1. Tutorial 2 Adventures in Threading presented by: Antonio Maiorano Paul Di Marco

2. Thread Scheduling in Java Execution of multiple threads in some order is called scheduling Java uses a simple, deterministic scheduling algorithm called fixed priority scheduling Threads of the same priority are chosen to run in round-robin fashion

3. When Threads Run Current thread runs until one of following occurs: –Higher priority thread goes on the ready queue (and thus, preempts current thread) –It yields, or it’s run() exits –On time-slicing supported systems, it’s time allotment expires

4. Programming Threads in Java There are four main functions of interest: –run() : the function the thread executes (you provide this) –start() : creates a new thread and puts it on the ready queue –join() : wait for this thread’s run() to complete –yield() : yields the cpu to the next thread

5. So… why Java 1.2.1 ??? Versions 1.2.1 and prior: Java VM controls thread scheduling, rather than the OS. Consequently: –No time-slicing –Does not take advantage of multiple processors –Must call yield() to allow other threads to run (of the same priority)

6. Sample Thread Class // ThreadTest.java class MyThread extends Thread { private String name; public MyThread(String name) { this.name = name; } public void run() { for (int i = 1; i <= 6; ++i) { System.out.println(name + ": " + i); if (i == 3) yield(); // Let other threads run } // CONTINUED…

7. Sample Thread Program public class ThreadTest { public static void main(String[] args) { MyThread t1 = new MyThread("Alpha"); MyThread t2 = new MyThread("Beta"); // Schedule threads to start t1.start(); t2.start(); System.out.println("This is in the main()"); // Wait for threads to finish try { t1.join(); t2.join(); } catch (InterruptedException e) {} System.out.println("End of main()"); }

8. Sample Output This is in the main() Alpha: 1 Alpha: 2 Alpha: 3 Beta: 1 Beta: 2 Beta: 3 Alpha: 4 Alpha: 5 Alpha: 6 Beta: 4 Beta: 5 Beta: 6 End of main() Does this make sense?

9. Programming Assignment 1 class Block represents a typical stack class AcquireBlock represents the “pop” operation class ReleaseBlock represents the “push” operation Objective: to see what can happen if push and pop operations are not atomic. (Ms = main store – British terminology)

10. public class Block { public static void main(String[] a) { System.out.println("Main thread starts executing."); System.out.println("Initial value of top = " + top + "."); System.out.println("Initial value of stack top = " + stack[top] + "."); System.out.println("Main thread will now fork two threads."); AcquireBlock t1 = new AcquireBlock(); ReleaseBlock t2 = new ReleaseBlock(); t2.start(); // added to ready queue t1.start(); // added to ready queue try { // main thread waits for thread termination t1.join(); t2.join(); } catch (InterruptedException e) { }; System.out.println("System terminates normally."); System.out.println("Final value of top = " + top + "."); System.out.println("Final value of stack top = " + stack[top] + "."); System.out.println("Final value of stack top-1 = " + stack[top-1] + "."); } class AcquireBlock extends Thread {... } class ReleaseBlock extends Thread {... } 1

11. public class Block {... } class AcquireBlock extends Thread {... } class ReleaseBlock extends Thread { public void run() { System.out.println("ReleaseBlock thread starts executing."); yield(); newtop = Block.top + 1; System.out.println("ReleaseBlock increments pointer to " + newtop + "."); Block.top++; yield(); System.out.println("ReleaseBlock thread returns Ms block " + block + " to position " + Block.top + "."); Block.stack[Block.top] = block; System.out.println("Rel: Current value of top = " + Block.top + "."); System.out.println("Rel: Current value of stack top = " + Block.stack[Block.top] + "."); System.out.println("ReleaseBlock thread terminates."); } 2

12. public class Block {... } class AcquireBlock extends Thread { public void run() { System.out.println("AcquireBlock thread starts executing."); yield(); System.out.println("AcquireBlock thread requests Ms block."); oldtop = Block.top; copy = Block.stack[oldtop]; System.out.println("AcquireBlock thread obtains Ms block " + copy + " from position " + oldtop + "."); block = Block.stack[oldtop]; newtop = Block.top - 1; System.out.println("AcquireBlock decrements pointer to " + newtop + "."); --Block.top; System.out.println("Acq: Current value of top = " + Block.top + "."); System.out.println("Acq: Current value of stack top = " + Block.stack[Block.top] + "."); System.out.println("AcquireBlock thread terminates."); } class ReleaseBlock extends Thread {... } 3

13. public class Block {... } class AcquireBlock extends Thread {... } class ReleaseBlock extends Thread { public void run() { System.out.println("ReleaseBlock thread starts executing."); yield(); newtop = Block.top + 1; System.out.println("ReleaseBlock increments pointer to " + newtop + "."); Block.top++; yield(); System.out.println("ReleaseBlock thread returns Ms block " + block + " to position " + Block.top + "."); Block.stack[Block.top] = block; System.out.println("Rel: Current value of top = " + Block.top + "."); System.out.println("Rel: Current value of stack top = " + Block.stack[Block.top] + "."); System.out.println("ReleaseBlock thread terminates."); } 4

14. public class Block {... } class AcquireBlock extends Thread { public void run() { System.out.println("AcquireBlock thread starts executing."); yield(); System.out.println("AcquireBlock thread requests Ms block."); oldtop = Block.top; copy = Block.stack[oldtop]; System.out.println("AcquireBlock thread obtains Ms block " + copy + " from position " + oldtop + "."); block = Block.stack[oldtop]; newtop = Block.top - 1; System.out.println("AcquireBlock decrements pointer to " + newtop + "."); --Block.top; System.out.println("Acq: Current value of top = " + Block.top + "."); System.out.println("Acq:Current value of stack top=" + Block.stack[Block.top]); System.out.println("AcquireBlock thread terminates."); } class ReleaseBlock extends Thread {... } 5

15. public class Block {... } class AcquireBlock extends Thread {... } class ReleaseBlock extends Thread { public void run() { System.out.println("ReleaseBlock thread starts executing."); yield(); newtop = Block.top + 1; System.out.println("ReleaseBlock increments pointer to " + newtop + "."); Block.top++; yield(); System.out.println("ReleaseBlock thread returns Ms block " + block + " to position " + Block.top + "."); Block.stack[Block.top] = block; System.out.println("Rel: Current value of top = " + Block.top + "."); System.out.println("Rel: Current value of stack top=" + Block.stack[Block.top]); System.out.println("ReleaseBlock thread terminates."); } 6

16. public class Block { public static void main(String[] a) { System.out.println("Main thread starts executing."); System.out.println("Initial value of top = " + top + "."); System.out.println("Initial value of stack top = " + stack[top] + "."); System.out.println("Main thread will now fork two threads."); AcquireBlock t1 = new AcquireBlock(); ReleaseBlock t2 = new ReleaseBlock(); t2.start(); // added to ready queue t1.start(); // added to ready queue try { // main thread waits for thread termination t1.join(); t2.join(); } catch (InterruptedException e) { }; System.out.println("System terminates normally."); System.out.println("Final value of top = " + top + "."); System.out.println("Final value of stack top = " + stack[top] + "."); System.out.println("Final value of stack top-1 = " + stack[top-1]); } class AcquireBlock extends Thread {... } class ReleaseBlock extends Thread {... } 7

17. PA1 Output Main thread starts executing. Initial value of top = 3. Initial value of stack top = d. Main thread will now fork two threads. ReleaseBlock thread starts executing. AcquireBlock thread starts executing. ReleaseBlock increments pointer to 4. AcquireBlock thread requests Ms block. AcquireBlock thread obtains Ms block $ from position 4. AcquireBlock decrements pointer to 3. Acq: Current value of top = 3. Acq: Current value of stack top = d. AcquireBlock thread terminates. ReleaseBlock thread returns Ms block e to position 3. Rel: Current value of top = 3. Rel: Current value of stack top = e. ReleaseBlock thread terminates. System terminates normally. Final value of top = 3. Final value of stack top = e. Final value of stack top-1 = c.