itec 502 컴퓨터 시스템 및 실습 chapter 5: cpu scheduling mi-jung choi [email protected]...

ITEC 502 컴퓨터 시스템 및 실습

Chapter 5:CPU Scheduling

Mi-Jung [email protected]

DPNM Lab., Dept. of CSE, POSTECH

ITEC502 컴퓨터 시스템 및 실습

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Contents

1. Basic Concepts2. Scheduling Criteria 3. Scheduling Algorithms

4. Multiple-Processor Scheduling5. Real-Time Scheduling6. Thread Scheduling


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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 wait until 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

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

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 a system call to

terminate execution

CPU burst distribution of a process– varies greatly from process to process and from computer

to computer


8

Scheduling:Introduction to Scheduling

Bursts of CPU usage alternate with periods of I/O wait– a CPU-bound process– an I/O bound process


9

Alternating Sequence of CPU & I/O Bursts

CPU burst time

I/O burst time

CPU burst time

CPU burst time


10

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


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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 process2. switches from running to ready state: when interrupt

occurs3. switches from waiting to ready: at completion of I/O4. terminates

Scheduling under 1 and 4 is non-preemptive Scheduling under 2 and 3 is preemptive


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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 by terminating 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, Mac OS X, 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


13

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


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Context Switch


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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)


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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


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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, Turnaround time


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First-Come, First-Served (FCFS) Scheduling

Process Burst TimeP1 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

P1 P2 P3

24 27 300


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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

P1P3P2

63 300


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FCFS Scheduling

P0

P1

P2

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)


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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


22

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 known as the Shortest-Remaining-Time-First (SRTF)

SJF is optimal – gives minimum average waiting time for a given set of processes


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Process Arrival Time Burst TimeP1 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

Example of Non-Preemptive SJF

P1 P3 P2

73 160

P4

8 12


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Example of Preemptive SJF

Process Arrival Time Burst TimeP1 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

0

P1 P3P2

42 11

P4

5 7

P2 P1

16


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Preemptive SJF Scheduling

P0

P1

P2

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)


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Non-preemptive SJF Scheduling

P0

P1

P2

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)


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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 stores the past history The parameter controls the relative weight of recent an

d past history in our prediction

nnn

n

thn

t

nt

1 :Define 4.10 number, real a is 3.

burst CPUnext for the valuepredicted 2.burst CPU oflenght actual 1.

1

1


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Prediction of the Length of the Next CPU Burst

In this example, 0 = 10, = ½ 1 = x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8 2 = x t1 + (1- ) x 1 = ½ x 4 + ½ x 8 = 6


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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


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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


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Process Burst Time PriorityP1 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

Example of Non-Preemptive Priority

P2 P1 P3

1610

P4

18

P5

6 19


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Preemptive Priority Scheduling

P0 (1)

P1 (2)

P2 (3)

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)


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Non-preemptive Priority Scheduling

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CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40

I/O(24) CPU (4)

P0 (1)

P1 (2)

P2 (3)


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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


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Example of RR with Time Quantum = 20

Process Burst TimeP1 53

P2 17

P3 68

P4 24 The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162


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RR Scheduling (1)

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)

P0 (1)

P1 (2)

P2 (3)

Time Quantum = 2


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RR Scheduling (2)

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)

P0 (1)

P1 (2)

P2 (3)

Time Quantum = 5


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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


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Round Robin (RR) Scheduling

The time quantum q must 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 100 milliseconds,

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


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Turnaround Time varies with the Time Quantum

The turnaround time depends 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


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Scheduling Algorithm with multi-Queues

Multi-level Queue Scheduling Multi-level Feedback Queue Scheduling


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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 slice scheduling – 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


43

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


44

Multi-level Queue Scheduling

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)

P0 (1)

P1 (0)

P2 (0)

Two-level Queues: SJF with priority 0, FCFS with priority 1 Fixed Priority Queue Preemptive


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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


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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 Q0 which is served RR. When it gains CP

U, 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


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Multilevel Feedback Queues


48

Multi-level Feedback Queue Scheduling

CPU (10) I/O (20) CPU(11)

CPU(6) I/O(17) CPU(9)

CPU(4) I/O(4) CPU (4)

0 10 20 30 40 50

I/O(24) CPU (4)

P0 (1)

P1 (0)

P2 (0)

Three-level Queues: – RR with quantum 3, RR with quantum 6, FCFS with priority 1


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Scheduling Algorithms Goals


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Scheduling in Batch Systems (1)

An example of shortest job first scheduling


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Scheduling in Batch Systems (2)

Three level scheduling


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Scheduling in Interactive Systems (1)

Round Robin Scheduling– list of runnable processes– list of runnable processes after B uses up its quantum


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Scheduling in Interactive Systems (2)

A scheduling algorithm with four priority classes


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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


55

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


56

Scheduling in Real-Time Systems

Schedulable real-time system Given

– m periodic events

– event i occurs within period Pi and requires Ci seconds

Then the load can only be handled if

1

1m

i

i i

C

P


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Policy versus Mechanism

Separate what is allowed to be done with how it is done– a process knows which of its children

threads are important and need priority

Scheduling algorithm parameterized– mechanism in the kernel

Parameters filled in by user processes– policy set by user process


58

Thread Scheduling (1)

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


59


Possible scheduling of user-level threads 50-msec process quantum threads run 5 msec/CPU burst


60


Possible scheduling of kernel-level threads 50-msec process quantum threads run 5 msec/CPU burst


61

Summary CPU scheduling is the task of selecting a waiting process from the

ready queue and allocating the CPU to it The CPU is allocated to the selected process by the dispatcher FCFS scheduling is simple, cause short processes to wait for long time SJF scheduling is provably optimal, providing the shortest averaging

waiting time. But predicting the length of the next CPU bursts is difficult

Priority scheduling allocates the CPU to the heights priority process Both priority and SJF may suffer from starvation. Aging is a technique

to prevent starvation RR scheduling is more appropriate for a time-shared system Major problem of RR scheduling is the selection of the time quantum FCFS is non-preemptive, RR is preemptive, SJF and Priority may be

preemptive and non-preemptive Multilevel queue allows different scheduling algorithm for each queue Multilevel feedback queue allow process to move from one queue to

another


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Review

Basic Concepts Scheduling Criteria Scheduling Algorithms

Multiple-Processor Scheduling Real-Time Scheduling Thread Scheduling

itec 502 컴퓨터 시스템 및 실습 chapter 5: cpu scheduling mi-jung choi [email protected]...

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