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Tutorial 4

Tutorial 4. Scheduling. Why do we need scheduling?. To manage processes according to requirements of a system, like: User responsiveness or Throughput Performance of a scheduler is determined mainly by: Context switch time Scheduling policy. 1, 2, 3, 4, 5, 6, Context Switch!.

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Tutorial 4

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  1. Tutorial 4 Scheduling

  2. Why do we need scheduling? • To manage processes according to requirements of a system, like: • User responsiveness or • Throughput • Performance of a scheduler is determined mainly by: • Context switch time • Scheduling policy

  3. 1, 2, 3, 4, 5, 6, Context Switch! • Switching from one process running on the CPU to another process • Saves all the registers of outgoing process (to memory), then loads all the registers of incoming process (from memory) • Can be time-costly; mostly hardware-dependent

  4. Scheduling • The mechanism that determines when the CPU will be allocated to processes, and in what order • Two classes of scheduling strategies: • Nonpreemptive (aka Batch) • Preemptive

  5. Non-preemptive policies • Allow any process to run to completion once it has been allocated to the CPU. Current process does not get interrupted. • Some examples: • First Come First Serve (FCFS) • Shortest Job Next (SJN) • Priority scheduling • Deadline scheduling

  6. Preemptive policies • Allow another process to interrupt current process if: • It has a higher priority • The time quantum has elapsed • Some examples: • Round Robin • Multiple-level Queues

  7. Scheduling examples • Given three threads, their execution times and I/O needs, apply scheduling policies • Threads are placed on ready queue in order: T1, T2 then T3 * Specific to Round Robin: • Time Quantum of 3ms • Context switch time considered negligible in this example

  8. First Come First Serve 1 2 3 2 3 3 CPU: 0 5 10 15 20 25 30 35 40 I/O: 2 3 3

  9. Shortest Job First (Nonpreemptive) 1 3 2 3 2 3 CPU: 0 5 10 15 20 25 30 35 40 I/O: 3 2 3

  10. Shortest Job First (Preemptive) 1 3 2 3 2 3 2 CPU: 0 5 10 15 20 25 30 35 40 I/O: 3 2 3

  11. Priority (Preemptive) 3 1 3 1 3 1 2 2 CPU: 0 5 10 15 20 25 30 35 40 I/O: 3 3 2

  12. 3 3 3 2 1 Round Robin 1 2 3 1 2 1 3 2 2 3 2 CPU: 0 5 10 15 20 25 30 35 40 I/O: 2 3 3

  13. Recapping Scheduling in Java Java 1.2.1 and prior (“green”) threads: • Threads are managed by Java VM • One thread runs at a time • Thread is only taken off the CPU when: • it yields • it blocks (on a resource) • it exits • a higher priority thread becomes runnable (not always)

  14. New-Style Scheduling in Java Post-1.2.1 versions of Java: • Threads are managed by the host OS • Allows multiple threads to run “concurrently” • Thread is taken off the CPU as in previous versions, but also • when its time quantum expires (if supported by OS)

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