Capítulo 5: Planificación de Procesos...

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5. Planificación de Procesos 1

Pontificia Universidad Católica del Perú. Ingeniería Informática. INF231 - Sistemas Operativos

Copyright © 2001, Viktor Khlebnikov v 0.1

Capítulo 5:

Planificaciónde Procesos(Scheduling)




The scheduler The scheduling algorithm

Process behavior:

AST, MOS2E, Fig. 2-37

Scheduler, Scheduling Algorithm and Process Behavior

A CPU-bound process

An I/O-bound process

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1. when a new process is created: the parent process orthe child process

2. when a process exits

3. when a process blocks on I/O, on a semaphore, ...

4. when an I/O interrupt occurs

5. when a hardware clock occurs:nonpreemptive scheduling algorithm

preemptive scheduling algorithm

AST, MOS2E, p. 135

When to Schedule




Three environments worth distinguishing are

1. Batch.

2. Interactive.

3. Real time.

AST, MOS2E, p. 136

Categories of Scheduling Algorithms

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All systemsFairness - giving each process a fair share of the CPUPolicy enforcement - seeing that stated policy is carried outBalance - keeping all parts of the system busy

Batch systemsThroughput - maximize jobs per hourTurnaround time - minimize time between submission and

terminationCPU utilization - keep the CPU busy all the time

AST, MOS2E, p. 137

Scheduling Algorithm Goals (1/2)




Interactive systemsResponse time - respond to requests quicklyProportionality - meet user’s expectations

Real-time systemsMeeting deadlines - avoid losing dataPredictability - avoid quality degradation in multimedia

systems

AST, MOS2E, p. 137

Scheduling Algorithm Goals (2/2)

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Proceso Duración de la ráfagaP1 24P2 3P3 3

Scheduling in Batch Systems: First-Come First-Served (FCFS)

P1 P2 P3

0

P2P1

24 27 30

tpr.espera = (0+24+27)/3 = 51/3 = 17

Efecto de convoy: one CPU-bound process runs for 1 sec, then it reads a disk block.Many I/O-bound processes have to perform 1000 disk reads to complete.Result: each I/O-bound process gets to read 1 block per second and will take 1000 secto finish. But in case of 10 msec preemption, the I/O-bound process would finish in10 sec and without slowing down the CPU-bound process very much.

nonpreemptiveP3

Gráfica de Gantt




Proceso Duración de la ráfagaP1 8P2 4P3 4P4 4

Scheduling in Batch Systems: Shortest Job First (SJF) (1/2)

P1 P2 P30

P2P1

8 12 16

tpr.respuesta =(8+12+16+20)/4 =56/4 = 14

nonpreemptive

P4P3

P420

P1P2 P30 4 8 12

tpr.respuesta =(4+8+12+20)/4 =44/4 = 11

P420

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Consider the case of 4 jobs, with run times of a, b, c, and d, respectively. Thefirst job finishes at time a, the second finishes at time a + b, and so on. Themean turnaround time is (4a + 3b + 2c + d)/4. ...

Proceso Duración de la ráfaga Tiempo de llegadaP1 2 0P2 4 0P3 1 3P4 1 3P5 1 3

Scheduling in Batch Systems: Shortest Job First (SJF) (2/2)

P1 P2 P30

P2P1

2 7 8

tmean turnaround =(2+6+4+5+6)/5 = 23/5 = 4.6versus 4.4for P2, P3, P4, P5, P1.

nonpreemptive

P4P3

P49

P5

P5

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Proceso Duración de la ráfaga Tiempo de llegadaP1 2 0P2 4 0P3 1 3P4 1 3P5 1 3

Scheduling in Batch Systems: Shortest Remaining Time First

P1 P2 P30

P2P1

2 6

tmean turnaround =(2+9+1+2+3)/5 =17/5 = 3.2

preemptive

P4P3

P49

P5

P5

5

P23 4

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Scheduling in Batch Systems: Three-Level Scheduling





Scheduling and Process State Transitions

WS, OSIDP, 3E, Fig. 9-1

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Queuing Diagram for Scheduling





Round-Robin Scheduling (1/4)

One of the oldest, simplest, fairest, and most widely usedalgorithms is round robin (time slicing).

Quantum, quanta. Process (context) switch.


The list of runnable processes. The list of runnable processesafter B uses its quantum.

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Queuing Diagram for Virtual Round-Robin Scheduling (4/4)





Priority Scheduling (1/7)


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Priority Scheduling (2/7)





MINIX Structure (3/7)

AST & ASW, OSDI2E, Fig. 2-26

Layer

4

3

2

1 Process management

Disktask

Ttytask

Clocktask

Memorymanager

Ethernettask . . . I/O tasks

Filesystem

Systemtask

Networkserver

. . . Server processes

. . .Userprocess

Userprocess

UserprocessInit User processes

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MINIX Scheduling (4/7)

AST & ASW, OSDI2E, Fig. 2-36




UNIX Scheduling (5/7)


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Mapping of Win32 priorities to Windows 2000 priorities (6/7)





Windows 2000 support 32 priorities for threads (7/7)


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Aging

AST, MOS2E, p. 146

Se predice como un promedio exponencial de las longitudesmedidas de las ráfagas de CPU anteriores.

Sea tn la longitud de la n-ésima ráfaga de CPU, y ττττn+1 el valorprevisto para la siguiente ráfaga. Entonces, para αααα, 0 ≤≤≤≤ αααα ≤≤≤≤ 1,se define

ττττn+1 = αααα tn + (1 - αααα) ττττn

Esta fórmula define un promedio esponencial.

Maduración o envejecimiento (aging).

αααα = 0.5




Other Scheduling Algorithms

AST, MOS2E, pp. 146-148

Guaranteed Scheduling: if there are n users logged in whileyou are working, you will receive about 1/n of the CPU power.

Lottery Scheduling: the system might hold a lottery 50 times asecond, with each winner getting 20 msec of CPU time as a prize.

Fair-Share Scheduling:

user 1 has four processes, A, B, C, and D, and user 2 has only1 process, E. If two users have each been promised 50% of theCPU:

A E B E C E D E A E B E C E D E

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Feedback Scheduling





A Comparison of Scheduling Policies (1/3)

Process Arrival Time Service Time

1 0 32 2 63 4 44 6 55 8 2


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