Scheduling

1. Scheduling • Scheduling is divided into various levels. • These levels are defined by the location of the processes • A process can be • available to be executed by the processor • partially or fully in main memory • in secondary memory • is not started yet CSE 331 Operating Systems Design

2. Types of Scheduling • Long Term Scheduling (batch processing) • The decision to add to the pool of processes to be executed. • Medium Term Scheduling (swapping) • The decision to add to the process in main memory. • Short Term Scheduling(CPU Scheduling) • The decision as to which process will gain the processor. • I/O Scheduling • The decision as to which process's I/O request shall be handled by a device. CSE 331 Operating Systems Design

3. CPU Scheduling • Select process(es) to run on processor(s) • Process state is changed from “ready” to “running” • The component of the OS which does the scheduling is called the scheduler CSE 331 Operating Systems Design

4. Basic Concepts (CPU Scheduling) • Maximum CPU utilization obtained with multiprogramming • CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. • CPU burst distribution CSE 331 Operating Systems Design

5. Alternating Sequence of CPU And I/O Bursts CSE 331 Operating Systems Design

6. Histogram of CPU-burst Times CSE 331 Operating Systems Design

7. Types of CPU Scheduling • A scheduling algorithm is NON-PREEMPTIVE (run to completion) if the CPU cannot be taken away by the OS. • A scheduling algorithm is PREEMPTIVE if the CPU can be taken away by the OS. CSE 331 Operating Systems Design

8. CPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. • CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state. 2. Switches from running to ready state. 3. Switches from waiting to ready. 4. Terminates. • Scheduling under 1 and 4 is nonpreemptive. • All other scheduling is preemptive. CSE 331 Operating Systems Design

9. Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: • switching context • switching to user mode • jumping to the proper location in the user program to restart that program • Dispatch latency – time it takes for the dispatcher to stop one process and start another running. (context switch overhead) CSE 331 Operating Systems Design

10. The Interrupting Clock- Timer • The OS sets the interrupting clock to generate an interrupt at some specified future time. • This interrupt time is the process quantum(time slice-ts, time quantum-tq). • Provides reasonable response times and prevents the system being held up by processes in infinite loops. CSE 331 Operating Systems Design

11. Scheduling Criteria • CPU utilization – keep the CPU as busy as possible • Throughput – # of processes that complete their execution per time unit • Turnaround time – amount of time to execute a particular process • Waiting time – amount of time a process has been waiting in the ready queue • Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) CSE 331 Operating Systems Design

12. Optimization Criteria • Fairness : each process should get a fair share of the CPU • Efficiency: keep CPU 100% utilized • Response time : should be minimized for interactive users • Turnaround : minimize batch turnaround times • Throughput : maximize number of jobs processed per hour CSE 331 Operating Systems Design

13. User-Oriented, Performance Criteria Criteria Aim Response Time low response time, maximum number of interactive users Turnaround Time time between submission and completion Deadlines maximize deadlines met CSE 331 Operating Systems Design

14. System-oriented, Performance Criteria • Criteria Aim • Throughput allow maximum number of jobs to complete • Processor maximize percentage of time processor is busy • utilization • Overhead minimize time processor busy executing OS CSE 331 Operating Systems Design

15. System oriented, other criteria Criteria Aim Fairness treat processes the same avoid starvation Enforcing Priorities give preference to higher priority processes Balancing Resources keep the system resources busy CSE 331 Operating Systems Design

16. Important Factors • I/O / CPU boundedness of a process • Is the process interactive or batch? • Process priority • Page fault frequency (Virtual Memory ) • Preemption frequency (time slice) • Execution time received • Execution time required to complete CSE 331 Operating Systems Design

17. Popular research area in 1970’s.. Assumptions • One program per user • One thread per program • Programs are independent CSE 331 Operating Systems Design

18. Scheduling Algorithms • FCFS -----------------------FCFS • Shortest Job First ------------------ SJF • Shortest Remaining Time • Highest Response Ratio Next • Round Robin -------------------------RR • Virtual Round Robin • Priority -------------------------------Priority • Priority Classes(Multilevel Queues) • Feedback Queues CSE 331 Operating Systems Design

19. FCFS (First Come First Serve) • Implementation: • As each process becomes ready, it joins the ready queue. • When the current process finishes, the oldest process is selected next. • Characteristics: • Simple to implement • Non-premptive • Penalizes short and I/O-bound processes CSE 331 Operating Systems Design

20. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 CSE 331 Operating Systems Design

21. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P2 , P3 , P1 . • The Gantt chart for the schedule is: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case. • Convoy effect short process behind long process CSE 331 Operating Systems Design

22. Approximating Load • Let λ = mean arrival rate • So 1/ λ = mean time between arrivals • And µ = mean service rate • So 1/ µ = mean service time (avg t(pi)) • CPU busy = ρ = λ * 1/ µ = λ / µ • Notice must have λ / µ(i.e., ρ < 1) • What if ρ approaches 1? CSE 331 Operating Systems Design

23. Predicting Wait Time in FCFS • In FCFS, when a process arrives, all inready list will be processed before this job • Let µbe the service rate • Let L be the ready list length Wavg(p) = L*1/ µ + 0.5* 1/ µ = L/ µ+1/(2 µ) Compare predicted wait with actual inearlier examples CSE 331 Operating Systems Design

24. Shortest-Job-First (SJF) • Sometimes known as Shortest Process Next (SPN) • Implementation: • The process with the shortest expected execution time is given priority on the processor CSE 331 Operating Systems Design

25. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) • Average waiting time = (0 + 6 + 3 + 7)/4 = 4 CSE 331 Operating Systems Design

26. SJF-continued • Characteristics: • Non-premptive • Reduces average waiting time over FIFO • Always produces the minimum average turnaround time • Must know how long a process will run • Possible user abuse • Suitable for batch environments. Not useful in a timesharing environment CSE 331 Operating Systems Design

27. Shortest Remaining Time (SRT) • Preemptive counterpart of SPN • Implementation: • Process with the smallest estimated run-time to completion is run next • A running process may be preempted by a new process with a shorter estimate run-time CSE 331 Operating Systems Design

28. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time = (9 + 1 + 0 +2)/4 =3 CSE 331 Operating Systems Design

29. SRT-continued • Characteristics: • Still requires estimates of the future • Higher overhead than SJF • No additional interrupts are generated as in Round Robin • Elapsed service times must be recorded CSE 331 Operating Systems Design

30. ( ) t = a + - a t t 1 . +1 n n n Determining Length of Next CPU Burst • Can only estimate the length. • Can be done by using the length of previous CPU bursts, using exponential averaging. CSE 331 Operating Systems Design

31. Prediction of the Length of the Next CPU Burst CSE 331 Operating Systems Design

32. Examples of Exponential Averaging •  =0 • n+1 = n • Recent history does not count. •  =1 • n+1 = tn • Only the actual last CPU burst counts. • If we expand the formula, we get: n+1 =  tn+(1 - )  tn-1+ … +(1 -  )j  tn-1+ … +(1 -  )n=1 tn 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor. CSE 331 Operating Systems Design

33. Highest Response Ratio Next (HRRN) • How do you get around the problem of Indefinite postponement? • Implementation: • Once a job gets the CPU,it runs to completion • The priority of a job is a function of the job's service time and the time it has been waiting for service priority =(time waiting + service time)/service time CSE 331 Operating Systems Design

34. Characteristics: • Nonpremptive • Shorter jobs still get preference over longer jobs • However aging ensures long jobs will eventually gain the processor • Estimation still involved CSE 331 Operating Systems Design

35. Round Robin (RR) • Implementation: • Processes are dispatched FIFO. But are given a fixed time on the CPU (quantum - time slice). • Characteristics: • Preemptive • Effective in time sharing environments • Penalizes I/O bound processes CSE 331 Operating Systems Design

36. Quantum Size • Some Options: • Large or small quantum • Fixed or variable quantum • Same for everyone or different • If quantum is to large RR degenerates into FCFS • If quantum is to small context switching becomes the primary job being executed • A good guide is: quantum should be slightly larger than the time required for a typical interaction (overhead 10%) For 100ms ts - context switch time < 10ms CSE 331 Operating Systems Design

37. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Example of RR with Time Quantum = 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response. CSE 331 Operating Systems Design

38. Time Quantum and Context Switch Time CSE 331 Operating Systems Design

39. Turnaround Time Varies With The Time Quantum CSE 331 Operating Systems Design

40. Virtual Round Robin (VRR) • A modification to the RR algorithm to remove the bias towards CPU bound processes. • Implementation: • Two “ready” queues, one called an AUX queue for storing “completed” IO processes • AUX queue has priority over READY queue • IO processes only runs for remaining time • Characteristics: • Performance studies indicate fairer than RR CSE 331 Operating Systems Design

41. Priority • Implementation: • Each process is assigned a priority and the scheduler always selects the highest priority process first • Characteristics: • High priority processes may run indefinitely, so decrease the priority of these processes at regular intervals • Assign high priority to system processes with known characteristics such as being I/O bound CSE 331 Operating Systems Design

42. Priority Classes Priority Class 4 Highest Priority Class 3 Priority Class 2 Priority Class 1 Lowest CSE 331 Operating Systems Design

43. Implementation: • Processes are grouped into priority classes • Round Robin is used within a class • When selecting process start with the highest class. If the class is empty, use a lower class • Characteristics: • If priorities are not adjusted from time to time, lower classes may starve to death CSE 331 Operating Systems Design

44. Multilevel Queue • Ready queue is partitioned into separate queues:foreground (interactive)background (batch) • Each queue has its own scheduling algorithm, foreground – RRbackground – FCFS • Scheduling must be done between the queues. • Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. • Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR20% to background in FCFS CSE 331 Operating Systems Design

45. Multilevel Queue Scheduling CSE 331 Operating Systems Design

46. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented this way. • Multilevel-feedback-queue scheduler defined by the following parameters: • number of queues • scheduling algorithms for each queue • method used to determine when to upgrade a process • method used to determine when to demote a process • method used to determine which queue a process will enter when that process needs service CSE 331 Operating Systems Design

47. Example of Multilevel Feedback Queue • Three queues: • Q0 – time quantum 8 milliseconds • Q1 – time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. CSE 331 Operating Systems Design

48. Multilevel Feedback Queues CSE 331 Operating Systems Design

49. I/O processes: • If the job requires I/O before quantum expiration it leaves the network and comes back at the same level queue • CPU bound processes: • If the quantum expires first, the process is placed on the next lower queue • This continues until it reaches the bottom queue CSE 331 Operating Systems Design

50. Dispatching: • A process is only placed on the CPU if all higher level queues are empty • A running process is preempted by a process arriving in a higher queue • Processes from lower level queues receive a larger quantum CSE 331 Operating Systems Design