SEG3550

1
SEG3550

• Introduction to
• Concurrent Programming in Java

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Introduction

• Between June 1985 and January 1987, a
  computerized radiation therapy machine called
  Therac-25 caused six known accidents involving
  massive overdoses with resultant deaths and
  serious injuries.
• Concurrent programming errors played an important
  part in these six accidents.

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Advantages of Concurrency

• For programs that interact with the environment
  to control some physical system, the parallelism
  and concurrency can be reflected in the control
  program structure.
• For example, a video on demand system that
  receiving data from the network and playing the
  movie. A matching server that receives many
  clients requests and matches the requests at the
  same time.

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Advantages of Concurrency

• Speed up response to user interaction by
  off-loading time-consuming tasks to separate
  processes.
• Throughput can be improved by using multiple
  processes to manage communication and device
  latencies. For example, network and disk access
  are slow compared to the processor speed.

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Whats a process?

• A process is the execution of a sequential
  program.
• The state of a process at any point in time
  consists of the values of explicit variables,
  declared by the programmer, and the implicit
  variables such as the program counter and the
  content of data/address registers.

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Process execution

• As a process executes, it transforms its state by
  executing statements.
• Each statement consists of a sequence of one or
  more atomic actions that make indivisible state
  changes.
• Each action causes a transition from the current
  state to the next state.
• The order in which actions are allowed to occur
  is determined by a transition graph that is an
  abstract representation of the program.

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State machine

• Consider the state machine for a light switch.
  Two actions on and off can operate on the
  light switch.
• Switch( on-gtoff-gtSwitch).
• on causes a transition from 0 to 1.
• off causes a transition from 1 to 0.

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State machine of a traffic light.

• TRAFFICLIGHT
• (red-gtorange-gtgreen-gtorange-gtTRAFFICLIGHT).

orange
red
green
0
1
2
3
orange
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Sequential Programming

• Traditional sequential programming
• Activities (procedures) are executed one after
  the other in a fixed order determined by the
  program and its inputs.
• The activities are not overlapped in time with
  another.

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Concurrent Programming

• Concurrent programming
• In concurrent programming, the execution of a
  concurrent program consists of multiple processes
  at the same time.
• Interleaving the concurrent execution does not
  require multiple processors.
• Interleaving instructions from multiple processes
  on a single processor can be used to simulate
  concurrency.

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Process in an OS point of view.

• A process in an operating system is a unit of
  resource allocation both for CPU time and for
  memory.
• A process in OS is represented by its code, data,
  and the state of machine registers.
• Each process in an operating system has its own
  address space and some special action must be
  taken to allow different processes to access
  shared data.

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OS Process

• The trading operating system process has a single
  thread of control. Just like the DOS age.
• Modern operating systems like Windows NT allows
  light weighted processes or threads for internal
  concurrency.
• A Process differs from a thread in such a way
  that an OS process has a data and a code segment
  however, it has multiple stacks for each thread.

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OS Process Thread
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Threading in Java.

• The operations to create and initialize threads
  and to subsequently control their execution are
  provided by the Java class Thread in the package
  java.lang.
• The program code executed by a thread is provided
  by the method run().
• Since Java does not support multiple inheritance,
  we can implement the Runnable interface.

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Extending the Thread Class.

• Class MyThread extends Thread
• Public void run()
• Put your code here.
• Thread a new MyThread()

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Implementing the Runnable

• Class MyRun implements Runnable
• Public void run
• ..
• Thread b new Thread(new MyRun())

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Thread Life Cycle

• Once created, start() causes a thread to call its
  run() and executes it as an independent activity.
• A thread terminates when the run() method returns
  or when it is stopped by stop(). A terminated
  thread cannot be restarted and it will be
  collected by the garbage collector.

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Thread Life Cycle

• The predicate isAlive() returns true if a thread
  has been started but not yet terminated.
• When started, a thread may be currently running
  on the processor, or it may be runnable but
  waiting to be scheduled. A running process may
  explicitly give up the processor using yield().

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Thread Life Cycle

• A thread may be non-runnable as a result of being
  suspended using suspend(). It can be made
  runnable again using resume().
• Sleep() causes a thread to be suspended for a
  given time and then automatically resume.