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  1. Chapter 5: Process Scheduling

  2. Outline • Basic Concepts • CPU-I/O Burst Cycle • CPU Scheduler • Preemptive Scheduling • Dispatcher • Scheduling Criteria • Scheduling Algorithms • First-Come, First-Serve Scheduling • Shortest-Job-First Scheduling • Priority Scheduling • Round-Robin Scheduling • Multilevel Queue Scheduling • Multilevel Feedback-Queue Scheduling

  3. Basic Concepts • CPU: One of the primary computer resource • Maximum CPU utilization obtained with multiprogramming • Several processes are kept in memory • CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. • CPU burst distribution determines how to schedule the processes.

  4. Alternating Sequence of CPU And I/O Bursts

  5. Histogram of CPU-burst Times Normally a process starts with frequent short CPU bursts and then it stabilizes and shifts to long but infrequent CPU bursts • I/O bound processes usually have manysmall CPU bursts • CPU bound programs usually have few but long CPU bursts CPU-bound program

  6. CPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. • Is it the same as Short-term scheduler? … Yes • 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.(Own will) • Scheduling Scheme given at 1 and 4 is nonpreemptive (Cooperative). • All other scheduling is preemptive.

  7. Preemptive vs Non PreemptiveScheduling • Non Preemptive scheduling • If a process is being executed then it keeps the CPU until one or four conditions of previous slide occur. • MS Windows 3.x which was sort of multitasking system used this method • Not at all efficient. • Preemptive • A process in running state can be brought back to ready state without any of the conditions of previous slide being met. • Can you guess how it is done? 

  8. Problems with preemptive • Although better than non-preemptive still suffers from synchronization problems • E.g. • Process P and Q share some data (can be memory location or a file) • P does some calculations changes some of the data but before it finishes its time expires • Q reads the data but is the data valid?

  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. • Should be minimum.

  10. Scheduling CriteriaFollowing criteria is kept under consideration while designing a scheduler • CPU utilization– Keep the CPU as busy as possible • Throughput – # of processes that complete their execution per unit time (Given Time). • Turnaround time– Amount of time to execute a particular process (Sum of ready, waiting, running, loading times,I/O) • 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 interactive environment) • Time it takes to start responding • Not the time it takes to output the response

  11. Optimization Criteria • So the best CPU Scheduling algorithm is which … • Maximizes CPU utilization • Maximizes throughput • Minimizes turnaround time • Minimizes waiting time • Minimizes response time

  12. Outline Basic Concepts CPU-I/O Burst Cycle CPU Scheduler Preemptive Scheduling Dispatcher Scheduling Criteria Scheduling Algorithms First-Come, First-Serve Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling

  13. 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

  14. 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. • CPU bound processes will get and hold the CPU • Convoy effect short process behind long process (Bad CPU utilization)

  15. Shortest-Job-First (SJR) Scheduling • Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time. • Two schemes: • nonpreemptive – once CPU given to the process it cannot be preempted until it completes its CPU burst. • preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). • SJF is optimal – gives minimum average waiting time for a given set of processes. • Shortest-next-cpu-burst algorithm

  16. Example of Non-Preemptive SJF ProcessBurst Time P1 6 P2 8 P3 7 P4 3 • SJF • Average waiting time = (3+16+9+0)/4=7 FCFS=10.25 8 P1 P3 P2 P4 0 3 16 24 9

  17. Shortest-Job-First (SJR) Scheduling • Good for long-term Scheduler in batch systems • User specifies process time during job submission • Lower value may mean faster response • Too low a value will cause a time-limit-exceeded error • Difficult to implement at the level of short-term CPU scheduler • Determining Length of Next CPU Burst • Can only predict/ estimate the length of next CPU burst. • Can be done by using the length of previous CPU bursts, using exponential averaging.

  18. Examples of Exponential Averaging n+1=αtn +(1- α) n tn: length of the nth CPU burst n+1: Our predicted Value α: weight of recent & past history 0<= α<=1 •  =0 • n+1 = n • Recent history does not count. •  =1 • n+1 = tn • Only the actual last CPU burst counts. • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.

  19. Prediction of the Length of the Next CPU Burst Initial guess for T(1) = 10 ms Actual 6 Determination of 2 Cycle based on actual (6) and last guess (10) T(2) = 0.5 x 6 + 0.5 x 10 = 8 ms Determination of 3 cycle based on actual(4) and last guess (8) T(3) = 0.5 x 4 + 0.5 x 8 = 6 ms … Now you can do it 

  20. For example: Suppose a process p is given a default expected burst length of 5 time units. When it is run, the actually burst lengths are 10,10,10,1,1,1 (although this information is not known in advance to any algorithm). The prediction of burst times for this process works as follows. Let e(1) = 5, as a default value. When process p runs, its first burst actually runs 10 time units, so, a(1) = 10. e(2) =0.5 * e(1) * 0.5 + a(1) = 0.5 * 5 + 0.5 * 10 = 7.5 This is the prediction for the second cpu burst The actual second cpu burst is 10. So the prediction for the third cpu burst is: e(3) = 0.5 * e(2) * 0.5 + a(2) = 0.5 * 7.5 + 0.5 * 10 = 8.75 e(4) = 0.5 * e(3) * 0.5 + a(3) = 0.5 * 8.75 + 0.5 * 10 = 9.38, So, we predict that the next burst will be close to 10 (9.38) because the recent bursts have been of length 10. At this point, it happens that the process starts having shorter bursts, with a(4) = 1, so the algorithm gradually adjusts its estimated cpu burst (prediction) e(5) = 0.5 * e(4) * 0.5 + a(4) = 0.5 * 9.38 + 0.5 * 1 = 5.19 e(6) = 0.5 * e(5) * 0.5 + a(5) = 0.5 * 5.19 + 0.5 * 1 = 3.10 e(7) = 0.5 * e(6) * 0.5 + a(6) = 0.5 * 3.10 + 0.5 * 1 = 2.05

  21. Example of Preemptive SJF(Shortest Remaining Time first) Process Arrival TimeBurst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 • SJF (preemptive) • Average waiting time = 6.5 • Non-Preemptive Average waiting time = 6.5 P1 P2 P1 P3 P4 17 0 1 10 26 5

  22. Outline Basic Concepts CPU-I/O Burst Cycle CPU Scheduler Preemptive Scheduling Dispatcher Scheduling Criteria Scheduling Algorithms First-Come, First-Serve Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling

  23. Priority Scheduling • A priority number (integer) is associated with each process • The CPU is allocated to the process with the highest priority (smallest integer  highest priority). • Preemptive • Non-preemptive • SJF is a priority scheduling where priority is the predicted next CPU burst time. • Problem  Starvation – low priority processes may never execute. • Solution  Aging – as time progresses increase the priority of the process.

  24. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high.

  25. 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.

  26. Time Quantum and Context Switch Time • If context switching time is approx 10 percent of the time quantum, then about 10 percent of the CPU time will be spent in context switching • Most modern systems have time quanta ranging from 10 to 100 milliseconds • Context switch typically takes less than 10 microseconds Remember: Context switch takes time 

  27. Turnaround Time Varies With The Time Quantum 80% of CPU bursts should be shorter than the time quantum • Average Turnaround time doesn’t necessarily improve as the time-quantum size increases)

  28. Outline Basic Concepts CPU-I/O Burst Cycle CPU Scheduler Preemptive Scheduling Dispatcher Scheduling Criteria Scheduling Algorithms First-Come, First-Serve Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling

  29. 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 RR • 20% to background in FCFS

  30. Multilevel Queue Scheduling

  31. 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

  32. Example of Multilevel Feedback Queue • Consider a case if we have 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 tail of 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 tail of queue Q2.

  33. Multilevel Feedback Queues

  34. Outline Basic Concepts CPU-I/O Burst Cycle CPU Scheduler Preemptive Scheduling Dispatcher Scheduling Criteria Scheduling Algorithms First-Come, First-Serve Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling Multi-Processor Scheduling

  35. Multiple-Processor Scheduling • CPU scheduling more complex when multiple CPUs are available. • Consider Homogeneous processors within a multiprocessor system. • Load sharing • Asymmetric multiprocessing– only one processor accesses the system data structures, alleviating the need for data sharing. • SMP: Each processor is self scheduling • May have separate ready queues

  36. Multiple-Processor Scheduling –Process Affinity • Cache memory? • The Data most recently accessed populates it • Process migration to another process • Invalidation & Repopulation • Some OS attempt to keep a process running on the same processor –Affinity • Soft Affinity: Attempt to keep a process running on the same processor but not guaranteeing • Hard Affinity: Process cannot migrate to another processor

  37. Multiple-Processor Scheduling –Load Balancing • Utilize the full benefits of multi-processors • Load balancing: Distribute the workload evenly across all processors. • Necessary where each processors has its own private queue of eligible processes • Not necessary where processors have common run queue • Modern OS --private queue or common? • Push Migration: move processes from overloaded to idle processors • Pull Migration: Idle processor pulls a waiting process from a busy processor • Any link with Process Affinity?

  38. Logical CPU Logical CPU Logical CPU Logical CPU Physical CPU Physical CPU System Bus Multiple-Processor Scheduling Symmetric Multithreading • Multiple logical rather than physical processors to run several threads concurrently (HPT) • Create logical processors on the same physical processor—a view of many Processors to OS • Each has its own machine-state registers • Own interrupt handling

  39. Multiple-Processor Scheduling Symmetric Multithreading • SMT: a feature provided by hardware • Os needn’t be designed differently • Performance gains –If OS is aware. But why? • **Scheduler**

  40. Outline Basic Concepts CPU-I/O Burst Cycle CPU Scheduler Preemptive Scheduling Dispatcher Scheduling Criteria Scheduling Algorithms First-Come, First-Serve Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling Multi-Processor Scheduling OS Examples –Self Study Algorithm Evaluation

  41. Algorithm Evaluation • How do we select the best algorithm? • Defining a criteria • CPU Utilization, response time, throughput • Criteria may include several measures • e.g. Maximize CPU utilization |and| Response time is 1 • e.g. Maximize Throughput |and| turnaround time • Analytic Evaluation Method • The algorithm and some system workload are used to produce a formula or number which gives the performance of the algorithm for that workload. • Deterministic modeling • Queuing models • Simulations • Implementation

  42. Deterministic Modeling • Predetermined workload –defines performance of each algorithm for that workload. Use of Gantt charts. • Simple and fast • Exact numbers for comparison • Answers apply only for the cases considered. • Performance figures may not be true in general • Suitable: If we are running the same program over and over again. We can easily select an algorithm. • Over a set of examples certain trends can be analyzed, e.g. If all the processes arrive at time 0, the SJF policy will always result in min waiting time • If the Arriving time is different the SJF may not always result in min average waiting time.

  43. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Deterministic Modeling • Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • Gantt chart • Average waiting time = (9 + 1 + 0 +2)/4 = 3

  44. Queuing Modeling • What if the processes that are run, vary from day to day? --Is deterministic method useful? • The distributions of CPU and I/O bursts can be determined/ approximated/ estimated. • Mathematical formula: probability of a particular CPU burst • Arrival times distribution can also be estimated. • Computer system viewed as a network of queues and servers: ready queue, I/O queue, event queues, CPUs, I/O device controllers, etc. e.g. CPU: ready queue, I/O system :device queue • Input: Arrival and service rates • Output: CPU utilization, average queue length, average waiting time, …

  45. Queuing Modeling Little’s Formula: n = λ* W where n = average queue length λ = average arrival rate W = average waiting time in a queue

  46. Queuing Modeling Let the average job arrival rate be 0.5

  47. Queuing Modeling • Complicated mathematics • Distributions (uniform, exponential, etc) for the arrival and departure rates can be difficult to work with • Assumptions may not be accurate • Approximation of the real system

  48. Simulation • To get more accurate evaluation • Workload generated by assuming some distribution and a random number generator, or by collecting data from the actual system. • The distributions can be defined mathematically or empirically.

  49. Simulation

  50. Simulation • Characteristics • Expensive: hours of programming and execution time • May be erroneous because of the assumptions about distributions