1. Distributed and Parallel Processing George Wells

2. Threads in Java Simplified lifecycle New Runnable Running Dea d Blocked Blocking event Unblocked Scheduler run() completes start() Thread scheduling is preemptive, but not necessarily time-sliced – Use Thread.sleep(x) or Thread.yield()

3. Thread Control Various methods: – isAlive() – getpriority() and setpriority(x) – join() – Thread.currentThread()

4. Exercise Write a class ( MyThread ) – Implements Runnable – run() consists of a loop (10x) printing the thread's name and sleeping for 1s Write a main program that creates 3 of these threads and runs them – Use join() to wait for them – Experiment with the setName method – Experiment with changing priorities and see what the effects are

5. Synchronisation Seen the effects of lack of synchronisation – Mandelbrot example Solution: – Use synchronized modifier on method Synchronized methods/blocks obtain a lock on the object before proceeding – No other thread can execute synchronised code on that object while the lock is held – Mutual exclusion (mutex) More to it than that!

6. Example: Producer-Consumer Specialised instance of the pipeline pattern – One process/thread produces data – Another uses (consumes) this data Uses some form of “communication channel” – We assume a simple queue

7. public class ArrayQueue { private T[] data; private int hd, tl; public ArrayQueue () { data = (T[])new Object[10]; hd = tl = -1; } // Constructor public void add (T item) { tl = (tl + 1) % data.length; data[tl] = item; if (hd == -1) // First item in queue hd = tl; } // add public T remove () { int tmpIndex = hd; if (hd == tl) // Was last element hd = tl = -1; else hd = (hd + 1) % data.length; return data[tmpIndex]; } // remove } // class ArrayQueue Queue Class

8. Main Program Producer class – Generates stream of random numbers – Places numbers in queue Consumer class – Retrieves numbers from queue – Displays square

9. Problem! No synchronisation But we need more than just synchronized methods – Need to handle queue empty/full conditions wait() – Relinquishes lock, and waits for notification notify() and notifyAll() – Release one/all waiting threads – It/they must wait to reacquire the lock

10. Queue Class The add method: public synchronized void add (T item) { while (((tl + 1) % data.length) == hd) wait(); // No space tl = (tl + 1) % data.length; data[tl] = item; if (hd == -1) // First item in queue hd = tl; notifyAll(); // Release any consumers } // add A little more complicated – wait() may throw InterruptedException

11. Queue Class The remove method: public synchronized T remove () { while (hd == -1) // No data wait(); int tmpIndex = hd; if (hd == tl) // Was last element hd = tl = -1; else hd = (hd + 1) % data.length; notifyAll(); // Release waiting producers return data[tmpIndex]; } // remove

12. Exercise Experiment with slowing either the producer or the consumer down – Use Thread.sleep() Experiment with multiple producers and/or consumers

13. Threads in Java Full lifecycle New Runnable Running Dea d Blocked Blocking event Unblocked Scheduler run() completes start() Blocked in lock pool Blocked in wait pool synchronized wait() notify() or interrupt() Acquires lock

14. Deadlock Major problem in parallel systems Thread 1 obtains lock on object A Attempts to obtain lock on object B Thread 2 obtains lock on object B Attempts to obtain lock on object A Hard to detect and prevent – Enforce rigid ordering of obtaining multiple locks – suspend, resume and stop methods deprecated