Introduction - University of Cambridge · Introduction. Introduction

Introduction

Introduction

This course covers three main topics:

� modular design of applications and systems software,using the facilities of the Java programming language asexamples,

� the need for and implementation of concurrency controland communication in inter-process and intra-processcontexts and

� the concept of transactions and their implementation anduses.

Concurrent Systems and Applications replaces the FurtherJava and Concurrent Systems courses this year

Where possible concrete examples and source code will beused to illustrate topics in concurrent systems

More background information and general principles will begiven than in the old Further Java course

Feedback’s useful at any point – either through the lab web-site, or e-mail [email protected] (or turn up at FN06).

Concurrent Systems and Applications 2001 – 2 Tim Harris

Notation

Many examples are illustrated using UML-style classdiagrams in which nodes represent classes and edgesbetween them denote different kinds of relationship betweenthose classes

operation1()operation2()

aggregated from

instantiates

refers to

extends

A

C

B

D E

The notation is consistent with Gamma et al ’s text book;others may vary


Concurrency

‘Concurrent systems’ just means those consisting of multiplethings that might be happening at the same time, e.g.

� Between the system as a whole and its user, externaldevices, etc.

� Between applications running at the same time on acomputer – whether through context switching by the OSor by genuine concurrency on a multi-processor machine

� Explicitly between multiple threads within an application

� Implicitly within an application, e.g. when receivingcall-backs through a user-interface tool-kit

� Other ‘housekeeping’ activities within an application,e.g. garbage collection


Books

These course notes are not intended as a complete referencetext – either to the subject or for practical programming inJava. For the latter, local documentation is available on theweb.http://www-uxsup.csx.cam.ac.uk/java/jdk-1.2.2/docs is most relevant.

� Bacon, J. (1997). Concurrent Systems. Addison-Wesley(2nd ed.)

� Bracha, G., Gosling, J., Joy, B. & Steele, G. (2000). TheJava Language Specification. Addison-Wesley (2nd ed.).http://java.sun.com/docs/books/jls/

� Lea, D. (1999). Concurrent Programming in Java.Addison-Wesley (2nd ed.)

� Gamma, E., Helm, R., Johnson, R., Vlissides, J. (1994).Design Patterns. Addison-Wesley


Outline

� Objects and classes– Terminology� class, object, method

– Structuring programs� encapsulation, inheritance� abstract classes, interfaces� design patterns

� GUIs (as examples of the above)– AWT, Swing

� Reflection and serialization– Class, Method, Field classes– newInstance(), clone()– Serializable, Externalizable

� Multi-threaded programming– Thread / process distinction– Thread control in Java� Creation� Why terminating threads is hard in this context� interrupt(), wait(), suspend(), resume()

– Thread scheduling (uni- and multi-processortechniques)


Outline (2)

� Communication within processes– Scenario: a shared address space– General problems:� Safety, liveness

– Simple atomic operations– Mutual-exclusion� Condition variables� Deadlock detection and avoidance

– Building other protocols (multi-reader etc)– Alternatives

� Communication between processes– Scenario: separate address spaces (and machines?)– General problems:� Parallel execution� Independent failures (of processes or links)� Maintaining consistent state� No global time

– Naming– Access control– IDLs and marshalling to portable data formats– Java RMI case study– Message passing� Sockets API


Outline (3)

� Transactions– Scenario: controlled through a transaction manager– Correctness criteria: serialisability, linearizability– ACID properties– Optimistic / pessimistic schemes– 2 phase-locking– Timestamp ordering– Cascading aborts– Logging

� Further Java topics– Memory models + volatile fields– Class loaders– Finalizers


Objects and classes

Object-oriented programming

Programs in Java are made up of objects, packaging togetherdata and the operations that may be performed on the data

For example, we could define:

class TelephoneEntry {1

String name;2

String number;3

4

TelephoneEntry(String name, String number) {5

this.name = name;6

this.number = number;7

}8

9

String getName () {10

return name;11

}12

13

TelephoneEntry duplicate() {14

return new TelephoneEntry(name, number);15

}16

}17

Programming with objects – 10 Tim Harris

Object-oriented programming (2)

This example shows a number of concepts:

� Lines 1-17 comprise a complete class definition. A classdefines how a particular kind of object works. Eachobject is said to be an instance of a particular class, e.g.line 15 creates a new instance of the TelephoneEntryclass

� Lines 2-3 are field definitions. These are ordinary’instance fields’ and so a separate value is held for eachobject

� Lines 5-8 define a constructor. This provides initializationcode for setting the field values for a new object

� Lines 10-12, 14-16 define two methods. These are’instance methods’ and so must be invoked on a specificobject


Object references

� A program manipulates objects through object references

� A value of an object reference type either (i ) identifies aparticular instance or (ii ) is the special value null

� More than one reference can refer to the same object, forexample:

TelephoneEntry te1 = new TelephoneEntry ("Tim",

"34476");

TelephoneEntry te2 = te1;

creates two references to the same object:

name = "Tim"te1

te2number = "34476"

� If te1.name is updated then that new value can also beaccessed by te2.name


Composition

� Placing a field of reference type in a class definition is aform of composition

� A new kind of data structure is defined in terms ofexisting ones, e.g.

class TEList

{

TelephoneEntry te;

TEList next;

}

� Used when modelling things related by a ‘has a’relationship

– e.g. a Car class might be expected to have a field oftype Engine and a field of type Wheels[]

� By convention field names are spelled with an initiallower-case letter and have names that are nouns, e.g.steeringWheel or leftChild


Overloaded methods

� The same name can be used for more than one method inany class. They are said to be overloaded

� ...however, they must have distinct parameter types todisambiguate which one to call,

� It is insufficient to merely have distinct return types, e.g.how would the following invocations behave?

void doSomething (String number) {

this.number = number;

}

String doSomething (String c) {

System.getRuntime().exec(c);

return "OK";

}

String s = o.doSomething ("rm -rf /");

o.doSomething ("12345");

� The choice would have to depend on the context inwhich an expression occurs


Overloaded methods (2)

� Calls to overloaded methods must also be unambiguous,e.g.

void f(int x, long y)

{

...

}

void f(long x, int y)

{

...

}

� Which should f(10,10) call? There is no best match

� However, unlike before, the caller can easily resolve theambiguity by writing e.g. f((long)10,10) to convertthe first parameter to a value of type long


Constructors

� Using constructors makes it easier to ensure that all fieldsare appropriately initialized

� If the constructor signature changes (e.g. an extraparameter is added) then other classes using the oldsignature will fail to compile: the error is detected earlier

� As with methods, constructors can be overloaded

� Unlike methods, constructors do not have a declaredreturn type or use the return statement

� A default constructor without any parameters is generatedautomatically if the programmer does not define any


Inheritance

� Previous examples have defined new classes in terms ofexisting ones using composition – e.g. a class Buscontaining a field of type Engine

� Inheritance is another way of combining classes – ittypically models an is a relationship, e.g. between a Carclass and a more general Vehicle class

� Inheritance defines a new sub-class in terms of anexisting super-class. The sub-class is intended to be amore specialized version of the super-class. It is typicallyused to

– add new fields– add new methods– provide new implementations of existing methods

� A programmer defines inheritance using the extendskeyword, e.g.

class NameNumberPlace extends NameNumber

{

String place;

}


Types and inheritance

� Reference types in Java are associated with particularclasses:

class A {

A anotherA; // Reference type A

}

� Such reference variables can also refer to any object of asub-class of the one named, e.g. if B extends A thenthe field anotherA could refer to an instance of B

� A particular object may be accessed through fields ofdifferent reference types over the course of its lifetime; it’sclass is determined at its time of creation

� Casting operations convert references to an objectbetween different reference types, e.g.

A ref1 = new B();1

B ref2 = (B) ref1; // super -> sub2

ref1 = ref2; // no cast needed: sub -> super3

� The cast in line 2 is needed because the variable ref1may refer to any instance of A or B. ref2 may only referto instances of B. Casts are checked at run-time in Java.


Arrays and inheritance

� If B extends A then how are B[] and A[] related?

� As might be expected, B[] is a sub-type of A[] , creatinga subtle problem:

A[] array1;1

B[] array2;2

array1 = new A[2];3

array2 = new B[2];4

array1[0] = new B(); // A[] <- B, ok5

array2[0] = new B(); // B[] <- B, ok6

array1[1] = new A(); // A[] <- A, ok7

array2[1] = new A(); // B[] <- A, fails8

� Line 8 fails at run-time: array2 refers to an object that isan array of references to things of type B and so an objectof class A is incompatible


Fields and inheritance

� A field in the sub-class is said to hide a field in thesuper-class if it has the same name. The hidden field canbe accessed by writing super.name rather thanthis.name .

� For example:

class A {

int x;

int y;

int z;

}

class B extends A {

String x;

int y;

void f () {

x = "Field defined in B";

y = 42; // B

super.x = 17; // A

super.y = 20; // A

z = 23; // A

}

}


Methods and inheritance

A class inherits methods from its superclass

� It can overload them by making additional definitionswith different signatures

� It can override them by supplying new definitions withthe same signature

class A {

int f () { }

}

class B extends A {

int f () {

System.out.println ("Override");

}

int f (int x) {

System.out.println ("Overload");

}

}

� When an overridden method is called, the code toexecute is based on the class of the target object, not thetype of the object reference


Methods and inheritance (2)

� Consequently, the type of an object reference does noteffect the chosen method in these examples. A commonmistake:

class A {1

void f () {2

System.out.println ("Super-class");3

}4

}5

class B extends A {6

void f () {7

System.out.println ("Sub-class");8

((A)this).f(); // Try to call original9

}10

}11

� As with fields, the super keyword is used:

super.f();9


Packages

� Java groups classes into packages. Classes within apackage are typically written by co-operatingprogrammers and expected to be used together

� Each class has a fully qualified name consisting of itspackage name, a full stop, and then the class name. e.g.uk.ac.cam.cl.tlh20.NameNumber

� The package keyword is used to select which package aclass definition is placed in, e.g.

package uk.ac.cam.cl.tlh20.examples;

class TelephoneEntry { ... }

� Definitions in the current package and java.lang canalways be accessed. Otherwise, the import keywordcan be used:

import java.util.*; // All from that package

import java.awt.Graphics; // Just named class


Modifiers

� This section looks at a number of modifiers that may beused when defining classes, fields and methods. Onlyaccess modifiers may be applied to constructors

<class-modifiers> class NameNumber {

<field-modifiers> String name;

<field-modifiers> String number;

NameNumber () {

/* Only access modifiers are allowed */

}

<method-modifiers> String getName () {

return name;

}

<method-modifiers> String getNumber () {

return number;

}

}


The final modifier

� A final method cannot be over-ridden in a sub-class –typically used because it allows faster calls to themethod, but also used for security

� A final class cannot be sub-classed at all

� The value of a final field is fixed after initialization –either directly or in every constructor, e.g.

class FinalField {

final String A = "Initial value";

final String B;

FinalField () {

B = "Initial value";

}

}


The abstract modifier

� Used on class and method definitions. An abstractmethod is one for which the class does not supply animplementation (and hence cannot be instantiated), e.g.

public class A {1

abstract int methodName ();2

}3

4

public class B extends A {5

int methodName () {6

return 42;7

}8

}9

� Abstract classes are used where functionality is movedinto a super-class, e.g. an abstract super-classrepresenting ‘sets of objects’ supporting iteration,counting, etc., but relying on sub-classes to provide theactual representation

� Note that fields cannot be abstract : they cannot beoverridden in sub-classes


The static modifier

� The static modifier can be applied to any method orfield definition. (It can also be applied to nested classes,discussed later)

� It means that the field/method is associated with the classas a whole rather than with any particular object

� For example, suppose the example TelephoneEntryclass maintains a count of the number of times that it hasever been instantiated: there is only 1 value for the wholeclass, rather than a separate value for each object

� Similarly, static methods are not associated with acurrent object – unqualified field names and the thiskeyword cannot be used

� static methods can be called by explicitly naming theclass within which the method is defined. The namedclass is searched, then its super-class, etc. Otherwise thesearch begins from the class in which the method call ismade


The static modifier (2)

class Example {1

static int instantiationCount = 0;2

3

String name;4

5

Example (String name) {6

this.name = name;7

instantiationCount ++;8

}9

10

String getName () {11

return String;12

}13

14

static int getInstantiationCount () {15

return instantiationCount;16

}17

}18


Access modifiers

� Previous examples have relied on the programmer beingcareful when implementing encapsulation

– e.g. to interact with classes through their methodsrather than directly accessing their fields

� Access modifiers can be used to ensure thatencapsulation is honoured and also, in some standardlibraries, to ensure that untrusted downloaded codeexecutes safely

Sam

ecl

ass

Sam

epa

ckag

e

Sub-

clas

ses

Any

whe

re

public

protected some

default

private


The protected modifier

� A protected entity is always accessible in the packagewithin which it is defined

� Additionally, it is accessible within sub-classes (B) of thedefining class (A), but only when actually accessed oninstances of B or its sub-classes

public class A {1

protected int field1;2

}3

4

public class B extends A {5

public void method2 (B b_ref, A a_ref) {6

System.out.println (field1);7

System.out.println (b_ref.field1);8

System.out.println (a_ref.field1);9

}10

}11

� Lines 7-8 are OK: this and b ref must refer toinstances of B or its sub-classes

� Line 9 is incorrect: a ref may refer to any instance of Aor its sub-classes


Other modifiers

� A strictfp method is implemented at run-time usingIEEE 754/854 arithmetic (see Numerical Analysis 1) –identical results are guaranteed on all computers. Can beapplied to classes () all methods are implicitlystrictfp )

� A native method is implemented in native code – e.g.to interact with existing code or for (perceived)performance reasons. The mechanism for locating thenative implementation is system-dependent

� There are three other modifiers to be covered later:

– synchronized and volatile are used inmulti-threaded applications

– transient is used with the serialization API


Interfaces

� There are often groups of classes that provide differentimplementations of the same kind of functionality

– e.g. the collection classes in Java 1.2 – HashSet andArraySet provide set operations, ArrayList andLinkedList provide list-based operations

� In that example there are some operations available onall collections, further operations on all sets and a thirdset of operations on the HashSet class itself

� inheritance and abstract classes can be used to movecommon functionality into super-classes such asCollection and Set

– Each class can only have a single super-class, soshould HashSet extend a class representing thehashtable aspects of its behaviour, or a classrepresenting the set-like operations available on it?

� More generally, it is often desirable to separate thedefinition of a standard programming interface (e.g.set-like operations) from their implementation using anactual data structure (e.g. a hash table)


Interfaces (2)

� Each Java class may only extend a single super-class,but it can implement a number of interfaces

interface Set {

boolean isEmpty();

void insert(Object o);

boolean contains(Object o);

}

class HashSet implements Hashtable, Set {

...

}

� An interface definition just declares methodsignatures and static final fields

� An ordinary interface may have public or defaultaccess. All methods and fields are implicitly public

� An interface may extend one or more super-interfaces

� A class that implements an interface must supplydefinitions for each of the declared methods (or bedeclared an abstract class)


Nested classes

� A nested class/interface is one whose definition appearsinside another class or interface

� There are four cases:

– inner classes in which the enclosed class is anordinary class (i.e. non-static )

– static nested classes in which the enclosed definitionis declared static

– nested interfaces in which an interface is declaredwithin an enclosing class or interface

– anonymous inner classes

� Beware: the term inner class is sometimes usedincorrectly to refer to all nested classes

inner classes � nested classes

� In general nested classes are used (i ) for programmingconvenience to associate related classes for readability(ii ) as a shorthand for defining common kinds ofrelationship (iii ) to provide one class with access toprivate members or local variables from its enclosingclass


Nested classes (2)

� An inner class definition associates each instance of theenclosed class with an instance of the enclosing class,e.g.

class Bus {1

Engine e;2

3

class Wheel {4

...5

}6

}7

� Each instance of Wheel is associated with an enclosinginstance of Bus. For example methods defined at Line 5can access the field e without qualification

� An instance of Bus must explicitly keep track of theassociated Wheel instances, if it wishes to do so

� As with static fields and static methods, a staticnested class is not associated with any instance of anenclosing class. They are often used to organise ‘helper’classes that are only useful in combination with theenclosing class. Nested interfaces are implicitly declaredstatic


Anonymous inner classes

� Anonymous inner classes provide a short-hand way ofdefining inner classes

class A {1

void method1 () {2

Object ref = new Object () {3

void method2 () { };4

};5

}6

}7

� An anonymous inner class may be defined using aninterface name rather than a class name, e.g.

interface Ifc {1

void InterfaceMethod ();2

}3

4

class A {5

void method1 () {6

Ifc i = new Ifc () {7

void InterfaceMethod () {8

};9

};10

}11

}12


GUIs in Java

Graphical interfaces

� The Abstract Window Toolkit (AWT) and Java FoundationClasses (JFC) provide facilities that can be used forcreating graphical applications in Java

� We’ll look at them for a number of reasons:

� ...firstly to introduce the facilities provided and show howthey can be used

� ...secondly because their design illustrates many of theobject-oriented features of the Java programminglanguage

These examples are intended to show the overall structure ofthese libraries, not to be a thorough reference


Graphical interfaces (2)

The Java Foundation Classes (JFC) extends the original AWTwith (e.g):

� Swing components

� Pluggable look and feel

� Accessibility API

� Java 2D rendering API

� Drag and drop

API specs are available on-line(http://www.java.sun.com/products/jfc) but the emphasishere is how these facilities differ architecturally from AWT

AWT GUI components each had peers responsible for theirdisplay, e.g. an instance of java.awt.Scrollbar has aninstance of a class implementingjava.awt.peer.ScrollbarPeer as its peer.


Graphics

� In both AWT and Swing, basic rendering primitives areavailable on instances of Graphics , e.g. using Javaapplets:

import java.awt.*;

public class E1 extends java.applet.Applet

{

public void paint (Graphics g) {

g.drawLine (0, 0, 100, 100);

}

}

In E1.html :

<html><body>

<applet code="E1.class" width=100 height=100>

</applet>

</body></html>


Graphics (2)

� Here the rendering is performed by making invocationson an object of type Graphics

� Simple primitives are available, e.g.

void setColor (Color c);

void copyArea (int x, int y, int w,

int h, int dx, int dy);

void drawLine (int x1, int y1,

int x2, int y2);

void drawArc (int x, int y, int w, int h,

int start, int end);

� More abstractly, an instance of Graphics represents thecomponent on which to draw (more on those later), atranslation origin, the clipping mask, the current font, thedrawing mode etc


Component hierarchy

void setMenuBar()

String getText()void setText(String s)

Component add(Component g)

void show()void dispose()

void init()void start()

void start()

void stop()void destroy()

String getDirectory()String getFile()

void paint(Graphics g)void setSize(int w, int h)

Container getContentPane()

ScrollPane

Panel

TextFieldTextArea

JButton, ...

Frame

TextComponentContainerButton

CheckBox Canvas

ScrollbarListLabelChoice

Window

DialogApplet

FileDialog

Component

JComponent

JFrame


Components

� In general a graphical interface is built up from AWTcomponents and containers

� Components represent the building blocks of theinterface, for example buttons, check-boxes or text boxes

� Each kind of component is modelled by a separate Javaclass (e.g. java.awt.Button ). Instances of thoseclasses provide particular things in particular windows –e.g. to create a button bar the programmer wouldinstantiate the Button class multiple times

� As you might expect, new kinds of component can becreated by sub-classing existing ones – e.g. sub-classingCanvas (a blank rectangular area of the screen) to definehow that component should be rendered by overriding itspaint method


Components (2)

� Containers are a special kind of component that cancontain other AWT components – as expected, theabstract class java.awt.Container extendsjava.awt.Component

� Containers implement an add method to placecomponents within them

� Containers are used to model top-level windows – forexample java.awt.Window (a plain window, withouttitle bar or borders) and java.awt.Frame (a‘decorated’ window with a title bar etc)

� Other containers allow the programmer to control howcomponents are organized – in the simplest casejava.awt.Panel

� In fact, java.applet.Applet is actually a sub-class ofPanel


Swing

� The java.awt.peer.* definitions are interfacesdefining operations that may be performed on the peer

� A particular implementation of AWT provides classesimplementing these interfaces, typically using native code

� Hence the look and feel follows that of the underlyingsystem

What are the problems here? Should portability include theexact mode of interacting with applications? What aboutdevices without existing graphical toolkits, or non-graphicalforms of input?

Swing GUI components are rendered in Java – i.e. drawnusing invocations on java.awt.Graphics .

� Exact control over appearance

� Flexibility over look and feel


Swing (2)

Superficially there’s a clear correspondence between Swingcomponents in javax.swing and AWT components injava.awt

Component JComponentButton JButton

List JList?? JProgressBar?? JPasswordField

JComponent extends Container (and transitivelyComponent )

But there’s no direct relationship between (e.g.) JButtonand Button


Containers

� Components are organized within a container under thecontrol of a layout manager, e.g.

import java.awt.*;1

public class Buttons extends Frame {2

public Buttons() {3

super();4

setLayout(new BorderLayout());5

add("North", new Button("North"));6

add("South", new Button("South"));7

add("East", new Button("East"));8

add("West", new Button("West"));9

add("Center", new Button("Center"));10

}11

public static void main (String args[]) {12

Buttons b = new Buttons();13

b.pack(); b.setVisible(true);14

}15

}16

� For Swing, change Frame ! JFrame and obtain thecontainer using getContentPane() , e.g.

getContentPane().setLayout( ... );5


Containers (2)

� BorderLayout , shown above, contains up to 5components

� CardLayout treats each component in the container asa card. 1 card is visible at a time. Methods first andnext flip through them

� FlowLayout lays out components in horizontalleft-to-right lines – e.g. for button bars. A new line isstarted when the current one becomes full

� GridLayout places components on a rectangular gridof equal-sized cells, e.g. setLayout (newGridLayout (3,2)) creates a 3x2 grid

� GridBagLayout is a more flexible layout manager: therectangular cells may vary in size and instances ofGridBagConstraints are used to describe howparticular cells scale

Usually nesting containers to define a spatial hierarchy ispreferable to using a complex layout manager: it promotesre-use of the nested components


Receiving input

� An event-based mechanism is used for delivering input toapplications

� Different kinds of event are represented by sub-classes ofjava.awt.AWTEvent . These are all in thejava.awt.event package. E.g. MouseEvent is usedfor mouse clicks, KeyEvent for keyboard input, etc.

� The system delivers events by invoking methods on aListener . E.g. instances of MouseListener are usedto receive MouseEvent :

public interface MouseListener

extends EventListener

{

public void mouseClicked(MouseEvent e);

...

}


Receiving input (2)

� Note that each kind of listener is represented by aninterface definition rather than by a class. Theprogrammer can therefore define one class thatimplements all of the listeners that they are interested in(typically the main part of the program, or the classrepresenting a particular part of the UI)

� Instances of AWTEvent have a getSource() methodthat returns the component generating the event, so asingle listener can disambiguate events from differentsources. Sub-classes add methods to obtain other details– e.g. getX() and getY() on a MouseEvent

� Components provide methods for registering listenerswith them, e.g. addMouseListener on Component


Receiving input (3)

� All components can generate:

1. ComponentEvent when it is resized, moved, shown orhidden

2. FocusEvent when it gains or loses the focus

3. KeyEvent when a key is pressed or released

4. MouseEvent when mouse buttons are pressed orreleased

5. MouseMotionEvent when the mouse is dragged ormoved

� Containers can generate ContainerEvent whencomponents are added or removed

� Windows can generate WindowEvent when opened,closed, iconified etc


Input using inner classes

� Anonymous inner classes can be used as an effective wayof handling some forms of input, e.g.

addActionListener (new ActionListener () {

public void actionPerformed (ActionEvent e)

{

...

}

});

� Recall that this defines an anonymous inner class thatimplements the ActionListener interface

� This idiom is useful because the inner class is able toaccess the fields and methods of the enclosing instance


Input using inner classes (2)

� A further idiom is to define inner classes that extendadapter classes from the java.awt.event package.The adapters provide ‘no-op’ implementations of theassociated interfaces – e.g. MouseMotionAdapter forMouseMotionListener

� The programmer just needs to override the methods forthe kinds of event that they are interested in: there is noneed to define empty methods for the entire interface

addMouseMotionListener

(new MouseMotionAdapter () {

public void mouseDragged (MouseEvent e)

{

...

}

};

� The anonymous inner class extends theMouseMotionAdapter class


Button / JButton

� Instances of java.awt.Button(javax.swing.JButton ) represent labelled buttons:

Button b = new Button ("Quit");

add (b);

� Input is delivered using ActionEvent supportinggetActionCommand (defaults to the button’s label) andgetModifiers (e.g. if SHIFT /CTRL/ALT were pressed).An ActionListener has a single actionPerformedmethod


Checkbox / JCheckBox

� A check box is a graphical component that can be ineither an on or and off state – mouse clicks changebetween the states. For example:

setLayout(new GridLayout(3, 1));

add(new Checkbox("one", null, true));

add(new Checkbox("two"));

add(new Checkbox("three"));

� Check boxes can be grouped together (only one may beon in each group) with setCheckboxGroup

� An ItemListener receives input events through anitemStateChanged method


Label / JLabel

� Label objects represent single lines of read-only text – i.e.changeable by the application, but not able to be editeddirectly by the user. For example:

setLayout(new FlowLayout(FlowLayout.CENTER,

10, 10));

add(new Label("Hi There!"));

add(new Label("Another Label"));

� The text string maybe passed to the constructor, orcontrolled using setText and getText methods


List / JList

� Instances of these classes represent scrollable lists of textitems. The programmer can control how many items arevisible at once. For example:

List lst = new List(4, false);

lst.add("Tinky-winky"); lst.add("Dipsy");

lst.add("La-la"); lst.add("Po");

� Input can be received using an ItemListener (whenentries are selected or deselected) and/or anActionListener (when the user double-clicks on anentry)


Scrollbar / JScrollBar

� These classes embody scroll bars, for example used in acolour selector:

new Scrollbar(Scrollbar.VERTICAL, 0, 1, 0, 255);

� The parameters control the orientation, initial value,‘bubble size’, minimum and maximum values

� An AdjustmentListener receives input events via aadjustmentValueChanged method


TextComponent / JTextComponent

� These are super-classes for TextArea (JTextArea ) andTextField (JTextField )

� They define methods getText and setText , controlover whether the text is editable by the user and whethersome portion of the text is selected

� TextArea and TextField are multiple and single-linetext regions, e.g.

TextField tf = new TextField ("Hello!", 20);

TextArea ta = new TextArea ("Goodbye!", 5, 20);

add("North", tf); add("South", ta);


Menus

� Menus can be defined using the Menu, MenuBar ,MenuComponent , MenuContainer , MenuItem andMenuShortcut classes

� In AWT menus are not components, but can be bound toinstances of Frame using the setMenuBar method

� In Swing JMenu sub-classes JMenuItem ,AbstractButton and hence JComponent

� Input is received using an ActionListener on a menuitem (typically the same listener would be used for manyitems)


Choice

� An instance of java.awt.Choice represents a pop-upmenu of choices – the current choice is displayed as thetitle of the menu. For example:

Choice ColorChooser = new Choice();

ColorChooser.add("Green");

ColorChooser.add("Red");

ColorChooser.add("Blue");

� An ItemListener is used for input events


Swing internals

Swing components use a ‘model-view-controller’architecture (derived from Smalltalk-80)

ViewModel

Controller

Display

Input devices

This separates three aspects of the component:

� The view, responsible for rendering it to the display

� The controller, responsible for receiving input

� The model, the underlying logical representation

Multiple views may be based on the same model (e.g. atable of numbers and a graphical chart). This separationallows views can hopefully be changed independently ofapplication logic


Model-View-Controller (2)

For simplicity the model and view are combined in Swing toform a delegate

The component itself (here JButton ) contains references tothe current delegate and current model

MotifButtonUI

ComponentUI

ButtonUI

JButton

ButtonModel

DefaultButtonModel

ItemSelectable

MacButtonUI

... (+ other models)

Delegate Model


Accessibility

Intended to allow interaction with Java applications throughtechnologies such as screen readers and screen magnifiers

Pacakge: javax.accessibility

User interface components should implement Accessible ,defining a single method

public AccessibleContextgetAccessibleContext()

This is implemented by the Swing components


Accessibility (2)

An instance of AccessibleContext describes and is usedto interact with a particular UI component. It definesmethods to retrieve associated instances of

� AccessibleAction – representing operations that maybe done on the component, named by strings

� AccessibleComponent – represents the current visualappearance of the component. Allows colours, fonts,focus settings to be over-ridden

� AccessibleSelection – e.g. items in a menu, tableor tabbed pane

� AccessibleRole – in terms of generic roles such asSCROLLPANEor SLIDER

� AccessibleState – e.g. CHECKED, FOCUSED,VERTICAL

� AccessibleText – represents textual information

� AccessibleValue – represents numerical values (e.g.scroll bar positions)


Design patterns

Design patterns

A number of common idioms frequently emerge inobject-oriented programming. Studying these design patternsprovides

� common terminology for describing programorganization in terms of inter-related classes

� examples of how to structure programs for flexibility andre-use

operation1()operation2()

aggregated from

instantiates

refers to

extends

A

C

B

D E


Abstract

factorypattern

CreateWindow()CreateScrollbar()



ClientFactory

AWindow BWindow

ScrollBar

AScrollBar BScrollBar

Window

BFactory AFactory

Programm

ingw

ithobjects

–68

TimH

arris

Abstract factory pattern (2)

� The Client invokes operations on an instance of abstractclass Factory

� Code for these methods is provided by one of a numberof sub-classes, e.g. AFactory or BFactory

� The factory class instantiates objects on behalf of theclient from one of a family of related classes, e.g.AFactory instantiates AWindow and AScrollBar

✔ New families can be introduced by providing the clientwith an instance of a new sub-class of Factory

✔ The factory can ensure classes are instantiatedconsistently; e.g. AWindow always with AScrollBar

✘ Adding a new operation involves co-ordinated change tothe Factory class and all its sub-classes


Singleton pattern

if (theInstance == null) theInstance = new Singleton();

return theInstance;

static getInstance()operation1()operation2()

static theInstance

Singleton

� The Singleton pattern solves the Highlander problem:there can be only one instance of a particular class– e.g. of a factory class in the previous Abstract Factory

pattern

� Clients invoke getInstance() to retrieve the uniqueinstance. The first invocation triggers instantiation of a(private) constructor

✔ More flexibile than a suite of static methods (allowssub-classing)

✔ Constraint can be relaxed in a single place – e.g. if a poolof instances are to be used

✘ We’ll return to the multi-threaded case later


Adapter pattern

operation()

operation()

operation2()

[process operands]adaptee -> operation2();[process result]

Target

Adapter

Adaptee

Client

� The Client wishes to invoke operations on the Targetinterface which the Adaptee does not implement

� The Adapter class implements the Target interface, interms of operations the Adaptee supports

✔ The adapter can be used with any sub-class of theadaptee (unlike sub-classing adaptee directly)


Visitor pattern

The Visitor pattern is one way of structuring operations thatwork on data structures comprising objects of differentclasses

visitA(A a)visitB(B b)



v->visitB(this)

v->visitA(this)

accept(Visitor v)

accept(Visitor v) accept(Visitor v)

Visitor

Visitor1 Visitor2

Element

ElementA ElementB

Client


Visitor pattern (2)

� The data structure is built from instances of ElementA,ElementB, etc., all sub-classes of Element

� These classes, or a separate object structure class, providesome mechanism for traversing the data structure

� The abstract Visitor class defines operationscorresponding to each sub-class of Element

� A concrete sub-class of Visitor is constructed for eachkind of operation on the data structure

✔ The methods implementing a particular operation arekept together in a single sub-class of Visitor

✔ Operations can be added and updated without changingthe data structure’s definition

✘ As with the Abstract Factory pattern, changing the datastructure requires changes to many classes


Observer pattern

attach(Observer o)detach(Observer o)notify()

for all o in observers o->update();

update(Subject s)

Subject

ConcreteSubjectB

ConcreteObserver1ConcreteSubjectA

Observer

✔ In Java Observer can be an interface rather than aconcrete class

✔ A many-to-many dynamically changing relationship canexist between subjects and observers

✘ The flexibility limits the extent of compile-timetype-checking

✘ If observers can change the subject then cascading orcyclic updates may occur


Reflection andserialization

Reflection

� Java provides facilities for reflection or introspection oftype information about objects at run time

� Given the name of a class, a program can...– find the methods and fields defined on that class,– instantiate the class to create new objects

� Given an object reference, a program can...– determine the class of the object it refers to,– invoke methods or update the values in fields.

� It is not possible to obtain or change the source code ofmethods– ...but at the end of the course we’ll look at class

loaders that provide a mechanism for dynamicallyloading new class definitions

� These facilities are often used ‘behind the scenes’ in theJava libraries, e.g. RMI, and in visual programdevelopments environments – presenting a graphicalrepresentation of the facilities provided by each class, orshowing the way in which classes are combined throughcomposition or inheritance


Reflection (2)

� Reflection is provided by a number of classes in thejava.lang and java.lang.reflect packages. Eachclass models one aspect of the Java programminglanguage

� An instance of Class represents a Java class definition.The Class associated with an object is obtained by thegetClass() method defined on it

� An instance of Field represents a field definition,obtained from the Class object by getFields()

� Instances of Method and Constructor representmethod and constructor definitions, similarly obtained bygetMethods() and getConstructors()

� Similarly for getSuperclass() andgetInterfaces()


Reflection (3)

� For example:

public class ReflExample1

{2

public static void main (String args[])3

{4

ReflExample re = new ReflExample ();5

Class reclass = re.getClass ();6

String name = reclass.getName ();7

System.out.println (name);8

}9

}10

� Line 5 creates a new instance of ReflExample

� Line 6 obtains the Class object for that instance

� Line 7 obtains the name of that class


Reflection (4)

� We could do the same in reverse:


{2


{4

try5

{6

Class c = Class.forName (args[0]);7

Object o = c.newInstance ();8

System.out.println (o);9

}10

catch (Exception e)11

{12

System.out.println (e);13

}14

}15

}16


Reflection (5)

� Here we’re taking a class name supplied as a parameterto the program and then instantiating it. For example

$ java ReflExample2 java.lang.Object

java.lang.Object@80cb54d

� We could have named any class on the command line

� By default a 0-argument constructor is called (and mustexist)

� Specific constructors are also modelled byConstructor objects and define a newInstancemethod


Fields and reflection

� We can invoke getFields() on a Class object toobtain an array of the fields defined on that class

� As a shortcut we can also use getField(...) , passingthe name required, to obtain information about anindividual field

� If there is a security manager then itscheckMemberAccess method must permit generalaccess for Member.PUBLIC andcheckPackageAccess must permit reflection withinthe package

� Only public fields are returned by getFields()

� A general getDeclaredFields() method providesfull access (subject to a checkMemberAccess test formember.DECLARED)

� Given an instance of Field we can use...– Class getDeclaringClass ()– String getName ()– int getModifiers ()– Class getType ()


Fields and reflection (2)


{2

public static int field1 = 17;3

public static int field2 = 42;4

5


{7

try8

{9

Class c = Class.forName (args[0]);10

java.lang.reflect.Field f;11

f = c.getField (args[1]);12

int value = f.getInt (null);13

System.out.println (value);14

}15

catch (Exception e)16

{17

System.out.println (e);18

}19

}20

}21


Fields and reflection (3)

� For example,

$ java ReflExample3 ReflExample3 field1

17

$ java ReflExample3 ReflExample3 field2

42

$ java ReflExample3 ReflExample3 incorrect

java.lang.NoSuchFieldException

� There are similar methods for setting the value of the field


Methods and reflection

� The reflection API represents Java methods as instances ofa Method class

� This has an invoke operation defined on it that calls theunderlying method, for example, given a reference mtoan instance of Method :

Object parameters[] = new Object [2];

parameters[0] = ref1;

parameters[1] = ref2;

m.invoke (target, parameters);

is equivalent to making the call

target.mth (ref1, ref2);

where mth is the name of the method being called


Methods and reflection (2)

� The first value passed to invoke identifies the object onwhich to make the invocation. It must be a reference toan appropriate object (target in the example), or nullfor a static method

� Note how the parameters are passed as an array of typeObject[] : this means that each element of the arraycan refer to any kind of Java object

� If a primitive value (such as an int or a boolean ) is tobe passed then this must be wrapped as an instance ofInteger , Boolean , etc For example newInteger(42)

� The result is also returned as an object reference and mayneed unwrapping – e.g. invoking intValue() on aninstance of Integer


Serialization

Reflection lets you inspect the definition of classes andmanipulate objects without knowing their structure atcompile-time

One use for this is automatically saving/loading datastructures

� starting from a particular object you could usegetClass() to find what it is, getFields() to findthe fields defined on that class and then use the resultingField objects to get the field values

� the data structure can be reconstructed by usingnewInstance() to instantiate classes and invocationson Field objects to restore their values

The ObjectInputStream and ObjectOutputStreamclasses automate this procedure

Beware: the term’s used with two distinct meanings. Hereit means taking objects and making a ‘serial’ representationfor storage. We’ll use it in a different sense when talkingabout transactions.


Serialization (2)

In its simplest form the writeObject() method onObjectOutputStream and readObject() method onObjectInputStream transfer objects to/from anunderlying stream, e.g.

FileOutputStream s = new FileOutputStream ("file");

ObjectOutputStream o = new ObjectOutputStream (s);

o.writeObject (drawing);

o.close ();

or

FileInputStream s = new FileInputStream ("file");

ObjectInputStream o = new ObjectInputStream (s);

Vector v = (Vector) o.readObject ();

o.close ();

� A real example must consider exceptions as well

� Fields with the transient modifier applied to them arenot saved or restored


java.io.Serializable

These methods attempt to transfer the complete structurereachable from the initial object

However, classes must implement thejava.io.Serializable interface to indicate that theprogrammer believes this is a suitable way of loading orsaving instance state, e.g. considering

� whether field values make sense between invocations –e.g. time stamps or sequence numbrs

� whether the complete structure should be saved/restored– e.g. if it refers to a data structure used as a cache

� any impact on application-level access control – e.g. ifsecurity checks were performed at instantiation time

The definition of Serializable is trivial:

public interface Serializable {

}


java.io.Serializable (2)

A 0-argument constructor must be accessible to the subclassbeing serialized: it’s used to initialize fields ofnon-serializable superclasses

More control can be achieved by implementingSerializable and also two special methods to save andrestore that particular class’s aspect of the object’s state:

private void writeObject(

java.io.ObjectOutputStream out)

throws IOException;

private void readObject(

java.io.ObjectInputStream in)

throws IOException, ClassNotFoundException;

If present these are called to save/restore object state.

Further methods allow alternative objects to be introduced ateach step, e.g. to canonicalize data structures:

ANY-ACCESS-MODIFIER Object writeReplace()

throws ObjectStreamException;

ANY-ACCESS-MODIFIER Object readResolve()

throws ObjectStreamException;


java.io.Externalizable

writeObject and readObject are fiddly to use: theymay require careful co-ordination within the class hierarchy.The documentation is unclear about the order in whichthey’re called on difference classes.

The interface java.io.Externalizable is more usefulin practice

public interface Externalizable

extends java.io.Serializable

{

void writeExternal(ObjectOutput out)

throws IOException;

void readExternal(ObjectInput in)

throws IOException,

ClassNotFoundException;

}

It is invoked using the normal rules of method dispatch

It is responsible for transferring the complete state of theobject on which it is invoked

But note: readExternal is called after instantiating thenew object

Multi-threaded programming – 90 Tim Harris

Multi-threadedprogramming

Threads and processes

Recall the two rôles of an operating system:

� to securely multiplex resources, i.e.– protect applications from each other, yet– share physical resources between them

� to provide an abstract virtual machine, e.g.– time-shareing CPU to provide virtual processors,– allocating and protecting memory to provide

per-process virtual address spaces,– present h/w independent virtual devices.– divide up storage space by using filing systems.

In the introduction we talked about explicit concurrency inapplications – how does this relate to these tasks of the OS?


Threads and processes (2)

Most OS introduce a distinction between processes (asdiscussed in Part 1A) and threads

For each process have a process control block (PCB):

� Identification (e.g. PID, UID, GID)

� Memory management information

� Accounting information

� (Refs to) one or more TCBs. . .

For each thread have a thread control block (TCB):

� Thread state

� Context slot (perhaps in h/w)

� Refs to user (and kernel?) stack

� Scheduling parameters (e.g. priority)


Concurrency

� Previous examples have been implemented using a singlethread that runs the main method of a program

� Java supports lightweight concurrency within anapplication – multiple threads can be running at the sametime

� Can simplify code structuring and aid interactiveresponse – e.g. one thread deals with user interaction,another thread deals with computation

– Easier to add additional tasks as new threads?

� Can benefit from multi-processor hardware

– e.g. the hammer.thor machine has 4 processors

� Implementation schemes vary substantially. We’ll look athow multiple threads are available to the Javaprogrammer, then at how they may be implemented andmanaged by the OS


Creating threads

� There are two ways of creating a new thread. Thesimplest is to define a sub-class of java.lang.Threadand to override the run() method, e.g.

MyThread

Thread

class MyThread extends Thread {1

public void run() {2

while (true) {3

System.out.println ("Hello from " +4

this);5

Thread.yield ();6

}7

}8

9


Thread t1 = new MyThread ();11

Thread t2 = new MyThread ();12

t1.start (); t2.start ();13

}14

}15


Creating threads (2)

� The run method of the class MyThread defines the codethat the new thread(s) will execute. Just defining such aclass does not create any threads

� Lines 11–12 instantiate the class to create two objectsrepresenting the two threads that will be executed

� Line 13 actually start the two threads executing

� The program continues to execute until all ordinarythreads have finished, even after the main method hascompleted

Hello from Thread[Thread-5,5,main]



etc...

A daemon thread will not prevent the application fromexiting:

t1.setDaemon(true);



� The second way of creating a new thread is to define aclass that implements the java.lang.Runnableinterface, e.g.

void run()

MyCode

Runnable

Thread

class MyCode implements Runnable {1

public void run() {2

while (true) {3

System.out.println ("Hello from " +4

Thread.currentThread());5

Thread.yield ();6

}7

}8


MyCode mt = new MyCode ();10

Thread t_a = new Thread (mt);11

Thread t_b = new Thread (mt);12

t_a.start (); t_b.start ();13

}14

}15



� As before, Lines 2–8 define the code that the new threadswill execute

� Lines 11–12 instantiate two Thread objects, passing areference to an instance of MyCode to them as their target

� Line 13 starts these two threads executing

� Note that here the run methods of the two threads arebeing executed on the same MyCode object, whereastwo separate MyThread objects were required

� The second way of creating threads is more complex, butalso more flexible

� Generally, fields in the class containing the run methodwill hold per-thread state – e.g. which part of a problem aparticular thread is tackling



� In some cases anonymous inner classes can be used tosimplify thread creation, e.g.

class Example {1


Thread t = new Thread () {3

public void run () {4

System.out.println ("Hello world!");5

}6

};7

8

t.start ();9

}10

}11

� Recall that Lines 3–7 define and instantiate a newanonymous class that extends Thread

� As before, line 9 actually starts the thread executing


Terminating a thread

� A thread can be forced to exit by invoking the stopmethod on it. This method throws an exception into thethread – the thread behaves as if the exception hadsuddenly been thrown at the point at which it wasexecuting

– Usually, an instance of java.lang.ThreadDeathis thrown. The programmer may pass some otherobject as a parameter to stop

Thread t = new MyThread ();

t.start ();

t.stop ();

� Beware: when using stop the exception may bedelivered to the target thread when it is executing afinally block

� stop is deprecated in newer version of Java: it shouldnot be used in new programs, in favour of theinterrupt() method on java.lang.Thread

� A thread is responsible for periodically callingisInterrupted()


Terminating a thread (2)

� In some situations a thread is interrupted immediately if itis blocked – e.g. sleep may throwInterruptedException . For example:

class Example {1


{3



try {6

do {7

Thread.sleep (1000); // sleep 1s8

} while (true);9

} catch (InterruptedException ie) {10

// Interrupted: exit11

}12

}13

};14

t.start (); // Start...15

t.interrupt (); // ...interrupt16

}17

}18

� If the thread didn’t block then line 9 could perhaps be

} while (!isInterrupted());9


Join

� The join method on java.lang.Thread causes thecurrently running thread to wait until the target threaddies

class Example {1

public void startThread (void)2

throws InterruptedException3

{4



System.out.println ("Hello world!");7

}8

};9

10

t.start (); // Start thread...11

t.join (0); // ...wait for it to exit12

}13

}14

� Line 12 waits for the thread started at Line 11 to finish.The parameter specifies a time in milliseconds (0) waitforever)

� The throws clause in line 3 is required: the call to joinmay be interrupted


Priority controls

� Methods setPriority and getPriority onjava.lang.Thread allow the priority to be controlled

� A number of standard priority levels are defined:MIN PRIORITY, NORMPRIORITY, MAXPRIORITY

� The programmer can also try to influence threadscheduling using the yield method onjava.lang.Thread . This is a hint to the system that itshould try switching to a different thread – note how itwas used in the previous examples

– In a non-preemptive system even low priority threadsmay continue to run unless they periodically yield

� Selecting priorities becomes complex when there aremany threads or when multiple programmers are workingtogether

Although it may work on some systems, the variation inbehaviour between different JVMs means that it is nevercorrect to use thread priorities to control access to shareddata in portable code


Thread scheduling

� The choice of exactly which thread(s) execute at anygiven time can depend both on the operating system andon the JVM

� Some systems are preemptive – i.e. they switch betweenthe threads that are eligible to run. Typically these aresystems in which the OS supports threads directly, i.e.maintaining separate PCBs and TCBs

� Other systems are non-preemptive – i.e. they only switchwhen the running thread yields, becomes blocked orexits. Typically these systems implement threads withinthe JVM

� The Java language specification says that, in general,threads with high priorities will run in preference to thosewith lower priorities

To write correct portable code it’s therefore important tothink about what the JVM is guaranteed to do – not justwhat it does on one system. Different behaviour may occurat different nodes within a distributed system


Thread scheduling (2)

For now assume a five-state model with threads supported bythe OS:

Exit

Running

New

Ready

Blocked

dispatch

timeoutor yield

releaseadmit

event-waitevent

� The operating system must:– decide if a new thread should be admitted– wake up blocked threads when appropriate– clean up after threads terminate– choose amongst runnable threads) schedule

� Typical scheduling objectives:– Maximise CPU utilisation– Maximise throughput– Minimise average response time

� Also want to minimise overhead (space + time).


Scheduler data structures

admitCPU

release

timeout or yield

dispatchReady Queue

event-wait (1)event (1)

Blocked Queue 1

event-wait (2)event (2)

Blocked Queue 2

event-wait (n)event (n)

Blocked Queue N

Inside scheduler maintain TCBs according to state:

� Runnable) “current thread ”

� Ready) on ready queue

� Blocked) on a blocked queue

Sometimes there will be multiple current threads (e.g. amulti-processor system) or multiple ready queues (e.g. fordifferent thread priorities)


When do we schedule?

Can choose a new thread to run when:

1. a running thread blocks (running ! blocked )

2. a timer expires (running ! ready )

3. a waiting thread unblocks (blocked ! ready )

4. a thread terminates (running ! exit )

� A non-preemptive system schedules on 1, 4 only:

✔ simple to implement

✘ open to denial of service

✘ poor priority concept

✘ doesn’t extend cleanly to MP

� Most modern systems use preemptive scheduling:✔ solves above problems

✘ introduces concurrency problems. . .


Static priority scheduling

� All threads are not equal) associate a priority witheach, e.g.

Highest Interrupt handlersDevice handlersPager and swapperOther OS daemonsInteractive jobs

Lowest Batch jobs

� Scheduling decision simple: just select runnable threadwith highest priority. (Be careful when seeing any systemthat refers to explicit priority numbers: lower numbersoften denote higher priority)

� Problem: how to resolve ties?– round robin with time-slicing– allocate quantum to each thread in turn.– Problem: biased towards CPU intensive jobs.� per-thread quantum based on usage?� ignore?

� Problem: starvation. . .


Dynamic priority scheduling

� Use same scheduling algorithm, but allow priorities tochange over time

� e.g. simple aging:– threads have a (static) base priority and a dynamic

effective priority– if thread starved for k seconds, increment effective

priority– once thread runs, reset effective priority

� e.g. computed priority:– First used in Dijkstra’s THE– time slots: . . . , t, t+ 1, . . .– in each time slot t, measure the CPU usage of threadj: uj

– priority for thread j in slot t+ 1:pjt+1 = f(uj

t ; pjt ; u

jt�1; p

jt�1; : : :)

– e.g. pjt+1 = pjt=2 + kujt

– penalises CPU bound! supports I/O bound.


Example: 4.3BSD Unix

� Priorities 0 (high) – 127 (low), user processes� 50,round robin within priorities, quantum 100ms.

� Priorities are based on usage and “nice” value:

Pj(i) = Basej +CPUj(i � 1)

nticks+ 2� nicej

gives the priority of process j at the beginning of intervali, where nicej 2 [�20; 20]. i.e. penalizes (recently)CPU bound processes in favour of I/O bound ones.

� CPUj(i) is incremented every tick in which process j isexecuting, and decayed each second using:

CPUj(i) =2� loadj

(2� loadj) + 1CPUj(i� 1) + nicej

� loadj(i) is the sampled average length of the run queuein which process j resides, over the last minute ofoperation

� so if e.g. load is 1)� 90% of 1 seconds CPU usage“forgotten” within 5 seconds


Example: Windows 2000

� Hybrid static/dynamic priority scheduling:– Priorities 16–31: “real-time” (static priority)– Priorities 1–15: variable (dynamic priority)– Priority 0: system (zero page thread)

� Default quantum 2 ticks (�20ms) on Professional, 12ticks (�120ms) on Server.

� Threads have base and current (� base) priorities.– On return from I/O, current priority is boosted by

driver-specific amount– Subsequently, current priority decays by 1 after each

completed quantum– Also get boost for GUI threads awaiting input– Yes, this is true:

HKLMnSYSTEMnCurrentControlSet n

Control nPriority-Control

� On Professional also get quantum stretching:– “. . . performance boost for the foreground application”

(window with focus)– All threads in the foreground application get double or

triple quantum– (More ad-hoc scheduler hacks to come)


Real-time systems

� Produce correct results and meet predefined deadlines.

� “Correctness” of output related to time delay it requires tobe produced, e.g.– nuclear reactor safety system– JIT manufacturing– video on demand

� Typically distinguish hard (HRT) and soft real-time (SRT):HRT: output utility = 100% before the deadline, 0 (or

less) after the deadline.SRT output utility = 100% before the deadline, (100 -kt)% if t seconds late.

� Building such systems is all about predictability.

� It is not about speed.


Real-time scheduling

� Basic model:– consider set of tasks Ti, each of which requires si

units of CPU time before a (real-time) deadline of di– often extended to cope with periodic tasks: require si

units every pi units

� Best-effort techniques give no predictability– in general priority specifies what to schedule but not

when or how much.– i.e. CPU allocation for thread ti, priority pi depends

on all other threads at tj s.t. pj � pi.– with dynamic priority adjustment becomes even more

difficult.

) need something different

� Three main approaches:1. static off-line scheduling2. static priority algorithms3. dynamic priority algorithms


Static off-line scheduling

Advantages:

� Low run-time overhead

� Deterministic behavior

� System-wide optimization

� Resolve dependencies early

� Can prove system properties

Disadvantages:

� Inflexibility

� Low utilisation

� Potentially large schedule

� Computationally intensive

In general, off-line scheduling only used when determinismis the overriding factor, e.g. MARS


Static priority algorithms

Most common is Rate Monotonic (RM)

� Assign static priorities to tasks at off-line (or at‘connection setup’), high-frequency tasks receiving highpriorities

� the tasks processed with no further rearrangement ofpriorities required () reduces scheduling overhead)

� optimal, static, priority-driven alg. for preemptive,periodic jobs: i.e. no other static algorithm can schedulea task set that RM cannot schedule

� Admission control: the schedule calculated by RM isalways feasible if the total utilisation of the processor isless than ln2

� for many task sets RM produces a feasible schedule forhigher utilisation (up to � 88%); if periods harmonic,can get 100%

� Predictable operation during transient overload


Dynamic priority algorithms

Most popular is Earliest Deadline First (EDF):

� Scheduling pretty simple:– keep queue of tasks ordered by deadline– dispatch the one at the head of the queue

� EDF is an optimal, dynamic algorithm:– It may reschedule periodic tasks in each period– If a task set can be scheduled by any priority

assignment, it can be scheduled by EDF

� Admission control: EDF produces a feasible schedulewhenever processor utilisation is� 100%

� Problem: scheduling overhead can be large

� Problem: if system overloaded, all bets are off


Multimedia scheduling

� Increasing interest in multimedia applications (e.g. videoconferencing, mp3 player, 3D games)

� Challenges OS since require presentation (or processing)of data in a timely manner

� OS needs to provide sufficient control so that appsbehave well under contention

� Main technique: exploit soft real-time scheduling

� Effective since:– The value of multimedia data depends on the

timeliness with which it is presented or processed) Real-time scheduling allows applications to receive

sufficient and timely resource allocation to handletheir needs even when the system is under heavy load

– Multimedia data streams are often somewhat tolerantof information loss

) informing applications and providing soft guaranteeson resources are sufficient.

� Still ongoing research area


Example: Atropos (CUCL)

Run

Wait

Block

Deschedule

Allocate

Unblock

Wakeup

EDF OPT IDLE

Interrupt orSystem Call

� Task requirements described by (p; s; x) triples

� System performs admission control

� Use a variant of EDF

� Expose CPU via activations

� Actual scheduling is easy (�200 lines C)


Multi-processors

Two main kinds of [shared-memory] multi-processor:

1. Uniform Memory Access (UMA), aka SMP

CPU CPU CPU CPU

Main Memory

Cache Cache Cache Cache

� all (main) memory takes the same time to access� scales only to 4, 8 processors

2. Non-Uniform Memory Access (NUMA)

Mem

CPU CPU CPU CPU

Cache

Mem

Cache

Mem

Cache

Mem

Cache

� rarer and more expensive� can have 16, 64, 256 CPUs . . .


Multi-processor operating systems

Multi-processor OSes may be roughly classed as eithersymmetric or asymmetric.

� Symmetric Operating Systems:– identical system image on each processor) convenient abstraction

– all resources directly shared) high synchronisation cost

– typical scheme on SMP (e.g. Linux, W2K).

� Asymmetric Operating Systems:– partition functionality among processors– better scalability (and fault tolerance?)– partitioning can be static or dynamic– common on NUMA (e.g. Hive, Hurricane)– NB: asymmetric 6) trivial “master-slave”

� Also get hybrid schemes, e.g. Disco:– (re-)introduce virtual machine monitor– can fake out SMP (but is this wise?)– can run multiple OSes simultaneously. . .


Multi-processor scheduling

� Objectives:– Ensure all CPUs are kept busy– Allow application-level parallelism

� Problems:– Preemption within critical sections:� threadA preempted while holding spinlock) other threads can waste many CPU cycles� similar situation with producer/consumer threads

(i.e. wasted schedule)

– Cache pollution:� if thread from different application runs on a given

CPU, lots of compulsory misses� generally, scheduling a thread on a new processor is

expensive� (can get degradation of factor or 10 or more)

– Frequent context switching:� if number of threads greatly exceeds the number of

processors, get poor performance


Multi-processor scheduling (2)

Consider basic ways in which one could adapt uniprocessorscheduling techniques:

� Central queue:✔ simple extension of uniprocessor case

✔ load-balancing performed automatically

✘ n-way mutual exclusion on queue

✘ inefficient use of caches

✘ no support for application-level parallelism

� Dedicated assignment:✔ contention reduced to thread creation/exit

✔ better cache locality

✘ lose strict priority semantics

✘ can lead to load imbalance

Are there better ways?


Multi-processor scheduling (3)

� Processor affinity:– modification of central queue– threads have affinity for a certain processor) can reduce cache problems

– but: load balance problem again– make dynamic? (cache affinity?)

� ‘Take’ scheduling:– pseudo-dedicated assignment: idle CPU “takes” task

from most loaded– can be implemented cheaply– nice trade-off: load high) no migration

� Co-scheduling / gang scheduling:– Simultaneously schedule “related” threads.) can reduce wasted context switches

– Q: how to choose members of gang?– Q: what about cache performance?


Example: Mach

� Basic model: dynamic priority with central queue.

� Processors grouped into disjoint processor sets:– Each processor set has 32 shared ready queues (one

for each priority level)– Each processor has own local ready queue: absolute

priority over global threads

� Increase quantum when number of threads is small– ‘small’ means #threads < (2� #CPUs)– idea is to have a sensible effective quantum– e.g. 10 processors, 11 threads� if use default 100ms quantum, each thread spends

an expected 10ms on runqueue� instead stretch quantum to 1s) effective quantum

is now 100ms.

� Applications provide hints to improve scheduling:1. discouragement hints: mild, strong and absolute2. handoff hints (aka “yield to”) — can improve

producer-consumer synchronization

� Simple gang scheduling used for allocation.


MP thread architectures

KernelThreads

User Threads

Process 2Process 1 Process 3

CPU2

Process 4

LWPs

CPU1

Hybrid schemes combine user and kernel threads, e.g.three-level scheduling in Solaris 2:

� 1 kernel thread$ 1 LWP$ n user threads

� user-level thread scheduler) lightweight & flexible

� LWPs allow potential multi-processor benefit:– more LWPs) more scope for true parallelism– LWPs can be bound to individual processors) could

in theory have user-level MP scheduler– kernel scheduler is relatively cache agnostic (although

have processor sets ( 6= Mach’s))

Overall: either first-class threads (Psyche) or scheduleractivations probably better for MP

Communication in concurrent systems – 125 Tim Harris

Communication inconcurrent systems

Concurrency

The next section of the course concerns different ways ofstructuring systems in which concurrency is present and, inparticular, co-ordinating multiple threads, processes andmachines accessing shared resources and data

Two main scenarios:

� Tasks operating with a shared address space – e.g.multiple threads created within a Java application

� Tasks communicating between address spaces – e.g.different processes, whether on the same or separatemachine

In each case we must consider

� How shared resources and data are named and referredto by the participants

� Conventions for representing shared data

� How access to resources and data is controlled

� What kinds of system failure are possible


Safety

In each of these environments we must ensure that thesystem remains safe – i.e. that ‘nothing bad happens’

� Unlike type-soundness, this cannot usually be checkedautomatically by compilers or tools (although some existto help)

� It’s often useful to think of safety in terms of invariants –things that must remain true, no matter how differentparts of the system evolve during execution– e.g. a ‘transfer’ operation between bank accounts

preserves the total amount in them

� We can then identify consistent object states in which allfields obey their invariants

� ...and aim that the system’s behaviour does not dependon objects in inconsistent states

� Therefore many of the problems we’ll see come down todeciding when different threads can be allowed access toobjects in various ways


Liveness

As well as safety, we’d also like liveness – i.e. ‘somethinggood eventually happens’. We often distinguish per-threadand system-wide liveness

Standard problems include:

� Deadlock – a circular dependency between processesholding resources and processes requiring them.Typically the resources will be access tomutual-exclusion locks

� Livelock – a thread keeps executing instructions, butmakes no useful progress, e.g. busy-waiting on acondition that will never become true

� Missed wake-up (wake-up waiting) – a thread misses anotification that it should continue with some operation

� Starvation – a thread is waiting for some resource butnever receives it – e.g. a thread with a very lowscheduling priority

� Distribution failures – of nodes or network connections ina distributed system

Communication within processes – 129 Tim Harris

Communication withinprocesses

Shared data

� Most useful multi-threaded applications will share databetween threads

� Sometimes this is straightforward e.g. data passed to athread through fields in the object containing the runmethod

� More generally, threads may share state through...– static fields in mutually-accessible classes, e.g.

System.out– objects to which multiple threads have references

� What happens to field o.x :

Thread A Thread B

o.x = 17; o.x = 42;

� Most fields accesses are atomic – the value read fromo.x after those updates will be either 17 or 42

� The only exceptions are numeric fields of type doubleor type long – some third value may be read in that case


Locks in Java

� Simple shared data structures can be managed usingmutual exclusion locks (‘mutexes’) and thesynchronized keyword to control access to criticalsections within an application

� The synchronized keyword can be used in two ways –either applied to a method or applied to a block of code

� For example, suppose we want to maintain an invariantbetween multiple fields:

class BankAccounts {

private int balanceA;

private int balanceB;

synchronized void transferToB (int v) {

balanceA = balanceA - v;

balanceB = balanceB + v;

}

}


Locks in Java (2)

� When a synchronized method is called, the thread musttake out a mutual exclusion lock on the object

� If the lock is already held by another thread then thecaller is blocked until the lock becomes available

� Locks operate on a per-object basis – that is, only onesynchronized method can be called on a particularobject at any time– ...similarly, it is OK for multiple threads to be calling

the same method, so long as they do so on differentobjects

� Locks are re-entrant, meaning that a thread may call onesynchronized method from another

� If a static synchronized method is called then thethread must acquire a lock on the class rather than on anindividual object

� The synchronized modifier cannot be used directly onclasses or on fields


Locks in Java (3)

� The second form of the synchronized keyword allowsit to be used within methods, e.g.

void methodA (Object x) {1

synchronized (x) {2

System.out.println ("1");3

}4

5

...6

7

synchronized (x) {8

System.out.println ("2");9

}10

}11

� The synchronized region at line 2 acquires a lock onthe object referred to by x , performs the printlnoperation at line 3 and then releases the lock at line 4

� The lock must be re-acquired at line 8

This kind of usage is good if an intervening operation, notrequiring mutual exclusion, may take a long time to execute:other threads may acquire the lock


Priority inversion

� All priority-based schemes can potentially suffer frompriority inversion:

� e.g. consider low, medium and high priority threadscalled Pl, Pm and Ph respectively.1. First Pl starts, and acquires a lock L.2. Then other two processes start.3. Ph runs since highest priority, tries to lock L and

blocks.4. Then Pm gets to run, thus preventing Pl from releasingL, and hence Ph from running.

� Usual solution is priority inheritence:– associate with every lock L the priority P of the

highest priority process waiting for it.– then temporarily boost priority of holder of the lock up

to P .– can use handoff scheduling to implement.

� Windows 2000 “solution”: priority boosts– checks if 9 ready thread not run � 300 ticks.– if so, doubles quantum & boosts priority to 15

� What happens in Java?


Deadlock

Suppose that a and b refer to two different shared objects,

Thread P Thread Q

synchronized (a)

synchronized (b)

{

...

}

synchronized (b)

synchronized (a)

{

...

}

� If P locks both a and b then it can complete its operationand release both locks, thereby allowing Q to acquirethem

� Similarly, Q may acquire both locks, then release themand then allow P to continue

✘ If P locks a and Q locks b then neither thread cancontinue: they are deadlocked waiting for the resourcesthat the other has


Deadlock (2)

Whether this deadlock actually occurs depends on thedynamic behaviour of the applications. We can show thisgraphically in terms of the threads’ progress:

lock(a) unlock(a)unlock(b)lock(b)

Progress of P

lock(b)

unlock(b)

lock(a)

unlock(a)

of QProgress (1)

(2)

(3)

The shaded areas indicate (left and right) that one thread isblocked by the other waiting to lock a and (above andbelow) to lock b

Paths (1) and (2) show how these threads may bescheduled without reaching deadlock

Deadlock is inevitable on path (3)


Deadlock (3)

If all of the following conditions are true then deadlockexists:

1. A resource request can be refused – e.g. a thread cannotacquire a mutual-exclusion lock because it is alreadyheld by another thread

2. Resources are held while waiting – e.g. when a threadblocks waiting for a lock it does not have to release anyothers it holds

3. No preemption of resources – e.g. once a thread acquiresa lock then it’s up to that thread to choose when torelease it

4. Circular wait – a cylce of threads exist such that eachholds a lock requested by the next process in the cycle,and that request has been refused

In the case of mutual exclusion locks in Java, 1–3 are alwaystrue, and so the existence of a circular wait leads to deadlock


Object allocation graphs

An object allocation graph shows the various tasks in asystem and the resources that they have acquired and arerequesting. We’ll use a simplified form in which resourcesare considered to be individual objects

P has acquired object a and is requesting object b:

a P b

P holds a and Q holds b:

a P

bQ

Should r2 be allocated to S or T?

S

Tr1 r2


Deadlock detection

Deadlock can be detected by looking for cycles (as in thesecond example on the previous slide)

Let A be the object allocation matrix, with one thread perrow and one column per object. A(i;j) indicates whetherthread i holds a lock on object j

Let R be the object request matrix. R(i;j) indicates whetherthread i is waiting to lock object j

We proceed by marking rows of A indicating threads that arenot part of a deadlocked set. Initially no rows are marked. Aworking vector W indicates which objects are available

1. Select an unmarked row i such that Ri � W – i.e. athread whose requests can be met. Terminate if none

2. Set W = W +Ai, mark row i, and repeat

This identifies when deadlock has occurred – we may beinterested in other properties such as whether deadlock is

� inevitable (must happen in some possible execution path)

� possible (may happen in some path)


Deadlock detection (2)

A =

0BB@

0 0 1 0 0

0 1 0 0 0

1 0 0 0 0

0 0 0 0 0

1CCA

R =

0BB@

0 1 0 0 1

0 0 1 0 1

0 0 0 0 1

1 0 0 0 1

1CCA

1. W = (0; 0; 0; 1; 1)

2. Thread 3’s requests can be met! it’s not deadlocked, socan continue and may release object 1

3. W = (1; 0; 0; 1; 1)

4. Thread 4’s requests can now be met! it’s notdeadlocked

5. W = (1; 0; 0; 1; 1)

✘ Nothing more can be done: threads 1 and 2 are bothdeadlocked


Deadlock avoidance

A conservative approach:

� Require that each process identifies the maxmium set ofresources that it may ever lock, C(i;j)

� When thread i requests a resource then construct ahypothetical allocation matrix A0 in which it has beenmade and a hypothetical request matrix B0 in whichevery other process makes its maximum request

� If A0 and B0 do not indicate deadlock then the allocationis safe

✔ It does avoid deadlock – may be preferable to deadlockrecovery

✘ Need to know maximum requests

✘ Run-time overhead

✘ What if there are no safe states?

✘ Objects are instantiated dynamically...


Deadlock avoidance (2)

It’s often more practical to prevent deadlock by carefuldesign. How else can we tackle the four requirements fordeadlock?

� Use locking schemes that allow greater concurrency –e.g. multiple-readers, single-writer in preference tomutual exclusion

� Do not hold resources while waiting – e.g. acquire allnecessary locks at the same time (not possible as aprimitive operation in Java if > 1 lock)

� Allow preemption of locks and roll-back (again, not aprimitive in Java if using built-in locks)

� Enforce a lock acquisition order, making it impossible forcircular waits to arise, e.g. lock 2 ‘bank account’ objectsin account number order


Limitations of mutexes

� Suppose we want a one-cell buffer with a put operation(store something if cell empty) and a remove operation(read something if anything there):

class Cell {1

private int value;2

private boolean full;3

4

public synchronized removeValue () {5

if (full) {6

full = false;7

return value;8

} else {9

/* ??? */10

}11

12

...13

14

}15

� What can we put in line 10? We could just write codethat keeps testing full (a ‘spin lock’), but at best thatwould be inefficient (also need to only hold the lock 6–9)


Limitations of mutexes (2)

� Another problem: what if we want to enforce some otherkind of concurrency control?

� e.g. if we identify read-only operations which can beexecuted safely by multiple threads at once

� e.g. if we want to control which thread gets next accessto the shared data structure

– perhaps to give preference to threads performingupdate operations

– or to enforce a first-come first-served regime

– or to choose on the basis of the threads’ schedulingpriority?

� All that mutexes are doing here is preventing more thanone thread from running the code on a particular objectat the same time


Suspending threads

� The suspend and resume methods onjava.lang.Thread allow one thread to temporarilystop and start the execution of another

Thread t = new MyThread ();

t.suspend ();

t.resume ();

� As with stop , the suspend and resume methods aredeprecated

� This is because the use of suspend can lead todeadlocks if the target thread is holding locks. It also risksrace conditions if multiple thread use them against thesame target

� They should be avoided even if the program does notexplicitly take out locks: many systems use additionallocks in their implementation


Condition variables

� Java provides condition variables for this kind of situation

� There is an individual condition variable associated witheach mutex

� Condition variables support two kinds of operation:

– a wait operation causes the current thread to block– notify operations that cause blocked thread(s) to

continue

� In each case there are important details about when theoperation can be used and, in the case of notify, whichblocked thread(s) are woken. We’ll first look at anexample and then at the details


Condition variables (2)

class Cell {1

private int value;2

private boolean full = false;3

4

public synchronized int removeValue () {5

while (!full) wait ();6

7

full = false;8

notifyAll ();9

return value;10

}11

12

public synchronized void putValue (int v) {13

while (full) wait ();14

15

full = true;16

value = v;17

notifyAll ();18

}19

}20



� Line 2 causes a thread executing removeValue to blockon the condition variable until the cell is full

� Line 4 updates the object to mark it empty

� Line 5 notifies any threads currently blocked on thecondition variable

� Similarly, line 10 causes a thread executing PutValue toblock on the condition variable until the cell is empty

� Lines 12–13 update the fields to mark the cell full andstore the value in it

� Line 14 notifies any threads currently blocked on thecondition variable



� wait , notify and notifyAll are operations definedon java.lang.Object

� A thread can only perform a wait or notify operation on acondition variable when it holds the associated mutex

� When a thread waits it atomically releases the mutex andbecomes blocked on the condition variable

� When a thread is notified it must re-acquire the mutexbefore it continues

lock

unlockwait

notifyWaiting for

the lock

the lockHolding

Waiting tobe notified



� There are two forms of notify operation:– notifyAll() notifies every thread blocked on the

condition variable– notify() notifies exactly one thread if any are

blocked

� Both require care to ensure correct use. WithnotifyAll() the programmer must ensure that everythread blocked on the condition variable can continuesafely– e.g. line 2 in the example surrounds the wait()

operation with a while loop – if a ‘getting’ thread isnotified when there is no work for it, then it just waitsagain

� notify() selects arbitrarily between the waitingthreads: the programmer must therefore be sure that theydo not mind which thread is chosen

notify() does not guarantee to wake the longest waitingthread



� The example shows a common way of using conditionvariables

� notifyAll() is always used and the applicationcontains extra code (the while loop) so that threadswait() again if they are woken prematurely

� Although only one thread is expected to continue,notify() would be incorrect here because the choicedoes matter: suppose there are some threads blocked inremoveValue() and some threads blocked inputValue()

� When it is safe to use, some programmers prefernotify() because it may be more efficient

� If using other programming languages then take care thattheir versions of these operations behave in the same way


N-slot buffer

class NSlotBuffer {

int spacesFree = SIZE; int spacesUsed = 0;

Object empty = new Object ();

Object full = new Object ();

void insert (int x) {

synchronized (full) {

while (spacesFree == 0) full.wait ();

spacesFree --;

...

}

synchronized (empty) {

spacesUsed ++; empty.notify ();

}

}

int remove () {

synchronized (empty) {

while (spacesUsed == 0) empty.wait ();

spacesUsed --;

...

}

synchronized (full) {

spacesFree ++; full.notify ();

}

}

}


N-slot buffer (2)

This example illustrates a couple of points,

� Firstly, it generalizes the previous one into allow up toSIZE insert() operations to be performed withoutintervening invocations of remove()

� Secondly, it shows how multiple objects can be used toindicate different conditions

Each Java object has an associated mutex and conditionvariable, so instances of java.lang.Object are oftenused for this purpose

Remember that a thread must acquire a lock on the objectbefore invoking wait() , notify() or notifyAll()

Do insert(...) and remove(...) still need to besynchronized ?


Design

Suppose that we wish to have a shared data structure onwhich multiple threads may make read-only access, or asingle thread may make updates

� How can this be implemented using the facilities of Java,– In terms of a well-designed OO structure?– In terms of the concurrency-control features?

One option is based on delegation and the Adapterpattern,

operation() operation()

operation()

MTImpl BasicImpl

Client Interface

� BasicImpl provides the actual data structureimplementation, conforming to Interface . The classMTImpl wraps each operation with appropriate code forits use in a multi-threaded application, delegating calls toan instance of BasicImpl .


Design (2)

� How does that compare with:

operation()

operation()

operation()

Client Interface

BasicImpl

MTImpl

✔ Sub-classes enforce encapsulation and mean that onlyone instance is needed

✔ Delegation may be easier, just usesuper.operation()

✘ Separate sub-classes are needed for e.g. MTImpl2

✘ Composition of wrappers is fixed at compile time


Design (3)

In each of these cases the class MTImpl will define methodsthat can be split into three sections:

1. An entry protocol responsible for concurrency control –usually waiting until it is safe for the operation tocontinue

2. Delegation to the underlying data structureimplementation (either by an ordinary method invocationon an instance of BasicImpl or a call using the superkeyword)

3. An exit protocol – generally selecting the next thread(s) toperform operations on the structure

This common structure often motivates further separation ofconcurrency control protocols from the data structure


Design (4)

operation() operation()

operation()

enter()exit()

MTImpl BasicImpl

Client Interface

CCInterface

CCImpl

MTImpl now just deals with delegation, wrapping eachinvocation on Interface with appropriate calls toenter() and exit() on a general concurrency-controlinterface (CCInterface ).

Sub-classes, e.g. CCImpl , provide specific entry/exitprotocols. A factory class may be used to instantiate andassemble these objects

✔ Concurrency-control protocols can be shared

✔ Only a single MTImpl class is needed per data structureinterface


Multiple readers, single writer

We’ll now look at implementing an example protocol,MRSW

class MRSWImpl1 implements MRSW {

int numReaders = 0;

int numWriters = 0;

A reader can enter when numWriters is zero. A writer canenter when both fields are zero:

synchronized void enterRead ()

throws InterruptedException

{

while (numWriters > 0)

wait ();

numReaders ++;

}

synchronized void enterWrite ()


{

while ((numWriters > 0) ||

(numReaders > 0))

wait ();

numWriters ++;

}


Multiple readers, single writer (2)

The exit protocols are more straightforward:

synchronized void exitRead () {

numReaders --;

notifyAll ();

}

synchronized void exitWrite () {

numWriters --;

notifyAll ();

}

}

✔ Simple design: (1) create a class containing the necessaryfields (2) write entry protocols that keep checking thesefields and waiting (3) write exit protocols that cause anywaiting threads to assess whether they can continue.

✘ notifyAll() may cause too many threads to be woken– the code is safe but may be inefficient

Is that efficiency likely to be a problem?

Could notify() be used instead?

� 1999 Paper 4 Q3


Giving writers priority

We may want to ensure writes are made as soon as possible– how can that be implemented?

class MRSWImpl2 implements MRSW {

int numReaders = 0;

int numWriters = 0;

int waitingWriters = 0;

synchronized void enterRead ()


{

while ((numWriters > 0) || (waitingWriters > 0))

wait ();

numReaders ++;

}

synchronized void enterWrite ()


{

waitingWriters ++;

while ((numWriters > 0) || (numReaders > 0))

wait ();

waitingWriters --;

numWriters ++;

}


First-come first-served ordering

Suppose now we want an ordinary lock that provides FCFSsemantics – the longest waiting thread is given access next

class FCFSImpl implements CCInterface {

int currentTurn = 0;

int nextTicket = 0;

Threads take a ticket and wait until it becomes their turn:

synchronized void enter ()


{

int myTicket = nextTicket ++;

while (currentTurn < myTicket)

wait ();

}

synchronized void exit ()

{

currentTurn ++;

notifyAll ();

}

}

� 2001 Paper 4 Q3


First-come first-served ordering (2)

✔ The implementation is simple

✘ notifyAll() will wake all threads waiting in enter()on this object – in this case we know that only one cancontinue

✘ What happens if the program runs for a long time andnextTicket overflows?

Resolving these issues must depend on the context the classis being used in, e.g.

� Lots of waiting threads and frequent contention: have anexplicit queue of per-thread objects and use notify()on the object at the head of the queue

� No undetected failures: would long s ever overflow here?


Primitives for concurrency

These examples have used the language-level mutexes andcondition variables exposed in Java.

Semaphores provide simpler operations on which thelanguage-level features could be based. In Java-stylepseudo-code:

class CountingSemaphore {

CountingSemaphore (int x) {

...

}

native void P();

native void V();

}

� P (sometimes called wait) decrements the value and thenblocks if it is less than zero

� V (sometimes called signal ) increments the value andthen, if it is zero or less, selects a blocked thread andunblocks it


Programming with semaphores

Typically the integer value is used to represent the number ofinstances of some resource that are available, e.g.:

class Mutex {

CountingSemaphore sem;

Mutex () {

sem = new CountingSemaphore (1);

}

acquire () {

sem.P();

}

release () {

sem.V();

}

}

� The mutex is considered unlocked when the value is 1 (itis initialized un-lcoked)

� ...and locked when the value is 0 or less

� How does this mutex differ from a Java-style one?


Programming with semaphores (2)

class CondVar {

int numWaiters = 0;

Mutex cv_lock = new Mutex();

CountingSemaphore cv_sleep =

new CountingSemaphore (0);

CVWait (Mutex m) {

cv_lock.acquire ();

numWaiters ++;

m.release ();

cv_lock.release ();

cv_sleep.P ();

m.acquire ();

}

CVNotify (Mutex m) {

cv_lock.acquire ();

if (numWaiters > 0) {

cv_sleep.V();

numWaiters --;

}

cv_lock.release ();

}

}


Programming with semaphores (3)

Why doesn’t Java just provide semaphores?

� They can be implemented using mutexes and conditionvariables

� Using semaphores directly is intricate – the programmermust ensure P() /V() are paired correctly

� Many OS provide mutexes and condition variablesdirectly

There are other general problems both with semaphores andwith the facilities in Java:

� wait() and notify() still need care from theprogrammer

� The usual interfaces do not provide isLocked() ,tryToLock() , tryToLockUntil(...) orlockAny(...) operations

— how could these be implemented?


Implementing semaphores

Uni-processor only: disable thread switches during theimplementation of P() and V()

All systems: rely on atomic primitive operations provided bythe processor to implement simple spin-locks

V() sem.lock()sem.val += 1if sem.val<=0 unblock a threadsem.unlock()

P() sem.lock()sem.val -= 1

sem.unlock()if sem.val<0 block thread

Each semaphore has an assocaited value, boolean lock fieldand blocked-thread queue. The block operation

1. adds the current thread to the blocked-thread queue

2. updates the thread control block (TCB)

3. unlocks the semaphore


Implementing semaphores (2)

How are lock and unlock implemented?

Almost all processors have atomic operations such as tas(test-and-set), cas (compare-and-swap) or ll/sc(load-linked / store-conditional, not covered here)

tas lock.val, 0, 1if failed

lock()

mov 0 -> lock.valunlock()

This spin lock is not a good solution in general:

✘ uni-processor non-preemptive case...

✘ threads waiting to acquire the lock are continuallyattempting tas operations – they do not block

Spin locks tend to be used when blocking is unlikely or foronly short durations, so the time spent spinning is much lessthan the time taken to block and unblock a thread


Mutexes without hardware support

� What can we do if there isn’t a cas or tas instruction,just atomic read and write? (e.g. the ARM7 only has aswap operation)

� ‘Bakery’ algorithm due to Lamport (1974)

for (j=0; j<i; j++) { while (taking[j]) { } while ((ticket[j] != 0) && (ticket[j] <= ticket[i])) { }}

exit() ticket[i] = 0;

taking[i] = true;

taking[i] = false;

enter()ticket[i]=max(ticket[0],..., ticket[n-1])+1

while (taking[j]) { } while ((ticket[j] != 0) &&

}

for (j=i; j<n; j++) {

(ticket[j] < ticket[i])) { }

2

1

� Threads enter the critical region in ticket order, usingtheir IDs as a tie-break


Event counts and sequencers

The bakery algorithm suffers the same efficiency concerns asa spin-lock using tas .

What happens if n changes?

However, a similar algorithm can easily be built from eventcount and sequencer primitives, proposed as an alternativeto semaphores

� An event count is represented by a positive integer,initialized to zero, supporting the following atomicoperations:

– advance() – increment the value by one, returningthe new value

– read() – return the current value– await(i) – wait until the value is greater than or

equal to i

� A sequencer is again represented by a positive number,initialized to zero, supporting a single atomic operation:

– ticket() – increment the value by one, returningthe old value


Event counts and sequencers (2)

� Mutual exclusion is easy: a thread takes a ticket enteringa critical region and then invokes await to receive itsturn

� The values returned by await can be used directly inimplementing a single-producer single-consumer N-slotbuffer: they give the modulo-N indices to read/write

� A general N-slot buffer is more difficult. Two sequencersare used to order producers and consumers to ensureslots are read/written in order

� Note that many operations on event counts andsequencers have a straightforward implementation usingcas – as before with semaphores, care is needed to avoidmissed wake-ups between await and advance

cas x,y,z atomically compares the contents oflocation x against the value y : if they match then z iswritten to x , otherwise x is unchanged. It’s a primitiveoperations on x86, IA-64 and SPARC processors


Monitors

A monitor is an abstract data type in which mutual exclusionis enforced between invocations of its operations. Oftendepicted graphically showing the internal state and externalinterfaces, e.g. in pseudo-code

if busy wait(free);busy=true;

allocator: monitor

busy: boolean

free: condition variable

busy = false;notify(free);

When looking at a definition such as this, independent of aspecific language, it’s important to be clear on whatsemantics are required of wait and notify

� Does notify wake at most one, exactly one or morethan one waiting thread?

� Does notify cause the resumed thread to continueimmediately (if so, must the notifier exit the monitor)?


Active objects

An active object achieves mutual exclusion betweenoperations by (at least conceptually) having a dedicatedthread that performs them on behalf of external callers, e.g.

loop

SELECT

when count < buffer-size

ACCEPT insert(param) do

[insert item into buffer]

end;

increment count;

[manage ref to next slot for insertion]

or when count > 0

ACCEPT remove(param) do

[remove item from buffer]

end;

decrement count;

[manage ref to next slot for removal]

end SELECT

end loop

� Guarded ACCEPTstatements provide operations andpre-conditions that must hold for their execution

� Management code occurs outside the ACCEPTstatements


Recap

e.g. Mutexes, ConditionLanguage-level features,

Variables

Application-specificconcurrency control(e.g. MRSW)

Primitive atomicoperations

Semaphores or event-counts & sequencers

Scheduler

support?

The details of exactly what is implemented where varygreatly between systems, e.g.

� Whether the thread scheduler is implemented inuser-space or in the kernel

� Which synchronization primitives can be used betweenaddress spaces

� Whether mutexes, condition variables are provideddirectly as primitives


Documents

Introduction - University of Cambridge · Introduction. Introduction