ITFN 2601 Introduction to Operating Systems

1. ITFN 2601Introduction to Operating Systems Lecture 4 Scheduling

2. Agenda • Scheduling • Batch • Interactive • Real-Time • Threads

3. Scheduling: When • New Process is Created • Parent Process • Child Process • Process Exits • When a process blocks • I/O Interrupt occurs • Clock Interrupts • Non-preemptive • Preemptive

4. Objectives of a Good Scheduling Policy • Fairness • Efficiency • Low response time (important for interactive jobs) • Low turnaround time (important for batch jobs) • High throughput

5. Scheduling • Throughput The amount of useful work accomplished per unit time. This depends, of course, on what constitutes “useful work.” One common measure of throughput is jobs/minute (or second, or hour, depending on the kind of job) • Utilization For each device, the utilization of a device is the fraction of time the device is busy. A good scheduling algorithm keeps all the devices (CPUs, disk drives, etc.) busy most of the time.

6. Scheduling • Turnaround The length of time between when the job arrives in the system and when it finally finishes • Response Time The length of time between when a job arrives in the system and when it starts to produce output. For interactive jobs, response time might be more important than turnaround. • Wait Time The amount of time the job is ready (runnable but not running). This is a better measure of scheduling quality than turnaround since the scheduler has no control of the mount of time the process spends computing or blocked waiting for I/O.

7. Preemption • Needs a clock interrupt (or equivalent) • Needed to guarantee fairness • Found in all modern general purpose operating systems • Without preemption, the system implements “run to completion (or yield)”

8. Scheduling Algorithms(Batch) • FIFO (First In First Out) • Fairest • Low throughput • High Turnaround • Shortest First • High Throughput • Low Turnaround • Unfair for Large Jobs

9. Scheduling Algorithms(Batch, cont) • Shortest Remaining • High Turnaround on Long Jobs • Unfair for Large Jobs • Multi-Scheduling (CPU or Memory Limited) • HIGH Turnaround (disk swaps) • Throughput highly variable, probably low • Fairness highly variable

10. First-Come First-Served • The simplest possible scheduling discipline is called First-come, first-served (FCFS). The ready list is a simple queue (first-in/first-out). The scheduler simply runs the first job on the queue until it blocks, then it runs the new first job, and so on. When a job becomes ready it is simply added to the end of the queue.

11. FCFS • Main advantage of FCFS is that it is easy to write and understand • No starvation • If one process gets into an infinite loop, it will run forever and shut out all the others • FCFS tends to excessively favor long bursts. CPU-bound processes

12. Shortest Job First (SJF) • Whenever the CPU has to choose a burst to run, it chooses the shortest one • Non-preemptive policy • Preemptive version of the SJN, called shortest remaining time first • Starvation is possible

13. Scheduling Algorithms(Interactive) • Round Robin • Fairest overall • Response time variable but finite • Priority Scheduling • Fair • “More Fair” for users with higher priorities • Response time inverse to priority

14. Round-Robin • Round-robin (RR). RR keeps all the processes in a queue and runs the first one, like FCFS. After a length of time q (called quantum), if the current burst hasn’t completed, it is moved to the tail of the queue and the next process is started

15. Round Robin • An important preemptive policy • Essentially the preemptive version of FCFS • The key parameter is the quantum size q • When a process is put into the running state, a timer is set to q • If the timer goes off and the process is still running, the OS preempts the process. • The process is moved to the ready state • The next job in the queue is selected to run

16. Round Robin • Quantum can’t be too large • Quantum can’t be too small • What quantum should we choose • Tradeoff • Small q makes system more responsive • Large q makes system more efficient since there’s less switching

17. Round Robin, Example

18. Priority Scheduling • Always run the highest priority process • Preemptive or non-preemptive • Priorities can be assigned externally to processes based on their importance • Assigned (and changed) dynamically

19. Other Interactive Scheduling • Multiple Queues • Shortest Process Next • Guaranteed Scheduling • Lottery Scheduling • Fair-Share Scheduling

20. Scheduling Algorithms(Interactive, cont) • Multi-Quantized • Response time proportionate to quanta • More bookkeeping • Shortest Process Next • Estimation of process length • Unfair for large jobs • Fast response for small jobs

21. Scheduling Algorithms(Interactive, cont) • Guaranteed Scheduling • Lottery Scheduling • Alotted time proportional to Job Size/Importance • Sharing • Fair by user, not necessarily fair by job • Responses become disproportionate

22. Scheduling Algorithms(Real-Time) • Small Jobs • High Priority • Periodic/Aperiodic • Schedulable? • Iff the sum of the ratios CPU Time to Period time is less than one • Sum(CPU/Period) <= 1 • Static/Dynamic?

23. Scheduler Mechanism • Some processes are intrinsically more important at some times than others • Time-dependent response • High-priority request • How can a process raise it’s scheduling priority?

24. Thread Scheduling • User-level threads • Process has a Thread Scheduler • Same concept as Process Scheduler • Conflicts with Process Scheduling • Kernel Level Threads • Kernel has Thread and Process Scheduler • Two Quanta’s • Thread Quanta • Process Quanta

25. Summary • Scheduler responsible for many goals • Scheduling algorithms complex • Know your math!