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Uniprocessor Scheduling

Uniprocessor Scheduling. Chapter 9. Aim of Scheduling. Minimize response time Maximize throughput Maximize processor efficiency. Types of Scheduling. Long-Term Scheduling. Determines which programs are admitted to the system for processing Controls the degree of multiprogramming

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Uniprocessor Scheduling

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  1. Uniprocessor Scheduling Chapter 9

  2. Aim of Scheduling • Minimize response time • Maximize throughput • Maximize processor efficiency

  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

  5. Long-Term Scheduling • Batch systems: • newly submitted jobs are routed to disk and held in a batch queue. • Time-sharing systems • the operating system will accept all authorized comers until the system is saturated

  6. Long-Term Scheduling • Decisions involved in taking a process from the queue: • A. Can the operating system can take on one or more additional processes? • B. Which job to be accepted and turned into a process • first-come-first-served basis, • priority, • expected execution time, • I/O requirements.

  7. Medium-Term Scheduling • Part of the swapping function • Based on the need to manage the degree of multiprogramming

  8. Short-Term Scheduling • Known as the dispatcher • Executes most frequently • Invoked when an event occurs • Clock interrupts • I/O interrupts • Operating system calls • Signals

  9. Short-Tem Scheduling Criteria • User-oriented • Response Time • Elapsed time between the submission of a request until there is output. • System-oriented • Effective and efficient utilization of the processor

  10. Short-Term Scheduling Criteria • Performance-related • Quantitative • E.G. response time and throughput • Not performance related • Qualitative • E.G. Predictability

  11. Priorities • Scheduler will always choose a process of higher priority over one of lower priority • Have 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

  12. Alternative Scheduling Policies Selection function • Selection function – determines which process, among ready processes, is selected next for execution. • based on priority, resource requirements, or the execution characteristics of the process • w = time spent in system so far, waiting and executing • e = time spent in execution so far • s = total service time required by the process, including e; generally, this quantity must be estimated or supplied by the user

  13. Alternative Scheduling PoliciesDecision mode • Decision mode – specifies the instants in time at which the selection function is exercised. • Non-preemptive:process continues to execute until • (a) it terminates or • (b) blocks itself to wait for I/O or to request some operating system services. • Preemptive:process may be interrupted and moved to the Ready state by the operating system.

  14. Decision Mode • Preemptive • Currently running process may be interrupted and moved to the Ready state by the operating system • Allows for better service since any one process cannot monopolize the processor for very long

  15. Process Scheduling Example

  16. 5 0 10 15 20 First-Come-First-Served(FCFS) • Each process joins the Ready queue • When the current process ceases to execute, the oldest process in the Ready queue is selected 1 2 3 4 5

  17. First-Come-First-Served(FCFS) • 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

  18. 5 0 10 15 20 Round-Robin • Uses preemption based on a clock • An amount of time is determined that allows each process to use the processor for that length of time 1 2 3 4 5

  19. Round-Robin • 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

  20. 5 0 10 15 20 1 2 3 4 5 Shortest Process Next • Non-preemptive policy • Process with shortest expected processing time is selected next • Short process jumps ahead of longer processes

  21. Shortest Process Next • Predictability of longer processes is reduced • If estimated time for process not correct, the operating system may abort it • Possibility of starvation for longer processes

  22. 5 0 10 15 20 1 2 3 4 5 Shortest Remaining Time • Preemptive version of shortest process next policy • Must estimate processing time

  23. 5 0 10 15 20 Highest Response Ratio Next (HRRN) • Choose next process with the highest ratio 1 2 3 4 5 time spent waiting + expected service time expected service time

  24. 5 0 10 15 20 1 2 3 4 5 Feedback • Penalize jobs that have been running longer • Don’t know remaining time process needs to execute

  25. Fair-Share Scheduling • User’s application runs as a collection of processes (threads) • User is concerned about the performance of the application • Need to make scheduling decisions based on process sets

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