4003-440-02 Operating Systems I Module 3. Thread Programming -- Lecture Notes Prof. Alan Kaminsky -- Winter Quarter 2012 Rochester Institute of Technology -- Department of Computer Science Introduction Reasons for writing a multi-threaded program To utilize multiple hardware processors (parallel programming) To keep executing other threads when one thread has to block To implement a simpler algorithm   Principal Java platform classes related to thread programming   Package java.lang Class Thread Interface Runnable   Package java.util.concurrent (JDK 1.5) Interface Callable<V> Interface Future<V> Interface ExecutorService Class Executors   Additional Java platform classes related to thread programming   Package java.lang Class ThreadGroup Class ThreadLocal<T> Class InheritableThreadLocal<T> A Sequential Program Class UC1 is the main program for the URL Counter Version 1. It counts lines, words, and bytes in the URLs given on the command line. It is a sequential (single-threaded) program.   Usage: java edu.rit.os1.UC1 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterSeq -- source code Class UC1 -- source code   Running time measurements Thread Creation, Alternative 1 Problem: Program UC1 spends a lot of time doing nothing, waiting for URL data to download   Solution: Do each URL in its own separate thread   Class UC2 is the main program for the URL Counter Version 2. It counts lines, words, and bytes in the URLs given on the command line. It is a multi-threaded program where each URL counter object is a subclass of class Thread.   Usage: java edu.rit.os1.UC2 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterThread -- source code Class UC2 -- source code   Pattern Write a subclass of class Thread, override the run() method Create an instance of the Thread subclass Call the thread's start() method Original thread returns from start() and continues executing New thread executes the thread's run() method When the run() method returns, the new thread terminates   "synchronized (System.out) { ... }" in class URLCounterThread   Running time measurements Waiting For Thread Termination Problem: Program UC2 prints its running time before any of the threads execute!   Solution: The main program must wait for all the threads to terminate before printing the running time   Class UC3 is the main program for the URL Counter Version 3. It counts lines, words, and bytes in the URLs given on the command line. It is a multi-threaded program where each URL counter object is a subclass of class Thread. Before returning, the main program waits for all the threads to terminate.   Usage: java edu.rit.os1.UC3 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterThread -- source code Class UC3 -- source code   Pattern To wait for a target thread to terminate, call join() on the target thread join() does not return until the target thread terminates The thread calling join() is blocked until the target thread terminates   Running time measurements Thread Creation, Alternative 2 Class UC4 is the main program for the URL Counter Version 4. It counts lines, words, and bytes in the URLs given on the command line. It is a multi-threaded program where each URL counter object implements interface Runnable but is not a subclass of class Thread. Before returning, the main program waits for all the threads to terminate.   Usage: java edu.rit.os1.UC4 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterRunnable -- source code Class UC4 -- source code   Pattern Write a class that implements interface Runnable, implement the run() method Create an instance of the runnable object Create an instance of class Thread, passing the runnable object to the thread constructor Call the thread's start() method Original thread returns from start() and continues executing New thread executes the runnable object's run() method When the run() method returns, the new thread terminates   Running time measurements Thread Creation, Alternative 3 Problem: That's a fair rigamarole to create all the threads and wait for them all to terminate -- isn't there a simpler way?   Solution: Use a thread pool -- new in JDK 1.5   Class UC5 is the main program for the URL Counter Version 5. It counts lines, words, and bytes in the URLs given on the command line. It is a multi-threaded program where each URL counter object implements interface Runnable but is not a subclass of class Thread. The main program uses a thread pool rather than creating threads directly.   Usage: java edu.rit.os1.UC5 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterRunnable -- source code Class UC5 -- source code   Pattern Write a class that implements interface Runnable, implement the run() method Create an ExecutorService (thread pool) using a static method of class Executors Several different kinds of thread pools are available We will generally use a cached thread pool, which reuses idle threads if possible and creates new threads if necessary Create an instance of the runnable object and pass it to the thread pool's execute() method Original thread returns from execute() and continues executing Thread pool executes the runnable object's run() method in a separate thread To wait for all threads to terminate, call shutdown() followed by awaitTermination() on the thread pool   Running time measurements Getting Results From Thread Executions Problem: Suppose we want to add up the counts for all the URLs   Solution: Use futures to return results back to the main thread -- new in JDK 1.5   Class UC6 is the main program for the URL Counter Version 5. It counts lines, words, and bytes in the URLs given on the command line. It is a multi-threaded program where each URL counter object implements interface Runnable but is not a subclass of class Thread. The main program uses a thread pool rather than creating threads directly. The main program also prints the total lines, words, and bytes in all the URLs.   Usage: java edu.rit.os1.UC6 url . . .   Package edu.rit.os1 Class URLCounter -- source code Class URLCounterCallable -- source code Class UC6 -- source code   Pattern Write a class that implements interface Callable<V>, implement the call() method to return a value of type V Create an ExecutorService (thread pool) using a static method of class Executors Create an instance of the callable object and pass it to the thread pool's submit() method submit() returns a future, an instance of interface Future<V>, to the original thread Save the future somewhere Thread pool executes the callable object's call() method in a separate thread and stores the call() method's return value in the corresponding future To get the callable object's return value, original thread calls get() on the future -- this blocks until the return value becomes available Must still call shutdown() on the thread pool   Running time measurements Segue Problem: But now we're back to processing the threads' results sequentially. Can't we accumulate the total counts concurrently also?   Problem: That's a fair rigamarole to store all the futures and then go back and process them all again -- isn't there a simpler way?   Solution: See Module 4, Thread Coordination. Operating Systems I • 4003-440-02 • Winter Quarter 2012 Course Page Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706 Home Page Copyright © 2008 Alan Kaminsky. All rights reserved. Last updated 27-Aug-2008. Please send comments to ark­@­cs.rit.edu.