Parallelism - Motivation

• Speed

• Responsiveness

• Clean design

1. Overview

Concurrency

• Concurrency refers to the ability of a program to perform operations in parallel.

  • Faster on hardware with multiple CPU’s, or CPU’s with parallel capability

  • Cleaner designs (sometimes) even when only a single CPU available

  • Allows useful work when waiting for I/O

1.1 Fundamental Ideas

Definitions

• Operations are concurrent if they could be executed in parallel.

  • Whether they are or not depends on hardware, compiler, etc.

• Operations that must occur in a fixed sequence are sequential.

• A process is a sequential calculation.

• Implemented in op sys as a separate execution state

  • execution counter & other registers

  • activation stack

  • possibly other data

Scheduler

A scheduler is

• responsible for deciding

  • which process gets the CPU,

  • and for how long

• Selection is made from among ready processes

Running Processes

• finish its computation
  • ⇒ terminated
• lose the CPU when its time slice runs out
  • ⇒ ready
• request a slow or unavailable resource
  • ⇒ blocked

Blocked Processes

• becomes ready when the resource is available

• still must wait for the scheduler to select it from among all the ready processes

Processes can Interact

Communication between process may be via

• messages

• shared variables

  • “shared” means visible to code of both processes

Synchronization

Synchronization

• relates two or more threads

• restricts the order in which the collection of 2 or more processes can perform events

• Critical to making concurrent software safe

1.2 Parallel versus Concurrent

Concurrent software can run in many hardware configurations.

• single processor

• multi-processor

• distributed

Single Processor

• one process can run while another is waiting for I/O

• time slicing

• reactive systems

  • e.g., user manipulates a windowed view of a lengthy calculation while that calculation is still in progress

Multi-Processor

• CPU’s may

  • work on distinct tasks,

  • or cooperate on a common goal

• Any single-CPU software designs can be mapped here

Distributed Processors

• “shared” data must be replicated

• much harder to control

  • in many cases, the cheapest hardware solution

• e.g. off-the-shelf workstations connected via a network

Looking Ahead

• We’ll concentrate on the shared memory models (single and multi-processors).

2. Spawning Processes

How do we get multiple processes in one program?

• Start with one, launch others

• May eventually need to assemble info from each spawned process into overall solution

2.1 Processes in Unix - Fork

• Usually, each time you type a command into the shell, a new process is launched to handle that command.

• Can also write programs that split into multiple processes

Concurrency in C++/Unix

Languages like C and C++ lack built-in support for concurrency.

• can sometimes do concurrent programming through special libraries

• tend to be OS and compiler-specific

Unix Model

• processes do not share memory but communicate via messages.

Process Control

Basic operations:

• fork() creates a copy of the current process, identical down to every byte in memory, except that

  • the fork()call returns 0 to the new copy (“child”) and

  • it returns the non-zero process ID of the child process to the original “parent”

• wait(int*) suspends the process until some child process completes.

Example: Forking a Process

 int status;
 if (fork() == 0) {
    /* child 1 */
     execl("./prog1", "prog1", 0);
 } else if (fork() == 0) {
    /* child 2 */
    execl("./prog2", "prog2", 0);
 } else {
    /* parent */
    wait(&status);
    wait(&status);
 }

This program forks into two processes. The child process runs a program prog1, then finishes. The parent moves into the first else, then forks again into two processes. The second child runs a program prog2, then finishes. The parent moves to the final else, where it waits for the two children to finish.

Example: Web-Launched Grading

 close STDOUT;
 close STDERR;
 $pid = fork();
 if (!defined($pid)) {
      die ("Cannot fork: $!\n");
 }
 
 if ($pid) {
   #parent: issue web page to confirm that assignment has been received
      <:\smvdots{}:>
 } else {
   #child: launch a possibly lengthy autograde program
      $gradeResults =  _autograde.pl -stop cleanup $grader $login_;
      $logFile = $assignmentDirectory . "grading.log";
      open (GRADELOG, ">>$logFile")
   }} die ("Could not append to $logFile: $!\n");
      print GRADELOG "$gradeResults\n";
      close GRADELOG;
 }

Here is part of the Perl script that gets launched when you submit programming assignments from the course’s web-based submissions pages:

This is run as a CGI script when you click the submit button on the assignment submission pages. Now, the actual auto-grading may take several minutes, but web browsers will time out long before then. So the script forks to create a new process. The new child runs the auto-grader. In the meantime, the writes out a quick “Thank you for submitting” page that the browser can show immediately.

Process Communication

Unix processes can communicate via a pipe, a communication path that appears to the program as a pair of files, one for input, the other for output.

Reading from an empty pipe will block a process until data is available (written into the pipe by another process).

Example of a Pipe

 char str[6];
 int mypipe[2];
 int status;
 pipe (mypipe); /* establishes a pipe.         */
                /* We read from mypipe[0] and  */
                /* write to mypipe[1]          */
 
 if (fork() == 0) {     /* child */
    read(mypipe[0], str, 6);
    printf (str);
 } else { /* parent */
    write (mypipe[1], "Hello", 6);
    wait (&status);
 }
 close mypipe[0];
 close mypipe[1];

This is “C” style I/O. Files are identified by an integer file handle, so out array of two ints is actually a pair of file handles. Each process can read from mypipe[0] and write to mypipe[1], though in this particular example one process does all the writing and the other all the reading.

Overall

This approach to process communication is

• relatively simple

• relatively safe - lack of shared memory reduces the possibilities for processes to interfere with one another

  • still possible though (e.g., files)

• awkward for programs that need to pass complex data

• “heavy weight” - each process is relatively expensive

Library vs. Language

Library-based concurrency tends to be non-portable.

• A big advantage to having concurrency built in as part of a language standard

  • But may lead to bad matches with some hardware configurations

• Only two such languages in common use: Ada and Java

2.2 Heavy & Light Weight Processes

A Unix process includes its entire memory image

• Such heavy-weight processes may consume a lot of system resource and be slow to start up

• Discourages programs from spawning large numbers of child processes

Threads

A thread is a light-weight process that

• Has its own execution location and activation stack

• Typically shares heap and static memory with its parent

Some OS’s support this directly

• Otherwise the Java runtime system implements multiple threads within a single heavy-weight OS process

3. Java Threads

• Java has built-in support for threads

• In fact, Java programs with GUIs already have 2 or more threads
  • The initial thread starts in main().
    • It usually builds the GUI and signals when it is to be displayed.
  • The event handler thread waits for input events and invokes the appropriate listeners when the events are detected.
    • repaint() calls are (usually?) handled as a simulated input event: queued up and eventually resulting in a paint().

Example: ThreadArt

• As an example of the use of threads in a Java GUI, let’s add some dynamic behavior to the StringArt program of the earlier lesson on Java GUIs

• The thread will
  • Increment a counter that ranges from 0 to 99 and then back to zero
  • Schedule a repaint() for each value of the counter

Using the counter to affect the drawing

public void drawLines(Graphics g, Dimension d)
{
    int dmin = (d.width < d.height) ? d.width : d.height;

    if (stepSize < 1)
        stepSize = 1;

    Point center = new Point();
    center.x = (double)d.width/2.0;
    center.y = (double)d.height/2.0;

    int k = Math.abs(cycleCounter - cycleLength/2);
    int theta = 60 * cycleCounter / cycleLength; 

    for (int i = 0; i < 60; ++i) {
        int radius = dmin/2;
        Point origin = ptOnCircle(6*i+thetai, radius, center);
        int j = i + stepSize;
        while (j >= 60)
            j -= 60;
        while (i != j) {
            Point destination = ptOnCircle(6*j+theta, radius, center);
            Color c = iinterpolate(colors[0], colors[1], k, cycleLength/2);i
            g.setColor(c);
            g.drawLine ((int)origin.x, (int)origin.y,
                    (int)destination.x, (int)destination.y);
            j += stepSize;
            while (j >= 60)
                j -= 60;
        }
    }
}

• The counter is used to
  • Compute an angle offset
    • Which is added to the angles at which all points a computed
  • Interpolate between two colors

3.1 Taking the Cheap Way Out

The easiest way to animate this code is to

• Add a Timer to the program
  • Adds an action event to the queue every 50 milliseconds
• An action listener then can increment the counter
  • and request a repaint

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JColorChooser;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.Timer;


/**
 * A simple example of GUI event handling in a Java application.
 * 
 * This can be run as a main program or as an applet.
 * 
 * @author zeil
 *
 */


public class ThreadArtByTimer extends JApplet {

    private boolean startedInAnApplet;

    // The Model
    private Color[] colors;
    private int stepSize = 5;

    private int cycleLength;
    private int cycleCounter;

    // The View & Controls
    private JFrame window;
    private JPanel canvas;
    private JButton colorChooser1;
    private JButton colorChooser2;
    private JTextField stepSizeIn;
    
    private Timer timer;


    private class ColorChooser implements ActionListener {
        private JButton button;
        private int colorNum;

        public ColorChooser (JButton button, int colorNum) {
            this.button = button;
            this.colorNum = colorNum;
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            Color chosen = JColorChooser.showDialog(window, "Choose a color", colors[colorNum]);
            if (chosen != null) {
                colors[colorNum] = chosen;
                setColor (button, chosen);
                canvas.repaint();
            }
        }
    };


    /**
     * Action that slowly changes the color of the drawing 
     *
     */
    public class ColorChanger implements ActionListener {

        public ColorChanger()
        {
        }
        
        

        @Override
        public void actionPerformed(ActionEvent arg0) {
            cycleCounter = (cycleCounter + 1) % cycleLength;
            canvas.repaint();
        }
        
    }




    public ThreadArtByTimer()
    {
        startedInAnApplet = false;
        window = null;
        colors = new Color[2];
        colors[0] = Color.red;
        colors[1] = Color.blue;
        cycleLength = 100;
        cycleCounter = 0;
    }


    public static void main (String[] args)
    {
        ThreadArtByTimer instance = new ThreadArtByTimer();
        instance.createAndShowGUI();  
    }

    public void createAndShowGUI() {
        window = new JFrame();
        // set up the components
        window.getContentPane().setLayout (new BorderLayout());

        canvas = new JPanel () {
            public void paint (Graphics g) {
                super.paint(g);
                drawLines (g, getSize());
            }
        };
        canvas.setBackground(Color.white);
        window.getContentPane().add (canvas, BorderLayout.CENTER);
        canvas.setPreferredSize(new Dimension(400, 400));

        JPanel controls = new JPanel();

        colorChooser1 = new JButton("Color 1");
        controls.add (colorChooser1);
        setColor(colorChooser1, colors[0]);
        colorChooser1.addActionListener (new ColorChooser(colorChooser1, 0));

        colorChooser2 = new JButton("Color 2");
        controls.add (colorChooser2);
        setColor(colorChooser2, colors[1]);
        colorChooser2.addActionListener (new ColorChooser(colorChooser2, 1));

        stepSizeIn = new JTextField (""+stepSize, 5);
        controls.add (stepSizeIn);
        stepSizeIn.addActionListener (new ActionListener()
        {
            public void actionPerformed(ActionEvent e) {
                try {
                    Integer newSize = new Integer(stepSizeIn.getText());
                    stepSize = newSize.intValue();
                    canvas.repaint();
                } catch (Exception ex) {};
            }
        });

        window.getContentPane().add (controls, BorderLayout.SOUTH);

        window.setDefaultCloseOperation((startedInAnApplet) ? JFrame.DISPOSE_ON_CLOSE : JFrame.EXIT_ON_CLOSE);


        timer = new Timer(50, new ColorChanger());
        timer.start();

        window.pack();
        window.setVisible(true);
    }

    /**
     * Sets the background color of a button to the indicated color.
     * Changes the foreground to wither black or white, depending on
     * which will give more contrast agasint the new background.
     * 
     * @param button
     * @param color
     */
    private void setColor(JButton button, Color color) {
        button.setBackground(color);
        int brightness = color.getRed() + color.getGreen() + color.getBlue(); // max of 3*255
        if (brightness > 3*255/2) {
            // This is a fairly bright color. Use black lettering
            button.setForeground (Color.black);
        } else {
            // This is a fairly dark color. Use white lettering
            button.setForeground (Color.white);
        }
    }

    // Applet functions

    public void init() {
        startedInAnApplet = true;
    }

    public void start() {
        if (window == null)
            createAndShowGUI();
    }

    public void stop() {
    }

    public void destroy() {
    }




    int interpolate (int x, int y, int i, int steps)
    {
        return (i * x + (steps-i)*y) / steps;
    }


    Color interpolate(Color c1, Color c2, int i, int steps)
    {
        return new Color (interpolate(c1.getRed(), c2.getRed(), i, steps),
                interpolate(c1.getGreen(), c2.getGreen(), i, steps),
                interpolate(c1.getBlue(), c2.getBlue(), i, steps));
    }


    class Point {
        double x;
        double y;
    }

    Point ptOnCircle (int degrees, int radius, Point center) 
    {
        Point p = new Point();
        double theta = Math.toRadians((double)degrees);
        p.x = center.x + (double)radius * Math.cos(theta);
        p.y = center.y + (double)radius * Math.sin(theta);
        return p;
    }

    public void drawLines(Graphics g, Dimension d)
    {
        int dmin = (d.width < d.height) ? d.width : d.height;

        if (stepSize < 1)
            stepSize = 1;

        Point center = new Point();
        center.x = (double)d.width/2.0;
        center.y = (double)d.height/2.0;

        int k = Math.abs(cycleCounter - cycleLength/2);
        int theta = 60 * cycleCounter / cycleLength; 
        
        for (int i = 0; i < 60; ++i) {
            int radius = dmin/2; //interpolate(dmin/4, dmin/2, k, cycleLength/2);
            Point origin = ptOnCircle(6*i+theta, radius, center);
            int j = i + stepSize;
            while (j >= 60)
                j -= 60;
            while (i != j) {
                Point destination = ptOnCircle(6*j+theta, radius, center);
                Color c = interpolate(colors[0], colors[1], k, cycleLength/2);
                g.setColor(c);
                g.drawLine ((int)origin.x, (int)origin.y,
                        (int)destination.x, (int)destination.y);
                j += stepSize;
                while (j >= 60)
                    j -= 60;
            }
        }
    }


}

Let’s Pretend That Never Happened

• Timers are easy to work with, but not always powerful enough
  • Not good if the recurring event may sometimes take a long time
    • Because it runs in the event handler thread, a time- consuming listener would “lock up” the GUI
  • Does not offer very flexible control options
• For educational purposes, therefore, let’s look at how we might move that timed action into a separate thread
  • not the event handler thread

3.2 Working with Threads

The Java Thread class

• Any subclasses of the system class Thread can have a function member run() that executes as a distinct process (per object).
  • Never call run() directory.
• A Thread object is not actually a new thread (process) unless we start() it
  • The start function
    • creates a new thread with its own activation stack, but sharing heap memory with the other threads
    • Starts that thread runnign with a call to the object’s run() function.

  • The thread terminates when control returns from run().

ThreadArt

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JColorChooser;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;


/**
 * A simple example of GUI event handling in a Java application.
 * 
 * This can be run as a main program or as an applet.
 * 
 * @author zeil
 *
 */


public class ThreadArt extends JApplet {

    private boolean startedInAnApplet;

    // The Model
    private Color[] colors;
    private int stepSize = 5;

    private int cycleLength;
    private int cycleCounter;
    private boolean running;

    // The View & Controls
    private JFrame window;
    private JPanel canvas;
    private JButton colorChooser1;
    private JButton colorChooser2;
    private JTextField stepSizeIn;
    
    private Animator colorChanger;


    private class ColorChooser implements ActionListener {
        private JButton button;
        private int colorNum;

        public ColorChooser (JButton button, int colorNum) {
            this.button = button;
            this.colorNum = colorNum;
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            Color chosen = JColorChooser.showDialog(window, "Choose a color", colors[colorNum]);
            if (chosen != null) {
                colors[colorNum] = chosen;
                setColor (button, chosen);
                canvas.repaint();
            }
        }
    };


    /**
     * Thread that slowly changes the color of the drawing 
     *
     */
    public class Animator extends Thread {  ➊

        public Animator()
        {
        }
        
        
        public void run()                  ➋
        {
            running = true;
            while (running) {              ➌
                try {
                    sleep(50);             ➍
                } catch (InterruptedException e) {
                  break;
                }
                cycleCounter = (cycleCounter + 1) % cycleLength; ➎
                canvas.repaint();
            }
        }
        
    }




    public ThreadArt()
    {
        startedInAnApplet = false;
        window = null;
        colors = new Color[2];
        colors[0] = Color.red;
        colors[1] = Color.blue;
        cycleLength = 100;
        cycleCounter = 0;
        running = false;
    }


    public static void main (String[] args)
    {
        ThreadArt instance = new ThreadArt();
        instance.createAndShowGUI();  
    }

    public void createAndShowGUI() {
        window = new JFrame();
        // set up the components
        window.getContentPane().setLayout (new BorderLayout());

        canvas = new JPanel () {
            public void paint (Graphics g) {
                super.paint(g);
                drawLines (g, getSize());
            }
        };
        canvas.setBackground(Color.white);
        window.getContentPane().add (canvas, BorderLayout.CENTER);
        canvas.setPreferredSize(new Dimension(400, 400));

        JPanel controls = new JPanel();

        colorChooser1 = new JButton("Color 1");
        controls.add (colorChooser1);
        setColor(colorChooser1, colors[0]);
        colorChooser1.addActionListener (new ColorChooser(colorChooser1, 0));

        colorChooser2 = new JButton("Color 2");
        controls.add (colorChooser2);
        setColor(colorChooser2, colors[1]);
        colorChooser2.addActionListener (new ColorChooser(colorChooser2, 1));

        stepSizeIn = new JTextField (""+stepSize, 5);
        controls.add (stepSizeIn);
        stepSizeIn.addActionListener (new ActionListener()
        {
            public void actionPerformed(ActionEvent e) {
                try {
                    Integer newSize = new Integer(stepSizeIn.getText());
                    stepSize = newSize.intValue();
                    canvas.repaint();
                } catch (Exception ex) {};
            }
        });

        window.getContentPane().add (controls, BorderLayout.SOUTH);

        window.setDefaultCloseOperation((startedInAnApplet) ? JFrame.DISPOSE_ON_CLOSE : JFrame.EXIT_ON_CLOSE);


        colorChanger = new Animator();   ➏
        colorChanger.start();

        window.pack();
        window.setVisible(true);
    }

    /**
     * Sets the background color of a button to the indicated color.
     * Changes the foreground to wither black or white, depending on
     * which will give more contrast agasint the new background.
     * 
     * @param button
     * @param color
     */
    private void setColor(JButton button, Color color) {
        button.setBackground(color);
        int brightness = color.getRed() + color.getGreen() + color.getBlue(); // max of 3*255
        if (brightness > 3*255/2) {
            // This is a fairly bright color. Use black lettering
            button.setForeground (Color.black);
        } else {
            // This is a fairly dark color. Use white lettering
            button.setForeground (Color.white);
        }
    }

    // Applet functions

    public void init() {
        startedInAnApplet = true;
    }

    public void start() {
        if (window == null)
            createAndShowGUI();
    }

    public void stop() {
    }

    public void destroy() {
    }




    int interpolate (int x, int y, int i, int steps)
    {
        return (i * x + (steps-i)*y) / steps;
    }


    Color interpolate(Color c1, Color c2, int i, int steps)
    {
        return new Color (interpolate(c1.getRed(), c2.getRed(), i, steps),
                interpolate(c1.getGreen(), c2.getGreen(), i, steps),
                interpolate(c1.getBlue(), c2.getBlue(), i, steps));
    }


    class Point {
        double x;
        double y;
    }

    Point ptOnCircle (int degrees, int radius, Point center) 
    {
        Point p = new Point();
        double theta = Math.toRadians((double)degrees);
        p.x = center.x + (double)radius * Math.cos(theta);
        p.y = center.y + (double)radius * Math.sin(theta);
        return p;
    }

    public void drawLines(Graphics g, Dimension d)
    {
        int dmin = (d.width < d.height) ? d.width : d.height;

        if (stepSize < 1)
            stepSize = 1;

        Point center = new Point();
        center.x = (double)d.width/2.0;
        center.y = (double)d.height/2.0;

        int k = Math.abs(cycleCounter - cycleLength/2);
        int theta = 60 * cycleCounter / cycleLength; 
        
        for (int i = 0; i < 60; ++i) {
            int radius = dmin/2;
            Point origin = ptOnCircle(6*i+theta, radius, center);
            int j = i + stepSize;
            while (j >= 60)
                j -= 60;
            while (i != j) {
                Point destination = ptOnCircle(6*j+theta, radius, center);
                Color c = interpolate(colors[0], colors[1], k, cycleLength/2);
                g.setColor(c);
                g.drawLine ((int)origin.x, (int)origin.y,
                        (int)destination.x, (int)destination.y);
                j += stepSize;
                while (j >= 60)
                    j -= 60;
            }
        }
    }


}

• ➊ Here we declare a Thread subclass

• ➋ … and override the run() function to manage our counter

• ➌ Loop almost forever

• ➍ The sleep function blocks the thread for at the indicated number of milliseconds

• ➎ This is what we came here to do.

  The repaint() will cause a near-future redraw of the canvas, which will use our new value of cycleCounter

• ➏Once the GUI has been set up, we create the new thread object and start() it.

4. Interleaving & Synchronization

• In what order do calculations take place?

• How Does Order Affect Correctness?

Interleavings

Given 2 processes whose code looks like:

Possible Orderings

First statement must be either a or x.

• If it’s a, then the 2nd statement executed must be either b or x

• If it’s x, then the 2nd statement executed must be either a or y

Possible Orderings (cont.)

Expanding that:

First statement must be either a or x.

• If it’s a, then the 2nd statement executed must be either b or x

  • If it’s b, then the 3rd must be either c or x.

  • If it’s x, then the 3rd must be either b or y.

• If it’s x, then the 2nd statement executed must be either a or y

and so on

Possible Orderings

abcxyz, abxcyz, abxycz, abxyzc, axbcyz, axbycz, axbyzc, axybcz, axybzc, axyzbc, xabcyz, xabycz, xabyzc, xaybcz, xaybzc, xayzbc, xyabcz, xyabzc, xyazbc, xyzabc

Although there are many possibilities, b never precedes a, c never precedes a or b, etc.

Interleavings

An interleaving of two sequences s and t is any sequence u

• formed from the events of s and t such that

  • the events of s retain their position relative to one another and

  • the events of t do likewise.

Describing Process Execution

• Describe each process as a sequence of “atomic” actions.

  • What constitutes “atomic” depends on hardware, compiler, etc.

• Each interleave of the two processes is a possible execution order for the program as a whole.

  • (assuming no synchronization takes place)

4.1 Safety and Parallel Programs

Shared Resoruces

Processes often compete for shared resources.

• I/O devices

• shared variables

• files

In these cases, some interleaves are dangerous.

Safety

We say that a nondeterministic program is safe if all of the answers it provides for any input are acceptable.

Example

Assuming that x starts with a value of –1 before the two processes are started, what will the value of x be after both have finished?

We could get: 1

• process 1 fetches x (–1)

• process 1 adds 1 to x and stores it (x == 0)

• process 2 fetches x (0)

• process 2 adds 1 to x and stores it (x == 1)

We could get: 0

• process 1 fetches x (–1)

• process 2 fetches x (–1)

• process 1 adds 1 to (it’s copy of) x and stores it (x == 0)

• process 2 adds 1 to (it’s copy of) x and stores it (x == 0)

Nondeterminism

• Now these two alternatives might or might not be bad.

• Some concurrent programs are specifically designed for a limited amount of nondeterminism:
  • The property of having multiple possible answers for the same input

• But there are other possibilities as well.

We could get: 65536

• process 1 fetches x (–1)

• process 1 adds 1 to (it’s copy of) x

• process 1 begins storing x. Stores the higher two bytes (x == 0000FFFFH)

• process 2 fetches x (65535)

• process 1 stores remaining 2 bytes of x (x == 0)

• process 2 adds 1 to (it’s copy of) x and stores it (x== 65536)

65536 : Really?

• Admittedly, this requires a different level of atomicity than we might have expected, but this is not at all impossible in a distributed system.

• And it’s a pretty good bet that we would not consider this an acceptable output of these two processes.
  • Which implies that this code is unsafe.

Simultaneous Access to Compound Data

More realistically, any kind of compound data structure is likely to be sensitive to simultaneous update by different processes.

More specifically,

• Simultaneous read-only access is probably safe.

• Simultaneous updates are likely to be a problem.

• Simultaneous reading and updating are often a problem as well.

Example: Simultaneous Access to a Queue

Suppose that we have a queue of customer records,

• implemented as a linked list

• currently holding one record
  

• Verify for yourself that, if either function runs without interruption, this code would work
  • No matter which of the two runs first.

Is there a potential problem with simultaneous access to these structures? The sequence of pictures below show that, for a linked list implementation of the queue, we could indeed get into trouble if a queue has a single element in it and one thread then tries to add a new element while another thread tries to remove one. (There may be other scenarios in which simultaneous access would also fail.)

Queues Interrupted

Our queue is now badly mangled.

4.2 Synchronization

Synchronization is a restriction of the possible interleavings of a set of processes.

• (to protect against interleaves that produce undesirable results.)

Synch & Shared Resources

Synchronization often takes the form of protecting a shared resource:

• Intended meaning of seize:

  • once a seize of some resource has begun, no other process can begin to seize the same resource until it has been released.

Synchronization Narrows the Options

• Now only two possible traces:

  • abcxyz and xyzabc

A Bullet, but not a Silver One

• Synchronization takes different forms

• It can relieve dangers of shared access
  • Thereby promoting safety
• But it introduces new problems
  • New kinds of programming errors (e.g., forgetting to release a seized resource), and
  • Liveness problems

5. Liveness Properties

• Synchronization promotes safety: do we get a “correct” answer?

• But it may adversely affect liveness: the rate of progress toward an answer

5.1 Deadlock

In deadlock, all processes are waiting on some shared resources, with none of them able to proceed.

Example: The Dining Philosophers

• N philosophers sitting at a round table with N plates of spaghetti and N forks, one between each pair of philosophers.

  • Each philosopher alternately thinks and eats.
  • Each needs two forks to eat.
  • They put down their forks when thinking.

Simulating the Philosphers

Represent each philosopher as an independent process:

 loop
    pick up left fork;
    pick up right fork;
    eat;
    release forks;
    think
 end loop;

• If all the philosophers are holding the left fork, the system deadlocks.
  • And the world has a few less philosophers in it!

Demo

Try running Sun’s Demo of the dining philosophers.

You may need to play a bit with the timing control at the bottom, but after a while you will almost certainly see the system get into deadlock.

Avoiding Deadlock

• One way to avoid deadlock is a policy of “ordered resource allocation”.
  • Number all the forks.
  • Require all the philosophers to pick up the lower-numbered of the two adjacent forks first.

This is a “classic” fix from the world of operating systems, where this problem often arose in conjunction with different programs requesting access to I/O devices. If one program grabbed control of the printer and then requested a card reader, while another program had already gotten control of the card reader and was asking for the printer, deadlock would result. The classic solution was to assign each I/O device a unique number and require programs to request the devices in ascending order.

In general…

This works very nicely in this specialized case, but there is no general technique for avoiding deadlock in arbitrary situations.

• You usually have to rely on careful analysis by the program designers.

5.2 Livelock

A system is in livelock if at least one process is not waiting, but the system makes no progress.

 loop
    pick up left fork;
    seize right fork
       if available;
    if seized then
      eat;
      release forks;
    else
      release left fork;
    end if;
    think;
  end loop;

 pick up left fork;
 release left fork;
 pick up left fork;
 release left fork;
 pick up left fork;
 release left fork;
    ⋮

5.3 Fairness

When multiple processes are waiting for a shared resource, and that resource becomes available, a scheduler must decide which of the waiting processes gets the resource.

• A scheduler is unfair if it consistently ignores certain processes.

What’s Fair?

Precise definition of fairness is difficult. Some possibilities:

• Any process that wants to run will be able to do so within a finite amount of time. (“finite-progress”)

• All processes awaiting a now-available resource have an equal chance of getting it.

  • Implies that it is possible for some processes to wait an arbitrarily long time.

Selfish Threads

Threads can contribute to unfairness by being “selfish”:

 class MyThread extends Thread {
   ⋮
   public void run()
   {
    while (true)
      ++counter;
   }
 }

Unless the run-time system preempts this thread, it will hog the CPU.

Yielding

 class MyThread extends Thread {
   ⋮
   public void run()
   {
    while (true) {
      ++counter;
      Thread.yield();
    }
   }
 }

Java allows threads to signal that they don’t mind losing the CPU by yielding to other threads.

Thread Priority

Java also allows programs to set relative priorities for different threads:

Thread cpuHog = new Thread() {
    ⋮
};
cpuHog.setPriority(Thread.MIN_PRIORITY);
cpuHog.start();

This could be useful if the GUI was sluggish because of the CPU cycles being burned up by this thread.

6. Safety

Concurrent programs are often non-deterministic.

• A critical section in a process is a section of code that must be treated as an atomic action if the program is to be correct.

• A concurrent program is safe if its processes are mutually excluded from being in two or more critical sections at the same time.

  • A fairly broad statement. Sometimes we can be more discerning by identifying groups of critical sections that can interfere with one another.
    • E.g., ones that work on a common shared resource

For example, if we have some critical sections that are writing data to the same output file, and other critical sections that are updating a shared display on the screen, our would be safe if processes were mutually excluded from the file output critical sections and mutually excluded from the screen update critical sections, but we might be able to tolerate one process writing to the file while another one updated the screen.

6.1 Mutual Exclusion Mechanisms

Semaphores

A semaphore is an ADT with two atomic operations, seize and release, and a hidden integer value.

seize()

• If the semaphore value is positive, it is decremented. The calling process continues.

• If the semaphore value is zero or negative, the calling process is blocked and must wait until the value becomes positive.

release()

• The semaphore value is incremented. The calling process continues.

  • In practice, the scheduler often selects a process blocked on this semaphore to be allowed to immediately attempt to seize it.

General Semaphores

• In its most general form, a semaphore can be initialized to any value k, thereby allowing up to k processes to seize it simultaneously.

  • Usually, we use binary semaphores, which are initialized to 1.

• Historically, the seize and release operations were originally called p and v.

Yep, those were semaphores

• assuming that the semaphore is not “x” itself but is something associated with or labeled as “x”.

Dining Philosophers with Semaphores

 Semaphore fork[N];
 Philosopher(i):
 loop
   int first = min(i, i+1%N);
   int second = max(i, i+1%N);
   fork[first].seize();
   fork[second].seize();
   eat;
   fork[first].release();
   fork[second].release();
   think;
 end loop;

Recap

Semaphores are relatively low-level approach to synchronization

• Although available in Java, should not be used often

• Monitors and synchronized sections are more elegant

Monitors

Monitors

A monitor is an ADT in which only one process at a time can execute any of its member functions.

• Thus all the ADT function bodies are treated as critical sections.

A Common Pattern

• A program processes data from an input source that supplies data at a highly variable rate
  • Sometimes slower than the program processes data
  • Sometimes faster, often in bursts
• The program should not “freeze up” when data is unavailable
  • may have a GUI

• The data source must be checked often enough that inputs are not lost/ignored

The Consumer-Producer Pattern

• Producer adds data to a queue

• Consumer removes data from a queue

• Each runs as a separate thread

What could go wrong?

Synchronizing the Queue

We have seen earlier that queues are likely unsafe for simultaneous access.

We avoid these simultaneous update problems by making the queue synchronized.

Monitored Queue

 class MonitoredQueue {
    private <: ... :>
    public synchronized
       void enter(Object) { ... }
    public synchronized
      Object front() { ... }
    public synchronized
      void leave() { ... }
    public synchronized
      boolean empty() { ... }
    public
       int maxSize() { ... }
  }

No two threads are allowed to be simultaneously in synchronized member functions of the same object.

This is across the whole interface. e.g., One thread cannot be inside the enter function while another is in the front() function of that queue.

If we have multiple synchronized queue objects however, one thread could be adding to queue A while another thread is adding to queue B.

Java Monitors

Monitors are the preferred synchronization technique in Java, where they are created by marking functions as “synchronized”.

• Each object of type MonitoredQueue is separately monitored.

      MonitoredQueue q1 = new MonitoredQueue();
      MonitoredQueue q2 = new MonitoredQueue();

• If process P1 calls q1.enter(x);, then another process P2 attempting to obtain q1.front() will be blocked until P1’s call to enter has completed.

• On the other hand, if process P1 calls q1.enter(x)

  • Process P2 can simultaneously execute q1.maxSize();

Monitored Statement Blocks

The synchronized member function declaration is a special case of synchronized statement blocks

• Suppose we had only a regular queue and were not willing/able to change its interface.

  We could still achieve a safe solution by marking the critical statements in the code as synchronized:

 class Consumer extends Thread {
    private Queue q;
 
   public void run() {
      while (true) {
          synchronized (q) {
            Customer c = q.front();
            q.leave();
          };
          ... process customer ...
        }
      }
   }
 }

 class Producer extends Thread {
    private Queue q;
 
   public void run() {
      while (true) {
        Customer c = fetchData();
        synchronized (line1) {
           q.enter(c);
        }
     }
 }

Synchronization on Objects

• When we mark statements as synchronized, we have to state explicitly what object we are synchronizing on.

  • When we mark member functions as synchronized, we don’t need to do so because, implicitly, the object being synchronized on is this.

• Important: To actually get synchronization, blocks of statements must synchronize on the same object.

7. Direct Control of Thread State

Waiting…

We often want to make a thread inactive until certain conditions are met.

Busy Wait

This is the bad way to do it:

public void run() {
   while (true) {
      if (conditionIsMet()) {
         doSomethingUseful();
      }
   }
}

This is a busy wait

• We continuously burn CPU cycles testing the condition until it becomes true
  • Makes machine very sluggish

Son of Busy Wait

Only marginally better:

public void run() {
   while (true) {
      if (conditionIsMet()) {
         doSomethingUseful();
      } else {
        sleep(100); // wait 0.1 seconds
      }
   }
}

• still a busy wait

• still a bad idea

Controlling the Process State

• block the thread and then

• make it ready when the condition is met.
  • (has to be done by some other thread)

wait()

If we have a synchronized lock on some object x, then x.wait() will

• Put the thread on a “waiting for x queue”

• Make the thread ineligible for any more CPU time (blocked)

• Release the lock on x

notifyAll()

If we have a synchronized lock on some object x, then x.notifyAll() will

• Make all threads on the “waiting for x queue” eligible for CPU time (ready)

Example: Adding a Pause Button to ThreadArt

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JColorChooser;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;

/**
 * A simple example of GUI event handling in a Java application.
 * 
 * This can be run as a main program or as an applet.
 * 
 * @author zeil
 *
 */


public class ThreadArt2 extends JApplet {

    private boolean startedInAnApplet;

    // The Model
    private Color[] colors;
    private int stepSize = 5;

    private int cycleLength;
    private int cycleCounter;
    private boolean running;
    
    // The View & Controls
    private JFrame window;
    private JPanel canvas;
    private JButton colorChooser1;
    private JButton colorChooser2;
    private JButton pause;
    private JTextField stepSizeIn;
    
    private Animator colorChanger;


    private class ColorChooser implements ActionListener {
        private JButton button;
        private int colorNum;

        public ColorChooser (JButton button, int colorNum) {
            this.button = button;
            this.colorNum = colorNum;
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            Color chosen = JColorChooser.showDialog(window, "Choose a color", colors[colorNum]);
            if (chosen != null) {
                colors[colorNum] = chosen;
                setColor (button, chosen);
                canvas.repaint();
            }
        }
    };


    /**
     * Thread that slowly changes the color of the drawing 
     *
     */
    public class Animator extends Thread {

        public Animator()
        {
        }
        
        
        public void run()
        {
            running = true;
            while (true) {
                try {
                    sleep(50);
                } catch (InterruptedException e) {
                  break;
                }
                synchronized (this) {
                    while (!running) {
                        try {
                            wait();                           ➌
                        } catch (InterruptedException e) {
                            return;
                        }
                    }
                }
                cycleCounter = (cycleCounter + 1) % cycleLength;
                canvas.repaint();
            }
        }
        
    }




    public ThreadArt2()
    {
        startedInAnApplet = false;
        window = null;
        colors = new Color[2];
        colors[0] = Color.red;
        colors[1] = Color.blue;
        cycleLength = 100;
        cycleCounter = 0;
        running = true;
    }


    public static void main (String[] args)
    {
        ThreadArt2 instance = new ThreadArt2();
        instance.createAndShowGUI();  
    }

    public void createAndShowGUI() {
        window = new JFrame();
        // set up the components
        window.getContentPane().setLayout (new BorderLayout());

        canvas = new JPanel () {
            public void paint (Graphics g) {
                super.paint(g);
                drawLines (g, getSize());
            }
        };
        canvas.setBackground(Color.white);
        window.getContentPane().add (canvas, BorderLayout.CENTER);
        canvas.setPreferredSize(new Dimension(400, 400));

        JPanel controls = new JPanel();

        colorChooser1 = new JButton("Color 1");
        controls.add (colorChooser1);
        setColor(colorChooser1, colors[0]);
        colorChooser1.addActionListener (new ColorChooser(colorChooser1, 0));

        colorChooser2 = new JButton("Color 2");
        controls.add (colorChooser2);
        setColor(colorChooser2, colors[1]);
        colorChooser2.addActionListener (new ColorChooser(colorChooser2, 1));

        stepSizeIn = new JTextField (""+stepSize, 5);
        controls.add (stepSizeIn);
        stepSizeIn.addActionListener (new ActionListener()
        {
            public void actionPerformed(ActionEvent e) {
                try {
                    Integer newSize = new Integer(stepSizeIn.getText());
                    stepSize = newSize.intValue();
                    canvas.repaint();
                } catch (Exception ex) {};
            }
        });

        pause = new JButton("Pause");             ➊
        controls.add (pause);
        pause.addActionListener(new ActionListener() {
            
            @Override
            public void actionPerformed(ActionEvent e) {
                if (running) {
                    running = false;              ➋
                    pause.setText("Resume");
                    pause.repaint();
                } else {
                    synchronized (colorChanger) {
                        running = true;           ➍
                        pause.setText("Pause");
                        pause.repaint();
                        colorChanger.notifyAll();
                    }
                }
            }
        });
        
        
        window.getContentPane().add (controls, BorderLayout.SOUTH);

        window.setDefaultCloseOperation((startedInAnApplet) ? JFrame.DISPOSE_ON_CLOSE : JFrame.EXIT_ON_CLOSE);


        colorChanger = new Animator();
        colorChanger.start();

        window.pack();
        window.setVisible(true);
    }

    /**
     * Sets the background color of a button to the indicated color.
     * Changes the foreground to wither black or white, depending on
     * which will give more contrast agasint the new background.
     * 
     * @param button
     * @param color
     */
    private void setColor(JButton button, Color color) {
        button.setBackground(color);
        int brightness = color.getRed() + color.getGreen() + color.getBlue(); // max of 3*255
        if (brightness > 3*255/2) {
            // This is a fairly bright color. Use black lettering
            button.setForeground (Color.black);
        } else {
            // This is a fairly dark color. Use white lettering
            button.setForeground (Color.white);
        }
    }

    // Applet functions

    public void init() {
        startedInAnApplet = true;
    }

    public void start() {
        if (window == null)
            createAndShowGUI();
    }

    public void stop() {
    }

    public void destroy() {
    }




    int interpolate (int x, int y, int i, int steps)
    {
        return (i * x + (steps-i)*y) / steps;
    }


    Color interpolate(Color c1, Color c2, int i, int steps)
    {
        return new Color (interpolate(c1.getRed(), c2.getRed(), i, steps),
                interpolate(c1.getGreen(), c2.getGreen(), i, steps),
                interpolate(c1.getBlue(), c2.getBlue(), i, steps));
    }


    class Point {
        double x;
        double y;
    }

    Point ptOnCircle (int degrees, int radius, Point center) 
    {
        Point p = new Point();
        double theta = Math.toRadians((double)degrees);
        p.x = center.x + (double)radius * Math.cos(theta);
        p.y = center.y + (double)radius * Math.sin(theta);
        return p;
    }

    public void drawLines(Graphics g, Dimension d)
    {
        int dmin = (d.width < d.height) ? d.width : d.height;

        if (stepSize < 1)
            stepSize = 1;

        Point center = new Point();
        center.x = (double)d.width/2.0;
        center.y = (double)d.height/2.0;

        int k = Math.abs(cycleCounter - cycleLength/2);
        int theta = 60 * cycleCounter / cycleLength; 
        
        for (int i = 0; i < 60; ++i) {
            int radius = dmin/2; //interpolate(dmin/4, dmin/2, k, cycleLength/2);
            Point origin = ptOnCircle(6*i+theta, radius, center);
            int j = i + stepSize;
            while (j >= 60)
                j -= 60;
            while (i != j) {
                Point destination = ptOnCircle(6*j+theta, radius, center);
                Color c = interpolate(colors[0], colors[1], k, cycleLength/2);
                g.setColor(c);
                g.drawLine ((int)origin.x, (int)origin.y,
                        (int)destination.x, (int)destination.y);
                j += stepSize;
                while (j >= 60)
                    j -= 60;
            }
        }
    }


}

• ➊ A new button

• ➋ that sets running to false

• ➌ which causes the animation thread to block itself, putting it a queue associated with the object colorChanger

  How does the thread ever get awakened?

• ➍ Next time we click the button, we set running to true and notify all threads (there should only be one) waiting on colorChanger to make themselves ready.

Note the placement of wait() calls inside loops. That’s a safety measure. If more than one thread is waiting to get something from the queue, and only one element is added to the queue, then all the waiting threads will be awakened by notifyAll(), but only one will actually get the data and the rest will immediately have to wait() again.

8. Summing Up

Making Things Parallel

• Divide the tasks into appropriate threads

• Examine the threads for shared variables (and other resources)
  • Consider whether simultaneous access to any of these shared resources could be unsafe.

• Add synchronization to prevent unsafe simultaneous access

• Analyze the synchronization for possible liveness problems