Java threads
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Java Threads. Instructor: Mainak Chaudhuri [email protected] Java threads. Two ways to create a thread Extend the Thread class and override the public run method Implement a runnable interface with a public run method class MyThread extends Thread { private int tid;

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Java threads

Java Threads

Instructor: Mainak Chaudhuri

[email protected]


Java threads1

Java threads

  • Two ways to create a thread

    • Extend the Thread class and override the public run method

    • Implement a runnable interface with a public run method

      class MyThread extends Thread {

      private int tid;

      private String name;

      private long startTime;

      public MyThread (String name, int tid) {

      super (name);

      this.tid = tid;

      this.name = new String(name);

      startTime = System.currentTimeMillis();

      } // next slide


Java threads2

Java threads

public void run () { // override

System.out.println (“<” + age() + “> Hi! This is ” + getName() + “.”);

}

public long age () {

return (System.currentTimeMillis () – startTime);

}

} // end class

class MyFirstThreadDemo {

public static void main (String arg[]) {

Integer n = new Integer (arg[0].trim());

int numThreads = n.intValue(); // next slide


Java threads3

Java threads

int i;

MyThread t[] = new MyThread[numThreads];

for (i=0; i<numThreads; i++) {

t[i] = new MyThread (“Thread ”+i, i);

t[i].start();

}

} // end main

} // end class


Java threads via interface

Java threads via interface

class MyThread implements Runnable {

private int tid;

private String name;

private long startTime;

public MyThread (String name, int tid) {

this.tid = tid;

this.name = new String(name);

startTime = System.currentTimeMillis();

}

public void run () {

System.out.println (“<” + age() + “> Hi! This is ” + Thread.currentThread().getName() + “.”);

} //next slide


Java threads via interface1

Java threads via interface

public long age () {

return (System.currentTimeMillis () – startTime);

}

} // end class

class MyFirstThreadDemo {

public static void main (String arg[]) {

Integer n = new Integer (arg[0].trim());

int numThreads = n.intValue();

int i; // next slide


Java threads via interface2

Java threads via interface

MyThread mt[] = new MyThread[numThreads];

Thread t[] = new Thread[numThreads];

for (i=0; i<numThreads; i++) {

mt[i] = new MyThread (“Thread ”+i, i);

t[i] = new Thread (mt[i], “Thread ”+i);

t[i].start();

}

} // end main

} // end class


Announcements

Announcements

  • Final exam on 19th November 1215 to 1515

  • Seating arrangement:

    Y4, Y5, Y7001 to Y7085L1 OROS

    Y7092 to Y7233L2 ERES

    Y7234 to Y7388L16 OROS

    Y7391 to Y7518L17 ERES

  • Please take seats 15 minutes before the scheduled time


Java thread states

Java thread states

  • Java threads have typically seven states

    • New: just after a thread is created

    • Runnable: after the start() method is invoked. The thread is placed in the runnable set or the ready queue. When scheduled, the run() method will be executed.

    • Running: executing the code in the run() method. May call its yield() method to put itself back in the runnable set.

    • Suspended: after the suspend() method is called. May be called by itself or by some other thread. It is placed back on runnable set only if some other thread calls its resume() method.


Java thread states1

Java thread states

  • Seven states (continued)

    • Blocked: after the sleep() method or the wait() method or the join() method is called to join with a thread (at a barrier) that is yet to arrive at the join point or when it does blocking I/O (e.g., reading from keyboard). The thread transitions back to runnable state when the blocking call is over.

    • Suspended-blocked: If a blocked thread is suspended by some other thread. It returns back to suspended or blocked state depending on whether the blocking finishes before a call to resume() or not.

    • Dead: On completion of the run() method or a call to stop() method.


Java thread scheduling

Java thread scheduling

  • Every Java thread gets a priority

    • Inherits the same priority from the creating thread. In our example, all threads have same priority as the “main thread”.

    • Priorities range from MIN_PRIORITY to MAX_PRIORITY defined in Thread class. The default priority is NORM_PRIORITY.

    • Use setPriority and getPriority methods to change and query a thread’s priority.

    • Among all the threads in the runnable set, the highest priority thread is allowed to run until it blocks, yields, gets suspended, or a new thread of higher priority enters the runnable set. In the last case the running thread goes back to runnable set


Java thread scheduling1

Java thread scheduling

  • Some implementations carry out a round-robin scheduling (e.g., JDK 1.1 for Windows 95/NT)

    • Every thread gets a time slice to execute

    • Once the time slice of the thread expires, it is put back in the runnable set and the next thread is given a chance (higher priority threads are considered first)

    • If a thread blocks, yields, or gets suspended before the time slice expires, the next thread is scheduled for execution


More example

More example

class MyThread extends Thread {

private int tid;

private String name;

private long startTime;

private long screamingInterval;

public MyThread (String name, int tid, long screamingInterval) {

super (name);

this.tid = tid;

this.name = new String(name);

startTime = System.currentTimeMillis();

this.screamingInterval = screamingInterval;

} // next slide


More example1

More example

public void run () { // override

long count = 0;

while (true) {

if (count % screamingInterval == 0) {

System.out.println (“<” + age() + “> Hi! This is ” + getName() + “.”);

}

count++;

}

}

public long age () {

return (System.currentTimeMillis () – startTime);

}

} // end class


More example2

More example

class MyThreadDemo {

public static void main (String arg[]) {

Integer n = new Integer (arg[0].trim());

int numThreads = n.intValue();

int i;

MyThread t[] = new MyThread[numThreads];

for (i=0; i<numThreads; i++) {

t[i] = new MyThread (“Thread ”+i, i, (i+1)*1000000);

//t[i].setPriority (t[i].getPriority()+i);

t[i].start();

}

} // end main

} // end class


Typical output snapshot

Typical output snapshot

<8408> Hi! This is Thread 0.

<9014> Hi! This is Thread 0.

<9374> Hi! This is Thread 1.

<9615> Hi! This is Thread 0.

<10214> Hi! This is Thread 0.

<10545> Hi! This is Thread 1.

<10813> Hi! This is Thread 0.

<11416> Hi! This is Thread 0.

<11720> Hi! This is Thread 1.

<12014> Hi! This is Thread 0.

<12613> Hi! This is Thread 0.

<12889> Hi! This is Thread 1.

<13213> Hi! This is Thread 0.


Physical resources

Physical resources

  • Parallelism ultimately depends on availability of multiple physical CPUs

    • You get true concurrency with “multiprocessors”

    • With single CPU, you get time-shared concurrency (at any point in time only one thread can run)

    • Today small-scale multiprocessors are commodity

    • Chip-multiprocessors or so called multi-core processors come with two, four, or eight processors on a single chip (soon 16 processors)


Data race

Data race

  • How to share a variable?

    • Create a single class containing the variables that you want to share

    • Create one instance of this object and attach multiple threads to it

    • Let us see what happens if we try to update a shared variable concurrently in multiple threads

    • This is known as a data race as multiple threads may have a race trying to update the same data


Data race1

Data race

class MyThread implements Runnable {

private long sum;

public MyThread () {

sum=0;

}

public void run () {

int i;

for (i=0; i<100000; i++) {

sum++;

}

System.out.println("<" + Thread.currentThread().getName() + ">: Sum=" + sum);

} // next slide


Data race2

Data race

public long GetSum () {

return sum;

}

} // end class

class MyDataRaceDemo {

public static void main (String arg[]) throws java.lang.InterruptedException {

Integer n = new Integer (arg[0].trim());

int numThreads = n.intValue();

int i; // next slide


Data race3

Data race

MyThread mt = new MyThread();

Thread t[] = new Thread[numThreads];

for (i=0; i<numThreads; i++) {

t[i] = new Thread (mt, "Thread "+i);

t[i].start();

}

for (i=0; i<numThreads; i++) {

t[i].join();

}

System.out.println("From main: Sum=" + mt.GetSum());

} // end main

} // end class


Data race4

Data race

  • If you run this program multiple times, you will get different outputs in different runs

    • A typical symptom of data race

    • The output depends on how the threads get interleaved when they run

    • A typical output:

      <Thread 0>: Sum=107927

      <Thread 1>: Sum=128853

      From main: Sum=128853


More data race example

More data race example

  • Let us try to write the program for summing an array

    class MyThread implements Runnable {

    private int array[];

    private long sum;

    private int numThreads;

    private int tid;

    public MyThread (long size, int numThreads) {

    int i;

    array = new int[size];

    for (i=0; i<size; i++) {

    array[i] = i;

    }

    sum = 0;

    tid = 0;

    this.numThreads = numThreads;

    } // next slide


More data race example1

More data race example

public void run () {

int i;

int myid = tid++; // data race on tid

for (i=myid*(array.length/numThreads); i<(myid+1)*(array.length/numThreads); i++) {

sum += array[i]; // data race on sum

}

}

public long GetSum() {

return sum;

}

} // end class


More data race example2

More data race example

class MyArraySumDemo {

public static void main (String arg[]) throws java.lang.InterruptedException {

Integer n = new Integer (arg[0].trim());

long size = n.intValue();

n = new Integer (arg[1].trim());

int numThreads = n.intValue();

int i;

MyThread mt = new MyThread(size, numThreads);

Thread t[] = new Thread[numThreads];

// next slide


More data race example3

More data race example

for (i=0; i<numThreads; i++) {

t[i] = new Thread (mt, "Thread "+i);

t[i].start();

}

for (i=0; i<numThreads; i++) {

t[i].join();

}

System.out.println("From main: Sum=" + mt.GetSum() + “[Expected=” + (size*(size-1))/2 + “]”);

} // end main

} // end class


More data race example4

More data race example

  • One typical output for size=100000 and numThreads=2

    From main: Sum=3692782245[Expected=4999950000]

  • Replacing sum by local sum and accumulating at the end reduces the chance of data race

    • Reduces the number of executed critical sections: more parallelism

    • But the program is still buggy and may produce wrong results once in a while


More data race example5

More data race example

class MyThread implements Runnable {

private int array[];

private long sum;

private int numThreads;

private int tid;

public MyThread (long size, int numThreads) {

int i;

array = new int[size];

for (i=0; i<size; i++) {

array[i] = i;

}

sum = 0;

this.numThreads = numThreads;

} // next slide


More data race example6

More data race example

public void run () {

int i;

long localSum = 0; // private variable

int myid = tid++; // data race on tid

for (i=myid*(array.length/numThreads); i<(myid+1)*(array.length/numThreads); i++) {

localSum += array[i]; // no data race

}

sum += localSum; // data race on sum

}

public long GetSum() {

return sum;

}

} // end class


More data race example7

More data race example

class MyArraySumDemo {

public static void main (String arg[]) throws java.lang.InterruptedException {

Integer n = new Integer (arg[0].trim());

long size = n.intValue();

n = new Integer (arg[1].trim());

int numThreads = n.intValue();

int i;

MyThread mt = new MyThread(size, numThreads);

Thread t[] = new Thread[numThreads];

// next slide


More data race example8

More data race example

for (i=0; i<numThreads; i++) {

t[i] = new Thread (mt, "Thread "+i);

t[i].start();

}

for (i=0; i<numThreads; i++) {

t[i].join();

}

System.out.println("From main: Sum=" + mt.GetSum() + “[Expected=” + (size*(size-1))/2 + “]”);

} // end main

} // end class


Fixing data races

Fixing data races

  • Every Java object has an in-built lock

    • We will make use of this to protect the critical sections

    • For every shared variable that is involved in a data race, we need to create a lock

    • It is possible to have a single lock to resolve multiple races, but that will limit concurrency (why?)

    • General solution: create a dummy object and use its lock to resolve a race

    • Java allows the programmer to mark critical sections with the keyword synchronized


Fixing array sum

Fixing array sum

class MyThread implements Runnable {

private int array[];

private long sum;

private int numThreads;

private int tid;

private Object tidLock; // dummy object

private Object sumLock; // dummy object

public MyThread (long size, int numThreads, Object tidLock, Object sumLock) {

int i;

array = new int[size];

for (i=0; i<size; i++) {

array[i] = i;

} // next slide


Fixing array sum1

Fixing array sum

sum = 0;

this.numThreads = numThreads;

this.tidLock = tidLock;

this.sumLock = sumLock;

}

public void run () {

int i;

int myid;

synchronized (tidLock) {

myid = tid++;

} // next slide


Fixing array sum2

Fixing array sum

for (i=myid*(array.length/numThreads); i<(myid+1)*(array.length/numThreads); i++) {

synchronized (sumLock) {

sum += array[i];

}

}

}

public long GetSum() {

return sum;

}

} // end class


Fixing array sum3

Fixing array sum

class MyArraySumDemo {

public static void main (String arg[]) throws java.lang.InterruptedException {

Integer n = new Integer (arg[0].trim());

long size = n.intValue();

n = new Integer (arg[1].trim());

int numThreads = n.intValue();

int i;

Object tidLock = new Object();

Object sumLock = new Object();

MyThread mt = new MyThread(size, numThreads, tidLock, sumLock);

Thread t[] = new Thread[numThreads];

// next slide


Fixing array sum4

Fixing array sum

for (i=0; i<numThreads; i++) {

t[i] = new Thread (mt, "Thread "+i);

t[i].start();

}

for (i=0; i<numThreads; i++) {

t[i].join();

}

System.out.println("From main: Sum=" + mt.GetSum() + "[Expected=" + (size*(size-1))/2 + "]");

} // end main

} // end class


Announcements1

Announcements

  • No class tomorrow

  • All the best for the final exam. Bye!


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