1. CS 3013 & CS 502 Summer 2006 Scheduling1 The art and science of allocating the CPU and other resources to processes

2. CS 3013 & CS 502 Summer 2006 Scheduling2 Bursts of CPU usage alternate with periods of I/O wait –a CPU-bound process –an I/O bound process Which process should have higher CPU priority? Which process should have higher I/O or disk priority?

3. CS 3013 & CS 502 Summer 2006 Scheduling3 Process Scheduling When to run scheduler 1.Process create 2.Process exit 3.Process blocks 4.System interrupt Non-preemptive – process runs until it blocks or gives up CPU (1,2,3) Preemptive – process runs for some time or until some asynchronous action interrupts, then scheduler selects a process to run (1-4)

4. CS 3013 & CS 502 Summer 2006 Scheduling4 Scheduling – Policies Issues –Fairness – don’t starve process –Priorities – most important first –Deadlines – task X must be done by time t –Optimization – e.g. throughput, response time Reality - No universal scheduling policy –Many models –Determine what to optimize - metrics –Select an appropriate one and adjust based on experience

5. CS 3013 & CS 502 Summer 2006 Scheduling5 Process Scheduling – System Needs

6. CS 3013 & CS 502 Summer 2006 Scheduling6 Scheduling – Metrics Simplicity – easy to implement Job latency – time from start to completion Interactive latency – time from action start to expected system response Throughput – number of jobs completed Utilization – keep processor and/or subset of I/O devices busy Determinism – insure that jobs get done before some time or event Fairness – every job makes progress

7. CS 3013 & CS 502 Summer 2006 Scheduling7 Batch System Scheduling Note: Number of Processes in Memory determines the degree of multiprogramming

8. CS 3013 & CS 502 Summer 2006 Scheduling8 Interactive System Scheduling First-Come, First-Served (FCFS) Round Robin (RR) Shortest Completion Time First (SCTF) Priority

9. CS 3013 & CS 502 Summer 2006 Scheduling9 Scheduling Policies – FCFS First Come, First Served (FCFS) –Easy to implement –Non-preemptive –Minimizes context switch overhead –Favors compute bound jobs –Short jobs penalized I.e., once a longer job gets the CPU, it gets in the way of a bunch of shorter jobs

10. CS 3013 & CS 502 Summer 2006 Scheduling10 Scheduling Policies – Round Robin Round Robin (RR) –Preemptive –Ready processes given a quantum of time when scheduled –Process runs until quantum expires or until it blocks (whichever comes first) –Suitable for interactive (timesharing) systems –Setting quantum is critical for efficiency

11. CS 3013 & CS 502 Summer 2006 Scheduling11 Scheduling Policies – Comparison 10 jobs each take 100 seconds – look at when jobs complete FCFS – job 1: 100s, job 2: 200s … job 10:1000s RR –1 sec quantum –Job 1: 991s, job 2 : 992s … RR good for short jobs – worse for long jobs

12. CS 3013 & CS 502 Summer 2006 Scheduling12 Scheduling Policies – RR example Utilization –A and B compute bound jobs:– 100 ms @ 100% CPU –C:– 1 ms CPU and 10 ms disk I/O –Quantum:– 100 ms CPU utilization = (100 + 100 +1)/210 = 95.7 % –Quantum:– 1 ms (assume 0 overhead) ABCAB(C-I/O)ABABAB ….AB CPU utilization = 100% –Smaller quantum can improve utilization – but consider overhead of context switches

13. CS 3013 & CS 502 Summer 2006 Scheduling13 Scheduling Policies – STCF STCF – shortest time to completion first or shortest job first –Can be preemptive –Good for throughput –Example jobs A = 100, B = 1, C = 2 msec Done B @ 1, C @ 3, A @ 103 Ave = 35, (for RR (1) about 37) –Reality – knowledge of job runtime??

14. CS 3013 & CS 502 Summer 2006 Scheduling14 Scheduling Policies – Priority Priority Scheduling –Preemptive –Process are given priorities and ranked Maybe done with multiple queues - multilevel –Highest priority runs next With multilevel queues –Select from highest queue –Round robin within queue –Recalculate priority – many algorithms E.g. increase priority of I/O intensive jobs E.g. favor processes in memory Must still meet system goals – e.g. response time

15. CS 3013 & CS 502 Summer 2006 Scheduling15 Scheduling Policy Problem Priority inversion –A has high priority, B has medium priority, C has lowest priority –C acquires a resource that A needs to progress –A attempts to get resource, fails and busy waits C never runs to release resource! or –A attempts to get resources, fails and blocks B (medium priority) enters system & hogs CPU C never runs! Priority scheduling can’t be naive

16. CS 3013 & CS 502 Summer 2006 Scheduling16 Solution Some systems increase the priority of a process/task/job to Match level of resource or Match level of waiting process

17. CS 3013 & CS 502 Summer 2006 Scheduling17 Scheduling Policies – Realtime Real Time System – processes have deadlines –Deadlines known –(usually) preemptive Static algorithm – periodic process behavior –Rate Monotonic Scheduling (RMS) – priority = 1/period Dynamic – aperiodic –Earliest Deadline First (EDF) – select process that must complete the soonest More about this subject later in the term

18. CS 3013 & CS 502 Summer 2006 Scheduling18 Scheduling – Examples Unix – multilevel - many policies and many policy changes over time Linux – multilevel with 3 major levels –Realtime FIFO –Realtime round robin –Timesharing Win/NT – multilevel –Threads scheduled – fibers not visible to scheduler –Jobs – groups of processes are given quotas that contribute to priorities

19. CS 3013 & CS 502 Summer 2006 Scheduling19 Scheduling Until now:– focus on processes and CPU Going forward:– focus on system resources memory, storage, I/O devices, web resources, … Space sharing –How can resources be split up? – e.g. disk, memory Time sharing –What gets to use something and for how long? –Whenever there are more requests than can be granted Typically resource can not be divided – I/O port, device, …

20. CS 3013 & CS 502 Summer 2006 Scheduling20 Scheduling General theme – what is the “best way” to run n processes on k resources? ( k < n) Conflicting Objectives – no one “best way” –Latency vs. throughput –Speed vs. fairness Incomplete knowledge –E.g. – does user know how long a job will take Real world limitations –E.g. context switching takes CPU time –Job loads are unpredictable

21. CS 3013 & CS 502 Summer 2006 Scheduling21 Scheduling (continued) Bottom line – scheduling is hard! –Know the models –Adjust based upon experience

22. CS 3013 & CS 502 Summer 2006 Scheduling22 Break (next topic)