Parallel programming in Java. Alexey A. Romanenko

Transcription

1 Parallel programming in Java Alexey A. Romanenko

2 Threads Threads use and exist within the process resources, yet are able to be scheduled by the operating system and run as independent entities within a process. A thread can possess an independent flow of control and be schedulable because it maintains its own. Thread is not a process

3 Thread

4 Multi-threading (1) Multi-threading is embedded in to Java. It supports by Java (syntax) and by Javamachine as well. Java class Thread provides user with wide functionality on launched and executed of parallel threads. Thread class provides low-level multithreading programming

5 Multi-threading (2) Threads synchronization is based on monitors. Each Java class has its own monitor which proves that shared data in parallel blocks are in consistent state

6 public class PI1 { static long num_steps = ; static double step; static double sum = 0.0; static int part_step; static class PITask extends Thread { int part; double x = 0.0; double sum = 0.0; public PITask(int part_number) {this.part = part_number; public void run(){ for (int i = part; i < num_steps; i += part_step) { x = (i + 0.5) * step; sum += 4.0 / (1.0 + x * x); public static void main(string[] args){ int i; double pi; step = 1.0 / (double) num_steps; part_step = Runtime.getRuntime().availableProcessors(); PITask[] part_sums = new PITask[part_step]; for (i = 0; i < part_step; i++) (part_sums[i] = new PITask(i)).start(); for (i = 0; i < part_step; i++) { try { part_sums[i].join(); catch (InterruptedException e) { sum += part_sums[i].sum; pi = step * sum; System.out.println(pi);

7 Notes to the example Create and start threads explicitly User should define parallel work in method run Launching thread with Thread.start() method. Not run()! Waiting for thread to complete with Thread.join() method Creating thread is expansive task

8 Scheduling Preemptive multitasking operating system interrupt the process and pass the execution to another one. Windows NT, Windows 2000+, Linux, Solaris Cooperative multitasking task (process) stop its execution and lets other process to continue their own. DOS, Windows 3.11

9 Selfish thread Native threads versus Green threads public void run(){ for (int i = part; i < num_steps; i += part_step) { x = (i + 0.5) * step; sum += 4.0 / (1.0 + x * x); Use yield() or sleep() to pass the execution to other threads

10 Thread priority Priority is a number from 1 to 10 Not exactly priority but relations between them is important Thread can pass the execution to other thread or be stopped by the scheduler. After that execution is passed to the thread with higher priority (if exist) Thread.setPriority() is used

11 Thread states Notify Wait sleep new Dead start run method exits Runnable Done sleep resume suspend I/O complete Block on I/O Blocked

12 Synchronization Use synchronized function modifier to read or write shared data. Monitor is used for thread synchronization Only one thread could enter into monitor Call of synchronized function is an attempt to enter into monitor. Function is blocked if monitor is occupied. Leaving the function is leaving the monitor

13 Threads interaction Interaction is based on wait-notify-notifyall mechanism This methods are implemented in Object class as final Calling of the methods should be performed only from synchronized methods Methods: wait stop current thread until notify() on the same object would call. notify wake up thread which called wait() first. notifyall wake up all threads

14 class Q { int n; synchronized int get() { System.out.println("Got: " + n); return n; synchronized void put(int n) { this.n = n; System.out. println("put: " + n); class Producer implements Runnable { Q q; Producer(Q q) { this.q = q; new Thread(this, "Producer").start(); public void run() { int i = 0; while (true) { q.put(i++); class Consumer implements Runnable { Q q; Consumer(Q q) { this.q = q; new Thread(this, "Consumer").start(); public void run() { while (true) { q.get(); class PC { public static void main(string args[]) { Q q = new Q(); new Producer(q); new Consumer(q);

15 Result С:\> java PC Put: 1 Got: 1 Got: 1 Got: 1 Got: 1 Got: 1 Put: 2 Put: 3 Put: 4 Put: 5 Put: 6 Put: 7 Access to Q.n is synchronized but ordering is not specified Additional action required

16 class Q { int n; boolean valueset = false; Improvement synchronized int get() { if (!valueset) try wait(); catch(interruptedexception e); System.out.println("Got: " + n); valueset = false; notify(); return n; synchronized void put(int n) { if (valueset) try wait(); catch(interruptedexception e); this.n = n; valueset = true; System.out.println("Put: " + n); notify();

17 Methods of Thread currentthread yield start run stop resume setpriority getpriority setname getname suspend

18 Group of threads ThreadGroup(name) ThreadGroup(parent_group, name) activecount() enumerate(thread[] list) getparent() interrupt() Thread(group, name) getthreadgroup()

19 Pipes Classes PipedInputStream, PipedOutputStream (bytes) PipedReader, PipedWriter (Unicode chars)

20 Extensions of Thread class java.util.concurrent standart package since Java standart 1.5. Pool of threads, and different synchronization primitives are provided by this package. EDU.oswego.cs.dl.util.concurrent contains some extentions of java.util.concurrent, FJTask is a part of this extension.

21 FJTask Implement fork-join model Designed for data based parallelism FJTask tasks are executed in a pool of thread FJTask allows one to use just basic thread operations like start(), yield(), join(). Setting priorities for threads is not allow. FJTask task should not contain difficult synchronization primitives and infinite loops

22 import EDU.oswego.cs.dl.util.concurrent.FJTask; import EDU.oswego.cs.dl.util.concurrent.FJTaskRunnerGroup; public class PI2 { static int num_steps = ; static double step; static double sum = 0.0; static int part_step; static class PITask extends FJTask { int i = 0; double sum = 0.0; public PITask(int i) { this.i = i; public void run() { double x = (i + 0.5) * step; sum += 4.0 / (1.0 + x * x); public static void main(string[] args) { int i; double pi; step = 1.0 / (double) num_steps; try { FJTaskRunnerGroup g = new FJTaskRunnerGroup( Runtime.getRuntime().availableProcessors()); PITask[] tasks = new PITask[num_steps]; for (i = 0; i < num_steps; i++) { tasks[i] = new PITask(i); g.invoke(new FJTask.Par(tasks)); for (i = 0; i < num_steps; i++) { sum += tasks[i].sum; pi = step * sum; System.out.println(pi); catch (InterruptedException ie) {

23 Notes to the example Work is defined in method run() Array of tasks is created and passed to FJTask.Par which means that these tasks should executes in parallel. invoke() method start the execution and wait for all tasks to complete. Tasks are executed on pool of threads. Number of threads is equal to the CPU number. It is recommended to assign a reasonable chunk of work to each task.