CSE120 Principles of Operating Systems. Prof Yuanyuan (YY) Zhou Scheduling

Transcription

1 CSE120 Principes of Operating Systems Prof Yuanyuan (YY) Zhou Scheduing

2 Announcement Homework 2 due on October 25th Project 1 due on October 26th 2 CSE 120 Scheduing and Deadock

3 Scheduing Overview In discussing process management and synchronization, we taked about context switching among processes/threads on the ready queue But we have gossed over the detais of exacty which thread is chosen from the ready queue Making this decision is caed scheduing In this ecture, we ook at: The goas of scheduing Starvation Various we-known scheduing agorithms Standard Unix scheduing agorithm 3 CSE 120 Scheduing and Deadock

4 Mutiprogramming In a mutiprogramming system, we try to increase CPU utiization and job throughput by overapping I/O and CPU activities Doing this requires a combination of mechanisms and poicy We have covered the mechanisms Context switching, how and when it happens Process queues and process states Now we ook at the poicies Which process (thread) to run, for how ong, etc. We refer to scheduabe entities as jobs (standard usage) coud be processes, threads, etc. 4 CSE 120 Scheduing and Deadock

5 Scheduing Deciding which process/thread shoud occupy the resource (CPU, disk, etc) (CPU) I want to pay it Whose turn is it? Process 1 Process 2 Process 3 5 CSE 120 Scheduing and Deadock

6 When does OS need to do schedue? A new process starts A running process exits A running process is bocked I/O interrupt (some processes wi be ready) Cock interrupt (every 10 miiseconds) 6 CSE 120 Scheduing and Deadock

7 Preemptive vs. Non-preemptive Non-preemptive scheduing: The running process keeps the CPU unti it vountariy gives up the CPU process exits switches to bocked state 1and 4 ony (no 3) Preemptive scheduing: Running The running process can be interrupted and must reease the CPU (can be forced to give up CPU) 3 Ready 4 1 Terminated Bocked 7 CSE 120 Scheduing and Deadock

8 What are the scheduing objectives? Anyone? CPU I want to pay Whose turn is it? Process 1 Process 2 Process 3 8 CSE 120 Scheduing and Deadock

9 Scheduing Objectives Fair (nobody cries) Priority (ady first) Efficiency (make best use of equipment) Encourage good behavior (good boy/gir) Support heavy oads (degrade gracefuy) Adapt to different environments (interactive, rea-time, muti-media) 9 CSE 120 Scheduing and Deadock

10 Performance Criteria Fairness Efficiency: keep resources as busy as possibe Throughput: # of processes that competes in unit time Turnaround Time (aso caed eapse time) amount of time to execute a particuar process from the time its entered Waiting Time amount of time process has been waiting in ready queue Response Time amount of time from when a request was first submitted unti first response is produced. predictabiity and variance Meeting Deadines: avoid osing data 10 CSE 120 Scheduing and Deadock

11 Different Systems, Different Focuses For a Fairness, poicy enforcement, resource baance Batch Systems Max throughput, min turnaround time, max CPU utiization Interactive Systems Min Response time, best proportionaity Rea-Time Systems predictabiity, meeting deadines 11 CSE 120 Scheduing and Deadock

12 Program Behaviors Considered in Scheduing Is it I/O bound? Exampe? Is it CPU bound? Exampe? I/O I/O compute I/O compute Batch or interactive environment Urgency Priority Frequency of page fauts Frequency of preemption How much execution time it has aready received How much execution time it needs to compete 12 CSE 120 Scheduing and Deadock

13 Singe Processor Scheduing Agorithms Batch systems First Come First Serve (FCFS) Short Job First Interactive Systems Round Robin Priority Scheduing Muti Queue & Muti-eve Feedback Shortest process time Guaranteed Scheduing Lottery Scheduing Fair Sharing Scheduing 13 CSE 120 Scheduing and Deadock

14 First Come First Serve (FCFS) Process that requests the CPU FIRST is aocated the CPU FIRST. Aso caed FIFO Non-preemptive Used in Batch Systems Rea ife anaogy: Fast food restaurant Impementation: FIFO queues A new process enters the tai of the queue The schedue seects from the head of the queue. Performance Metric: Average Waiting Time. Given Parameters: Duration(in ms), Arriva Time and Order 14 CSE 120 Scheduing and Deadock

15 FCFS Exampe Process Duration Order Arriva Time P P P The fina schedue: P1 (24) P2 (3) P3 (4) 0 P1 waiting time: 0 P2 waiting time: 24 P3 waiting time: The average waiting time: ( )/3 = CSE 120 Scheduing and Deadock

16 Probems with FCFS Non-preemptive Not optima AWT Average waiting time can be arge if sma jobs wait behind ong ones Soution? You have a basket, but you re stuck behind someone with a fu shopping cart at Von s. Express ane (12 items or ess) 16 CSE 120 Scheduing and Deadock

17 Shortest Job First (SJF) 17 Schedue the job with the shortest eapse time(duration) first Scheduing used in Batch Systems Two types: Non-preemptive & Preemptive Requirement: the eapse time needs to know in advance Optima if a the jobs are avaiabe simutaneousy (provabe). Gives the best possibe AWT (average waiting time) Rea ife anaogy? Express ane in supermarket Shortest important task first --The 7 Habits of Highy Effective Peope CSE 120 Scheduing and Deadock

18 Non-preemptive SJF: Exampe Process Duration Order Arriva Time P P P P P4 (3) P1 (6) P3 (7) P2 (8) P4 waiting time: 0 P1 waiting time: 3 P3 waiting time: 9 P2 waiting time: 16 The tota time is: 24 The average waiting time (AWT): ( )/4 = 7 CSE 120 Scheduing and Deadock

19 Comparing to FCFS Process Duration Order Arriva Time P P P P P1 (6) P2 (8) P3 (7) P4 (3) P1 waiting time: 0 P2 waiting time: 6 P3 waiting time: 14 P4 waiting time: The tota time is the same (why?) The average waiting time (AWT): ( )/4 = (comparing to 7) 24

20 SJF is not aways optima Is SJF optima if a the jobs are not avaiabe simutaneousy? Process Duration Order Arriva Time P P P1 (10) P2 (2) 0 2 (p2 arrives) P1 waiting time: 0 P2 waiting time: 8 The average waiting time (AWT): (0+8)/2 = 4 20 CSE 120 Scheduing and Deadock

21 Preemptive SJF Aso caed Shortest Remaining Time First Schedue the job with the shortest remaining time required to compete Requirement: the eapse time needs to be known in advance 21 CSE 120 Scheduing and Deadock

22 Preemptive SJF: Same Exampe Process Duration Order Arriva Time P P P1 (2) P2 (2) P1 (8) P1 waiting time: 4-2 =2 P2 waiting time: 0 The average waiting time (AWT): (0+2)/2 = 1 22 No CPU waste!!! CSE 120 Scheduing and Deadock

23 A Probem with SJF Starvation In some condition, a job is waiting for ever Exampe: SJF Process A with eapse time of 1 hour arrives at time 0 But every 1 minute, a short process with eapse time of 2 minutes arrive Resut of SJF: A never gets to run 23 CSE 120 Scheduing and Deadock

24 Interactive Scheduing Agorithms 24 Usuay preemptive Time is siced into quantum (time intervas) Scheduing decision is aso made at the beginning of each quantum Performance Criteria Min Response time best proportionaity Representative agorithms: Priority-based Round-robin Muti Queue & Muti-eve Feedback Shortest process time Guaranteed Scheduing Lottery Scheduing Fair Sharing Scheduing CSE 120 Scheduing and Deadock

25 Priority Scheduing 25 Each job is assigned a priority. FCFS within each priority eve. Seect highest priority job over ower ones. Rationa: higher priority jobs are more mission-critica Exampe: DVD movie payer vs. send emai Rea ife anaogy? Boarding at airports Probems: May not give the best AWT indefinite bocking or starvation a process CSE 120 Scheduing and Deadock

26 Set Priority Two approaches Static (for system with we known and reguar appication behaviors) Dynamic (otherwise) Priority may be based on: Cost to user. Importance of user. Aging Percentage of CPU time used in ast X hours. 26 CSE 120 Scheduing and Deadock

27 Priority Scheduing: Exampe Process Duration Priority Arriva Time P P P P P2 (8) P4 (3) P3 (7) P1 (6) P2 waiting time: 0 P4 waiting time: 8 P3 waiting time: 11 P1 waiting time: 18 The average waiting time (AWT): ( )/4 = 9.25 (worse than SJF) CSE 120 Scheduing and Deadock

28 Priority in Unix 28 CSE 120 Scheduing and Deadock

29 Nobody wants to Be nice in Unix 29 CSE 120 Scheduing and Deadock

30 priority priority More on Priority Scheduing For rea-time (predictabe) systems, priority is often used to isoate a process from those with ower priority. Priority inversion is a risk uness a resources are jointy schedued A soution: priority inheritance x->acquire() x->acquire() x->reease() time x->acquire() How can this be avoided? x->acquire() 30 time CSE 120 Scheduing and Deadock

31 Round-robin One of the odest, simpest, most commony used scheduing agorithm Seect process/thread from ready queue in a round-robin fashion (take turns) Rea ife anaogy? Probem: Do not consider priority Context switch overhead 31 CSE 120 Scheduing and Deadock

32 Round-robin: Exampe Process Duration Order Arriva Time P P P Suppose time quantum is: 1 unit, P1, P2 & P3 never bock P1 P2 P3 P1 P2 P3 P1 P2 P3 P2 0 P1 waiting time: 4 P2 waiting time: 6 P3 waiting time: 6 10 The average waiting time (AWT): (4+6+6)/3 = CSE 120 Scheduing and Deadock

33 Time sice too arge FIFO behavior Poor response time Time sice too sma Time Quantum Too many context switches (overheads) Inefficient CPU utiization Heuristic: 70-80% of jobs bock within timesice Typica time-sice 10 to 100 ms Time spent in system depends on size of job. 33 CSE 120 Scheduing and Deadock

34 Combining Agorithms Scheduing agorithms can be combined Have mutipe queues Use a different agorithm for each queue Move processes among queues Exampe: Mutipe-eve feedback queues (MLFQ) Mutipe queues representing different job types Interactive, CPU-bound, batch, system, etc. Queues have priorities, jobs on same queue schedued RR Jobs can move among queues based upon execution history Feedback: Switch from interactive to CPU-bound behavior 34 CSE 120 Scheduing and Deadock

35 Muti-eve Feedback Agorithm: Exampe 35 CSE 120 Scheduing and Deadock

36 Unix Scheduer The canonica Unix scheduer uses a MLFQ 3-4 casses spanning ~170 priority eves (the higher the better) Timesharing: first 60 priorities System: next 40 priorities Rea-time: next 60 priorities Interrupt: next 10 (Soaris) Priority scheduing across queues, RR within a queue The process with the highest priority aways runs Processes with the same priority are schedued RR Processes dynamicay change priority Increases over time if process bocks before end of quantum Decreases over time if process uses entire quantum 36 CSE 120 Scheduing and Deadock

37 Motivation of Unix Scheduer The idea behind the Unix scheduer is to reward interactive processes over CPU hogs Interactive processes (she, editor, etc.) typicay run using short CPU bursts They do not finish quantum before waiting for more input Want to minimize response time Time from keystroke (putting process on ready queue) to executing keystroke hander (process running) Don t want editor to wait unti CPU hog finishes quantum This poicy deays execution of CPU-bound jobs But that s ok 37 CSE 120 Scheduing and Deadock

38 Scheduing Summary Scheduer (dispatcher) is the modue that gets invoked when a context switch needs to happen Scheduing agorithm determines which process runs, where processes are paced on queues Many potentia goas of scheduing agorithms Utiization, throughput, wait time, response time, etc. Various agorithms to meet these goas FCFS/FIFO, SJF, Priority, RR Can combine agorithms Mutipe-eve feedback queues Unix exampe 38 CSE 120 Scheduing and Deadock