the ready queue and next ready job is selected to run A B C D E

  10. 5 0 10 15 20 A B C D E Shortest Process Next (SPN) • Nonpreemptive • Process with shortest expected processing time is selected next. • For batch jobs, user is required to estimate the running time. If estimated time for process not correct, the OS may abort it. • For interactive jobs, OS tries to predict it. • Short process jumps ahead of longer processes • Possibility of starvation for longer processes

  11. Process Scheduling Example

  12. 5 0 10 15 20 A B C D E Shortest Remaining Time (SRT) • Preemptive version of shortest process next policy • Achieves better turnaround time as compared to SPN, because a short job is given immediate preference to a running longer job • Risk of starvation for long jobs • Must estimate the remaining processing time  Not easy!

  13. 5 0 10 15 20 Highest Response Ratio Next (HRRN) • Nonpreemptive • An important performance criteria to minimize is: Normalized TurnAround Time = TAT / ServiceTime • Choose next process with the highest response ratio R (so that it does not get any higher!): A B C D E

  14. 5 0 10 15 20 A B C D E Feedback (FB) • Hard to predict the remaining time a process needs to execute. Instead, we can penalize jobs that have been running longer • Each subsequent time that a job is preempted, it is moved to a lower-priority ready queue. This way, a longer process will gradually drift downward in the hierarchy of queues. • starvation may occur if new jobs enter the system frequently

  15. A Comparison of Scheduling Policies

  16. Characteristics of Various Scheduling Policies

  17. Traditional UNIX Scheduling • Designed to provide good response time for interactive users while ensuring that low-priority background jobs do not starve • Multilevel feedback using round robin within each of the priority queues. 1-second preemption. • Priorities are recomputed once per second. It is based on process type and execution history • In decreasing order of priority, the bands are as follows: • Swapper • Block I/O device control • File manipulation • Character I/O device control • User processes

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