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Chapter 5: Process Scheduling

Chapter 5: Process Scheduling. Chapter 5: Process Scheduling. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling. Basic Concepts. Process Scheduling is the basis of multi-programmed operating system

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Chapter 5: Process Scheduling

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  1. Chapter 5: Process Scheduling

  2. Chapter 5: Process Scheduling • Basic Concepts • Scheduling Criteria • Scheduling Algorithms • Multiple-Processor Scheduling • Real-Time Scheduling • Thread Scheduling

  3. Basic Concepts • Process Scheduling is • the basis of multi-programmed operating system • Terminology • CPU scheduling, Process scheduling, Kernel Thread scheduling • used interchangeably, we use process scheduling

  4. Basic Concepts • In a single-processor system • Only one process can run at a time • Any others must waituntil the CPU is free and can be rescheduled. • When the running process goes to the waiting state, • the OS may select another process to assign CPU to improve CPU utilization. • Every time one process has to wait, another process can take over use of the CPU • Process scheduling is • a fundamental function of operating-system. • to select a process from the ready queue and assign the CPU • Maximum CPU utilization obtained with multiprogramming • by process scheduling

  5. Diagram of Process State from ch.3 • It is important to realize that only one process can be running on any processor at any instant. • Many processes may be ready and waiting states.

  6. Process Scheduling from ch.3 • Two types of queues: one ready queue, a set of device queues • Two types of resources: CPU, I/O • Arrow indicates the flow of processes in the system

  7. CPU - I/O burst Cycle • Process execution consists of • a cycle of CPU execution (CPU burst) and I/O wait (I/O burst) • Process alternate between these two states • Process execution begins with a CPU burst, which is followed by an I/O burst, and so on. • Eventually, the final CPU burst ends with an system call to terminate execution. • CPU burst distribution of a process • varies greatly from process to process and from computer to computer

  8. CPU burst time I/O burst time CPU burst time CPU burst time Alternating Sequence of CPU & I/O Bursts

  9. Histogram of CPU-burst Times • CPU burst distribution is generally characterized • as exponential or hyper-exponential • with large number of short CPU burst and small number of long CPU burst • I/O bound process has many short CPU bursts • CPU bound process might have a few long CPU bursts.

  10. Process Scheduler • is one of OS modules. • selects one of the processes in memory that are ready to execute, and allocates the CPU to the selected process. • CPU scheduling decisions may take place when a process: 1. switches from running to waiting state: I/O request, invocation of wait() for the termination of other process 2.switches from running to ready state: when interrupt occurs 3.switches from waiting to ready: at completion of I/O 4.terminates • Scheduling under 1 and 4 is non-preemptive • Scheduling under 2 and 3 is preemptive

  11. Non-preemptive vs. Preemptive • Non-preemptive scheduling • Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU • either byterminating or by switching to the waiting state. • used by Windows 3.x • Preemptive scheduling • Current running process can be switched with another at any time • because interrupt can occur at any time • Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX) • incurs a cost associated with access to shared data among processes • affects the design of the OS kernel • Certain OS (UNIX) waits either for a system call to complete or for an I/O block to take place before doing a context switch. • protects critical kernel code by disabling and enabling the interrupt at the entry and exit of the code.

  12. Dispatcher • Dispatcher module is a part of a Process Scheduler • 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

  13. Context Switch from ch. 3

  14. Scheduling Criteria • Based on the scheduling criteria, the performance of various scheduling algorithm could be evaluated. • Different scheduling algorithms have different properties. • CPU utilization– ratio (%) of the amount of time while the CPU was busy per time unit. • Throughput– # of processes that complete their execution per time unit. • Turnaround time– the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O • Waiting time– Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm • Response time– In an interactive system, amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

  15. Optimization Criteria • It is desirable to maximize: • The CPU utilization • The throughput • It is desirable to minimize: • The turnaround time • The waiting time • The response time • However in some circumstances, it is desirable to optimize the minimum or maximum values rather than the average. • Interactive systems, it is more important to minimize the variance in the response time than minimize the average response time.

  16. Process Scheduling Algorithms • First-Come, First-Served Scheduling (FCFS) • Shortest-Job-First Scheduling (SJF) • Priority Scheduling • Round-Robin Scheduling • Our measure of comparison is the average waiting time. • CPU utilization, Throughput, Turn arround time,

  17. 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

  18. P2 P3 P1 0 3 6 30 FCFS Scheduling 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

  19. FCFS Scheduling 0 10 20 30 40 50 P0 I/O (20) CPU(11) CPU (10) P1 CPU(9) CPU(6) I/O(17) P2 CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P1 CPU(9) P2 CPU(4) P1 CPU(6) P0 CPU(10) P2 CPU(4) P2 CPU(4) P0 I/O(6) P0 CPU(11) 10 14 20 24 30 41 50 54

  20. FCFS Scheduling • FCFS scheduling algorithm is non-preemptive • Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O. • is particularly troublesome for time-sharing systems. • Convoy effect occurs. • When one CPU-bound process with long CPU burst and many I/O-bound process which short CPU burst. • All I/O bound process waits for the CPU-bound process to get off the CPU while I/O is idle • All I/O- and CPU- bound processes executes I/O operation while CPU is idle. • results in low CPU and device utilization

  21. Shortest-Job-First (SJF) 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: • non-preemptive– once CPU given to the process it cannot be preempted until 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 knownas the Shortest-Remaining-Time-First (SRTF) • SJF is optimal– gives minimum average waiting time for a given set of processes

  22. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF ProcessArrival 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

  23. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF ProcessArrival 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

  24. Preemptive SJF Scheduling 0 10 20 30 40 50 P0 I/O (20) CPU(11) CPU (10) P1 CPU(9) CPU(6) I/O(17) P2 CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P1 CPU(6) P0 CPU(1) P2 CPU(4) P2 CPU(4) P2 CPU(4) P0 CPU(9) P1 I/O(4) P1 CPU(9) P2 I/O(2) P0 I/O(1) P0 CPU(11)

  25. Non-preemptive SJF Scheduling 0 10 20 30 40 50 P0 I/O (20) CPU(11) CPU (10) P1 CPU(9) CPU(6) I/O(17) P2 CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P1 CPU(6) P1 CPU(9) P2 CPU(4) P0 CPU(10) P2 CPU(4) P2 CPU(4) Idle (6) P0 CPU(11)

  26. 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 • The value of tn contains our most recent information. • n+1 stores the past history • The parameter  controls the relative weight of recent and past history in our prediction.

  27. Prediction of the Length of the Next CPU Burst • In this example, 0 = 10,  = ½ • 1 =  x t0 + (1- ) x 0 = ½ x 6 + ½x 10 = 8 • 2 =  x t2 + (1- ) x 2 = ½ x 4 + ½x 8 = 6

  28. Examples of Exponential Averaging •  = 0 • n+1 = n = n-1 = n-2 . … = 0 • 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-j+ … + (1 -  )n +1 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

  29. 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

  30. Example of Non-Preemptive Priority ProcessBurst TimePriority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 • Priority Scheduling (non-preemptive) • Average waiting time = (0 + 1 + 6 + 16 + 18)/5 = 8.2 P2 P5 P4 P1 P3 16 6 18 19 0 1

  31. Preemptive Priority Scheduling 0 10 20 30 40 50 P0(1) I/O (20) CPU(11) CPU (10) P1(2) CPU(9) CPU(6) I/O(17) P2(3) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P2 I/O(2) P2 CPU(4) P0 I/O(2) P0 CPU(11) P1 CPU(9) P1 CPU(6) P0 CPU(10) P2 CPU(4) P2 CPU(4) Idle: P2(I/O4) I/O is same to idle (원래 I/O에는 idle이 들어가야 합니다.)

  32. Non-preemptive Priority Scheduling 0 10 20 30 40 50 P0 (1) I/O (20) CPU(11) CPU (10) P1 (2) CPU(9) CPU(6) I/O(17) P2 (3) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) 이 경우는 특수 case로 preemptive나 non-preemptive나 답이 같습니다. (즉 앞장과 답이 같습니다.)

  33. Round Robin (RR) Scheduling • 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 (= q time units) in chunks of at most n x q time units at once. • No process waits more than (n-1) x q time units. • Performance depends on the size of the time quantum. • q large  RR is same as FIFO • q small  provides high concurrency: each of n processes has its own processor running at 1/n the speed of the real processor

  34. 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

  35. RR Scheduling 0 10 20 • Time Quantum = 2 • (9번째 P1과 10번째 P2는 바뀌어도 상관 없음) • (20번째 P0와 21번째 P2도 바뀌어도 상관 없음) 30 40 50 P0(1) I/O (20) CPU(11) CPU (10) P1(2) CPU(9) CPU(6) I/O(17) P2(3) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P0 CPU(2) P1 CPU(2) P1 CPU(2) P2 CPU(2) P0 CPU(2) P1 CPU(2) P2 CPU(2) P0 CPU(2) P1 CPU(2) P0 CPU(2) P2 CPU(2) P0 CPU(2) P2 CPU(2) P1 CPU(2) Idle P1 I/O (11) P1 CPU1) P0 CPU(2) P1 CPU(2) P1 CPU(2) P0 CPU(2) P0 CPU(2) P2 CPU(2) P0 CPU(2) P2 CPU(2) P0 CPU(1) P0 CPU(2)

  36. RR Scheduling 0 10 20 • Time Quantum = 5 30 40 50 P0(1) I/O (20) CPU(11) CPU (10) P1(2) CPU(9) CPU(6) I/O(17) P2(3) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4) P0 CPU(5) P1 CPU(5) P1 CPU(4) Idle P1 I/O(15) P0 CPU(1) P2 CPU(4) P1 CPU(5) P0 CPU(5) P2 CPU(4) P0 CPU(5) P0 CPU(5) P2 CPU(4) P1 CPU(1)

  37. Time Quantum and Context Switch Time • The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum. • quantum = 12 time units, finished in less than 1 time quantum • quantum = 6 time units, requires 2 quanta, 1 context switch • quantum = 1 time units, requires 10 quanta, 9 context switch

  38. Round Robin (RR) Scheduling • Thetime quantum qmust be large with respect to context switch, otherwise overhead is too high • If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching • Most modern OS have time quanta ranging from 10 to 100milliseconds, • The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum.

  39. Turnaround Time varies with the Time Quantum • Theturnaround timedepends on the size ofthe time quantum • The average turnaroundtime can be improvedif most processes finish their next CPU burst in a single time quantum. • When SJF and RR used • If quantum = 6 and 7, average turnaround time = 10.5 • If quantum = 1, average turnaround time = 11

  40. Scheduling Algorithm with multi-Queues • Multi-level Queue Scheduling • Multi-level FeedbackQueue Scheduling

  41. Multilevel Queue • Ready queue is partitioned into separate queues:foreground (interactive)background (batch) • The processes are permanently assigned to one queue, generally based on some property, or process type. • Each queue has its own scheduling algorithm • foreground–RR • background–FCFS • Scheduling must be done between the queues • Fixed priority scheduling - serve all from foreground then from background, Possibility of starvation. • Time slicescheduling– 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

  42. Multilevel Queue Scheduling • No process in the batch queue could run unless the queues with high priority were all empty. • If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.

  43. Multi-level Queue Scheduling 0 10 20 • Two-level Queues: SJF with priority 0, FCFS with priority 1 • Fixed Priority Queue • Preemptive 30 40 50 P0 (1) I/O (20) CPU(11) CPU (10) P1 (0) CPU(9) CPU(6) I/O(17) P2 (0) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4)

  44. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented in 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

  45. Example of Multilevel Feedback Queue • Three queues: • Q0–RR with time quantum 8 milliseconds • Q1–RR time quantum 16 milliseconds • Q2–FCFS • Scheduling • A new job enters queue Q0which is servedRR. 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 RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. • The job is serverd based on FCFS in queue Q2

  46. Multilevel Feedback Queues

  47. Multi-level FeedbackQueue Scheduling 0 10 20 • Three-level Queues: • RR with quantum 3, RR with quantum 6, FCFS with priority 1 30 40 50 P0 (1) I/O (20) CPU(11) CPU (10) P1 (0) CPU(9) CPU(6) I/O(17) P2 (0) CPU(4) CPU (4) CPU (4) I/O(24) I/O(4)

  48. Multiple-Processor Scheduling • CPU scheduling more complex when multiple CPUs are available • Homogeneous processors within a system or heterogeneous processors within a system • Asymmetric multiprocessing vs. Symmetric multiprocessing (SMP) • Symmetric Multiprocessing (SMP)– each processor makes its own scheduling decisions. • Asymmetric multiprocessing– only one processor accesses the system data structures, alleviating the need for data sharing. • Load sharing on SMP system • keeps the workload evenly distributed across all processors in an SMP system. • Push migration vs. Pull migration

  49. Real-Time Scheduling • Hard real-time systems – required to complete a critical task within a guaranteed amount of time • Soft real-time computing – requires that critical processes receive priority over less fortunate ones

  50. Thread Scheduling • On operating systems that support kernel-level thread, it is kernel-level threads, not processes, that are being scheduled by the operating system. • Local Scheduling– How the threads library decides which thread to put onto an available LWP • Global Scheduling– How the kernel decides which kernel thread to run next

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