CPU????(CPU Scheduling) ~

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CPU????(CPU Scheduling)

• ???? ????
• ?? job scheduling
• ?? CPU scheduling
• ?? swapping

?? ??(Basic Concepts)

• CPU-I/O ??? ??(burst cycle)
• cycle CPU ??(CPU burst) lt--gt I/O ??(I/O burst)
• CPU burst ??
• I/O bound program ?? ?? CPU burst ??
• CPU bound program ?? ?? ? CPU burst ??
• CPU ????
• ?? ????(short-term scheduler) ready queue?? ??
• FIFO(First-In First-Out)?
• ???? ?
• ??
• ?????

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Alternating Sequence of CPU And I/O Bursts
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Histogram of CPU-burst Times
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CPU????(CPU Scheduling)

• ?? ????(Preemptive Scheduling)
• ??(preemptive) ????
• ??????(timer)??
• ?? ???? ?? ???? ??? ??
• ???(non preemptive) ?? ???(cooperative) ????
• MS-Windows, ?? ????(timer) ??
• ?? ?? I/O?? ?? CPU??
• Kernel? ?? ??
• case 1(?? Unix) system call ?? ?? I/O ??? ? ??
  ????? ????
• ??? ????? ?? ????? ??
• case 2 interrupt ? ?? interrupt enable(?????
  ??) ?? ?? system call? preemptible
• critical section(?? ??? ???? ?? ??)? ?? ??
  interrupt disable ?? ?
• CPU scheduling decision time
• running -gt waiting non preemptive
• running -gt ready (interrupt) preemptive
• waiting -gt ready (I/O ??) preemptive
• halt non preemptive

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CPU????(CPU Scheduling)

• ???(Dispatcher)
• ?? ??
• ??? ??? ??
• ????? ??? ??? ???? ???? ???
• (dispatch latency? ??? ?)

???? ??(Scheduling Criteria)

• ???(CPU utilization) 40 90
• ???(throughput) ??? ???? ??/?? ??
• ????(turnaround time) system in -gt system out
  ?? ??
• ????(waiting time) ready queue?? ??? ??
• ????(response time) ??? ????? ? ????? ??

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???? ????(Scheduling Algorithm)

• ?? ?? ?? ??(average waiting time)
• ?? ???(First-Come, First-Served) ????
• ??? ????
• FIFO queue First-inlt- tail First-outlt- head
• p141-142 ?(Gantt chart)
• ?? ??(convoy effect) ? job??? ???? ??
  ???(CPU-bound process? ???? I/O bounded process??
  ???)
• non-preemptive?(time-sharing??? ??)
• ?? ?? ??(Shortest-Job-First) ????
• Shortest Next CPU Burst Scheduling
• ?? CPU burst ??? ?? ?? ??????
• ? ?? ???? ??
• non-preemptive SJF p143 ?
• preemptive SJF Shortest Remaining Rime First
  Scheduling p145 ?
• ??(Optimal)
• ?? ?? ??(starvation)
• long-term scheduling? ??(???? ??? ??? ?? ? ??)
• short-term scheduling ?? ?? ?? CPU burst ?? ???
  ????
• ?? CPU ??? ?? ?? ??

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First-Come, First-Served (FCFS) Scheduling

• Example Process Burst 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

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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

P1
P3
P2
6
3
30
0
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Example of Non-Preemptive SJF

• Process Arrival Time Burst 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

P1
P3
P2
P4
7
3
16
0
8
12
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Example of Preemptive SJF

• Process Arrival Time Burst 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

P1
P3
P2
P4
P2
P1
11
16
0
4
2
5
7
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Determining Length of Next CPU Burst

• CPU ??? ?? ??? ? ?? ??? ?? ??
• ?? CPU ??? ???? ??? ??(exponential averaging)??
  ??

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Examples of Exponential Averaging

• ? 0
• ?n1 ?n
• ??? ?? CPU ??? ??? ?? ??
• ? 1
• ?n1 tn
• ??? ?? CPU ??? ??? ??
• ?? ????
• ?n1 ? tn (1 - ?) ?n
• ? tn (1 - ?) (? tn-1 (1 - ?) ?n-1 )
• ? tn (1 - ?) ? tn-1 (1 - ?) 2 ?n-1
• ? tn (1 - ?) ? tn-1
• (1 - ? ) j ? tn-1
• (1 - ? ) n1 ?0
• ?? (1 - ?)? 1?? ????, ???? ? ??? ?? ???? ??
  ?(weight)? ? ???

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???? ????(Scheduling Algorithm)

• ????(Priority) ????
• ?? ?? ??? ??????
• ready queue priority queue -gt heap??? ??
• FCFS equal-priority
• SJF p 1/T (T ?? CPU ??? ?? ?? ?)
• priority??(OS??)
• ?? ??(time limits)
• ??? ???
• ?? ?? ?
• (average I/O burst)/(average CPU burst) ?? ?
• priority??(OS??)
• ???? ???
• ??? ??? ??? ??
• ?? ??
• ??? ?? ?
• preemptive ? ?? non-preemptive ? ??
• ??? ?? ??(blocking) ?? ?? ??(starvation)
• CPU? ??? ??? ?? ????? system crash ? ?????
• (?) IBM 7094 at MIT 1973, 1967 job

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???? ????(Scheduling Algorithm)

• ?? ??(Round-Robin) ????
• FCFS preemption (time slice ??)
• ready queue ?? FIFO queue
• preemptive ?
• time sharing??time quantum? ??? ??
• 1 time quantum gt context switching time
• 80 CPU burst lt 1 time quantum
• p148, p150 ?
• ??? ?(Multilevel Queue) ????
• ? ????? ?? ??? ?? ?? ?? ? ? ??? ??? ?? p151 ??
  5.6
• ? queue? ??? ??? scheduling algorithm ??
• foreground (interactive) queue RR????
• background (batch) queue FCFS????
• queue? ??? scheduling ?? ?? ?? ?? ????(fixed
  priority preemptive scheduling)
• ? ??? CPU time slice ?? ?
• 80 for RR
• 20 for FCFS
• ???? ?? ??? ???? ??

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Example RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is
• Typically, higher average turnaround than SJF,
  but better response.

0
20
37
57
77
97
117
121
134
154
162
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How a Smaller Time Quantum Increases Context
Switches
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Turnaround Time Varies With The Time Quantum
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Multilevel Queue Scheduling
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???? ???? (Scheduling Algorithm)

• ??? ?? ?(Multilevel Feedback Queue) ????
• ????? ?? ? ??? ?? ?? 6.7
• ?? ????(I/O bound, interactive processes)? ??
• ? ????? ?? ?? ?? ??
• aging(?? ???? ???? ?? ?? ?? ??)
• preemptive ?(? ??)
• the most sophisticated, the most complex
• ?? ??? -gt ?? ???? ?? ????(configure)
• ?? ??
• ? ?? ???? ????
• ?? ?? ??? ?? ?? ??
• ?? ?? ??? ?? ?? ??
• ?? ?? ??? ???
• HRN(Highest-Response-ratio Next) ????
• 1971 Brinch Hansen
• SJF? ?? ??
• dynamic priority (time waiting service time)
  / (service time)
• ?? ??? ????? time waiting? ????? priority ???
• short process??? priority ??

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Multilevel Feedback Queues
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Example of Multilevel Feedback Queue

• Three queues
• Q0 time quantum 8 milliseconds
• Q1 time quantum 16 milliseconds
• Q2 FCFS
• Scheduling
• FCFS queue Q0 ? ?? ??? ??? 8 milliseconds ?? CPU?
  ??? ??? ??? ??? ???? queue Q1?? ??
• Q1? ??? FCFS? 16 additional milliseconds? ?? ???
  ??? ??? ???? queue Q2? ??

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Unix ???? ????? ?

• CPUdecay(CPU)CPU/2
• ???? (??? CPU ???)/2 (?? ?? ??? ???? 60)
• Unix ???? ?? ????? 0 (???? ?? ?? ?? ???? ??)

??
????A
????B
????C
????
????
????
CPU??
CPU??
CPU??
0
0 1 2 60 30
60
60
60
0
0
1
60
60
75
0
0 1 2 60 30
2
60
75
67
15
0 1 2 60 30
3
75
67
63
7 8 9 67 33
15
67
63
15
4
76
7 8 9 67 33
63
76
68
16
7
5
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?? ???? ????(Multiple-Processor Scheduling)

• ?? ?? ????(homogeneous multiprocessor)
• ????? ?? ?? ?? ????(any processes)? ??
• ?? ??(load sharing)
• ??? ?? ?(separate ready queue)
• ?? ?? ?(common ready queue)
• ?? ?? ????(heterogeneous multiprocessor)
• ????? ???(dedicated) ????? ??
• distributed systems
• ?? ?? ? ?? ?? ???? ???(common ready queue on
  homogeneous multiprocessor)? ????
• ? ????? ??? ????(self-scheduling) symmetric
  (SMP)
• master-slave(asymmetric) asymmetric
• master? kernel system data structure? ?? ??
• ? ????? ?? ???? ????
• master server all system activity
• client user code only

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Real-Time Scheduling

• hard real-time system
• ??????? ???? ???? ?????? ???
• ?? H/W??? ???? ?? S/W? ???
• soft real-time system
• ?? ????? ??(general-purpose computer system???
  ??)
• multimedia, high-speed interactive graphics?(soft
  real-time computing? ??)
• soft real-time OS ??? ????
• 1. ???? ????? ?? ?(must have priority scheduling)
• ??? ????? ??? ?? ??? ???? ?
• 2. ????? ????? ???? ? (the dispatch latency must
  be small)
• ???? OS context switching ??? ???? system call?
  ????? I/O? ?? block??? ???? ?
• -gt ??
• ? system call? preemptible??
• ? system call?? preemption points(? ?? ?? ???
  ????? ?? check, ??? interrupt)
• ? kernel??? preemptible??
• Kernel data??? ?? synchronization ??.
• (?) Solaris 2
• ???? ??(priority inversion)
• ? kernel data ?? ?? ?? control? ?? ?? ??

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Real-Time Scheduling

• ???? ??(priority inversion)
• kernel data ???? ????? ?? ????? ????? ???? ?????
  ???? ???
• ???? ?? ????(priority inheritance protocol)
• kernel data ???? ?? ?? ??? ????? ????? ????? ??
  ????? ?? ??, ??? ??? ???
• ?? ??(complict phase)? ?? ?? 6.8
• 1. ???? ?? ?? ????? ??
• 2. ?? ??(?? ?? ?? ????? ?? ?? ?? ????? ???? ???
  ???)
• (?) Solaris 2
• dispatch latency nonpreemptible 100 ms
• dispatch latency preemptible 2 ms

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Dispatch Latency
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Rate Monotonic vs. EDF
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Thread Scheduling

• 2 thread levels
• User level process local scheduling
• threads library? ?? ??? LWP? thread? ???? ??
• Kernel level system global scheduling
• kernel? ?? ??? kernel thread? ???? ??
• Solaris 2 Scheduling
• priority-based process scheduling
• 4 classes (?? 6.9 ??)
• real time ??? ????
• system ???? ??? ? ??
• interactive multi-level feedback queue
  scheduling
• windowing application? ?? ????
• time sharing default class, multi-level feedback
  queue scheduling
• the higher the priority, the smaller the time
  slice
• interactive processes
• the lower the priority, the larger the time slice
• CPU-bound processes
• Scheduler? class-specific priorities? global
  priorities? ??
• ???? ?? ?? round-robin

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Solaris 2 Scheduling
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Java Thread Scheduling

• JVM? ????? ?
• Preemptive, Priority-Based Scheduling Algorithm
  ??
• ???? ??? FIFO Queue ?? (RR algorithm)
• Time slicing
• time-sliced a thread runs until
• time quantum
• exit the Runnable state
• preempted
• not time-sliced a thread runs until
• exit the Runnable state
• preempted
• yield() method? ??? ?? (time slicing ??)
• cooperative multitasking

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Time-Slicing

• Since the JVM Doesnt Ensure Time-Slicing(??? ??
  ??), the yield() Method May Be Used
• while (true)
• // perform CPU-intensive task
• . . .
• Thread.yield()
• This Yields Control to Another Thread of Equal
  Priority.

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Java Thread Scheduling

• Thread priority
• ??? ???? thread? Runnable thread? ???
• Thread ??? defalut priority(??? ??) -gt
  setPriority()? ??
• Thread.MIN_PRIORITY 1
• Thread.MAX_PRIORITY 10
• Thread.NORM_PRIORITY 5 (default priority)
• setPriority(Thread.NORM_PRIORITY 2)
• Java-Based Round-robin Scheduler thread with
  priority 6 (?? 6.11 ??)
• schedulers queue? addThread() priority 2
• ?? ?? ???? priority 4
• ????? 1 time quantum ?? ??? CPU? priority 4?
  thread?
• 1 time quantum ? ??? ??? ????? priority 4?
  thread? ??(preempt)
• CircularList class? queue? ???? ???? isEmpty()
  ???? ?? ??? ?? ????? ?? queue ???? ?? busy-wait
  ??
• JVM? ?? ??? thread? ????? ??
• ?? ?? ??? thread? Runnable State? ???? ?
• ?? ????? thread? Runnable State? ???? ?
• Note the JVM Does Not Specify Whether Threads
  are Time-Sliced or Not.

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Round-robin Scheduler
public void run() Thread current this.setPrio
rity(6) while (true) //get the next
thread current (Thread)queue.getNext() if
((current ! null) (current.isAlive()))
current.setPriority(4) schedulerSleep() cur
rent.setPriority(2) private CircularList
queue private int timeSlice private static
final int DEFAULT_TIME_SLICE 1000

• public class Scheduler extends Thread
• public Scheduler()
• timeSlice DEFAULT_TIME_SLICE
• queue new CircilarList()
• public Scheduler(int quantum)
• timeSlice quantum
• queue new CircilarList()
• public void addThread(Thread t)
• t.setPriority(2)
• queue.addItem(t)
• private void scheculerSleep()
• try
• thread.sleep(timeSlice)
• catch (InterruotedException e)

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TestThread

• class TestThread extends Thread
• private String name
• public TestThread(String id)
• name id
• this.setPriority(Thread.NORM_PRIORITY)
• public void run()
• /
• The thread does something
• /
• while (true)
• for (int i 0 i lt 500000 i)
• System.out.println("I am thread " name)

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TestScheduler

• public class TestScheduler
• public static void main(String args)
• Thread.currentThread().setPriority(Thread.MAX_P
  RIORITY)
• Scheduler CPUScheduler new Scheduler()
• CPUScheduler.start()
• TestThread t1 new TestThread("Thread 1")
• t1.start()
• CPUScheduler.addThread(t1)
• TestThread t2 new TestThread("Thread 2")
• t2.start()
• CPUScheduler.addThread(t2)
• TestThread t3 new TestThread("Thread 3")
• t3.start()
• CPUScheduler.addThread(t3)

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???? ??(Algorithm Evaluation)

• ??? ???(Deterministic Modeling)
• ????(workload)? ?? ?? ?? p158-159 ?? ??
• ????
• ???? ?
• CPU ??? ?? ? ?? ??? ?? ??
• ?? ??(Queueing Models)
• CPU-I/O ??? ? ??? ???? ????? ??? ??? ????
• ???? ???? ?? ???, ?? ? ??, ?????? ? ? ??
• Little? ?? (?? ? ??) (?? ?) x (???? ??)
• ?? ??? ?????? ???? ?
• ?? ??(Simulations)
• ????? ????? clock variable, system state
  variables? ???? ??
• ?? ??? ???(??, ??, ???), ?? ????? ??
• ?? ?? ?? ?? ?? ?? trace tapes ??
• ???? ??? ?? ?
• ??(Implementation)
• ?? ???? ?? ?? ??? ??? ??
• ?? ??? ?? ???? ????(flexible scheduling
  algorithm)
• tunable scheduling

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Evaluation of CPU Schedulers by Simulation