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Concurrent Programming: Concepts, Java Threads, and Fork/Join Model

This lecture explores the concepts of concurrent programming, Java threads, and the fork/join model. It covers thread lifetime, communication, synchronization, and the creation of threads using both inheritance and implementing interfaces. It also introduces the fork/join model in Java.

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Concurrent Programming: Concepts, Java Threads, and Fork/Join Model

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  1. The University of North Carolina at Chapel Hill Lecture 37: Beyond Sequential Programs COMP 144 Programming Language Concepts Spring 2002 Felix Hernandez-Campos April 24 COMP 144 Programming Language Concepts Felix Hernandez-Campos

  2. Concurrent Programming • The previous lectures dealt with sequential programs • Programs with a single active execution context • It is also possible to develop programs with more than one execution context • They are concurrent programs • For example, • http://java.sun.com/docs/books/tutorial/essential/threads/ • We will refer to an execution context as a thread COMP 144 Programming Language Concepts Felix Hernandez-Campos

  3. Concurrent ProgrammingMotivation • Capture the logical structure of the problem • E.g., servers and graphical applications • Cope with independent physical devices • E.g., multiple processors in a real-time control system • Execute a single program in more than one processors • Parallel computing • E.g., scientific simulations such as weather prediction COMP 144 Programming Language Concepts Felix Hernandez-Campos

  4. Concurrent ProgrammingLanguage Support • Most languages support some sort of concurrent execution • We will discuss a number of approaches • The most important issues in concurrent programming are • Thread lifetime • Communication • Synchronization • Let’s illustrate these concepts briefly with Java COMP 144 Programming Language Concepts Felix Hernandez-Campos

  5. Concurrent ProgrammingJava Threads • We will follow the Java Tutorial • http://java.sun.com/docs/books/tutorial/essential/threads/ • Threads are the units of execution context in Java Concurrent Program Sequential Program COMP 144 Programming Language Concepts Felix Hernandez-Campos

  6. Thread Lifetime COMP 144 Programming Language Concepts Felix Hernandez-Campos

  7. Creating ThreadsExtending Class Thread public class SimpleThread extends Thread { public SimpleThread(String str) { super(str); } public void run() { for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { sleep((long)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); } } COMP 144 Programming Language Concepts Felix Hernandez-Campos

  8. Creating ThreadsImplementing Interface Runnable • See • http://java.sun.com/docs/books/tutorial/essential/threads/clock.html • Java API • Thread • http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Thread.html • Runnable • http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Runnable.html COMP 144 Programming Language Concepts Felix Hernandez-Campos

  9. Thread Lifetime • http://java.sun.com/docs/books/tutorial/essential/threads/lifecycle.html COMP 144 Programming Language Concepts Felix Hernandez-Campos

  10. Co-begin • Co-Begin (Algol-68, Occam, SR) • Sequential begin a := 3, b := 4 end • Concurrent par begin a := 3, b := 4 end COMP 144 Programming Language Concepts Felix Hernandez-Campos

  11. Co-begin • Co-Begin par begin p(a, b, c), begin d := q(e, f); r(d, g, h) end, s(i, j) end p(a, b, c) d:=q(e, f) r(d, g, h) s(i, j) COMP 144 Programming Language Concepts Felix Hernandez-Campos

  12. Parallel Loops • Each iteration is executed concurrently • SR co (i := 5 to 10 -> p(a, b, i) oc • Occam par i = 5 for 6 p(a, b, i) COMP 144 Programming Language Concepts Felix Hernandez-Campos

  13. Parallel Loops • Fortran • OpenMP !$OMP PARALLEL DO do i=1,n-1 A(i) = B(i) + C(i) enddo !$OMP END PARALLEL DO • HPF forall (i = 1:n-1) A(i) = B(i) + C(i) end forall COMP 144 Programming Language Concepts Felix Hernandez-Campos

  14. Data dependency Parallel Loops • Fortran • HPF forall (i = 1:n-1) A(i) = B(i) + C(i) A(i + 1) = A(i) + A(i + 1) end forall COMP 144 Programming Language Concepts Felix Hernandez-Campos

  15. Data dependency Parallel Loops • Fortran • HPF forall (i = 1:n-1) A(i) = B(i) + C(i) A(i + 1) = A(i) + A(i + 1) end forall • Data dependencies make sequences of statements inherently sequential • The two statement must execute sequentially • Updates to A(i+1) should not be seen by A(i) in the subsequent operation COMP 144 Programming Language Concepts Felix Hernandez-Campos

  16. Launch-At-Elaboration • Ada procedure P is task T is … end T; begin --P … end P; COMP 144 Programming Language Concepts Felix Hernandez-Campos

  17. Fork/Join • In previous mechanisms, the creation of threads is implicit, while it is explicit in the fork-join models • The fork join model is more general • E.g., Ada task type T is … begin … end T; pt : access T := new T; • Java threads follow the fork/join model Forks a new thread COMP 144 Programming Language Concepts Felix Hernandez-Campos

  18. Fork/Join • (a) Threads are always properly nested with co-begin statements • (b) The fork/join mechanism is more general • In Java, • Fork by creating a Thread and executing start() • Join by executing join() on the thread COMP 144 Programming Language Concepts Felix Hernandez-Campos

  19. Reading Assignment • Read Scott • Ch. 12 intro • Ch. 12.2 COMP 144 Programming Language Concepts Felix Hernandez-Campos

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