Java Threads Instruct or: M Maina k Ch k Chaudh dh i ur ac.in in 1

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1 Java Threads Instructor: t Mainak Chaudhuri mainakc@cse.iitk.ac.iniitk ac 1

2 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 2

3 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()); 3 int numthreads = n.intvalue(); // next slide

4 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(); t t() // end main // end class 4

5 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 5

6 Java threads via interface public long age () { return (System.currentTimeMillis () starttime); // end class class MyFirstThreadDemo { public static ti void main (String arg[]) { Integer n = new Integer (arg[0].trim()); int numthreads = n.intvalue(); 6 int i; // next slide

7 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 7 // end class

8 Announcements Final exam on 19 th November 1215 to 1515 Seating arrangement: Y4, Y5, Y7001 to Y7085 L1 OROS Y7092 to Y7233 L2 ERES Y7234 to Y7388 L16 OROS Y7391 to Y7518 L17 ERES Please take seats 15 minutes before the scheduled time 8

9 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: d 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 9 resume() method.

10 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 d or blocked state depending on whether the blocking finishes before a call to resume() or not. Dead: On completion of the run() method 10 or a call to stop() method.

11 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 it and getpriority it 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 h priority it enters the runnable set. In the last case the running 11 thread goes back to runnable set

12 Java thread scheduling Some implementations carry out a roundrobin 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 d before the time slice expires, the next thread is scheduled for execution 12

13 More example class MyThread extends Thread { private int tid; pi private Stin String nm; 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 13

14 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 14

15 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)* ); //t[i].setpriority (t[i].getpriority()+i); t[i].start(); // end main 15 // end class

16 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. 16

17 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) 17

18 Data race How to share a variable? Create a single class containing n ng the variables a 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 18

19 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); 19 // next slide

20 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(); 20 int i; // next slide

21 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 t tl main: Sum=" + mt.getsum()); // end main // end class 21

22 Data race If you run this program multiple times, you will get different outputs in different runs A typical symptom m of data race The output depends on how the threads get interleaved when they run A typical output: <Thread 0>: Sum= <Thread 1>: Sum= From main: Sum=

23 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; 23 // next slide

24 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); id mth ds); i++) { sum += array[i]; // data race on sum public long GetSum() { return sum; // end class 24

25 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]; 25 // next slide

26 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( println("from main: Sum=" + mt.getsum() + [Expected= + (size*(size-1))/2 + ] ); ]) // end main 26 // end class

27 More data race example One typical output for size= and numthreads=2 From main: Sum= [Expected= ] 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 27

28 More data race example class MyThread implements Runnable { private int array[]; pi private ln long sm; 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 28

29 More data race example public void run () { int i; long localsum ls = 0; // private variable int myid = tid++; // data race on tid for (i=myid*(array (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 29

30 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]; 30 // next slide

31 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( println("from main: Sum=" + mt.getsum() + [Expected= + (size*(size-1))/2 + ] ); ]) // end main 31 // end class

32 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 i 32 sections with the keyword synchronized

33 Fixing array sum class MyThread implements Runnable { private int array[]; pi private ln long sm; 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; 33 // next slide

34 Fixing array sum sum = 0; this.numthreads = numthreads; this.tidlock = tidlock; this.sumlock sumlock = sumlock; public void run () { int i; int myid; synchronized (tidlock) { myid = tid++; // next slide 34

35 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 35

36 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 36

37 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( println("from main: Sum=" + mt.getsum() + "[Expected=" + (size*(size-1))/2 + "]"); ]) // end main 37 // end class

38 Announcements No class tomorrow All the best for the final exam. Bye! 38

CIS233J Java Programming II. Threads

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