Threads

1
Threads
2
Process

• A process is a program in execution
• It has
• Code
• Memory
• Other resources
• Files
• Printer
• Processor

3
Multiprogramming

• Many processes run concurrently
• OS
• For each user
• Browser
• Editor
• Compiler
• Etc.
• These processes must share physical resources

4
An Analogy

• A collection of threads is to a single process as
  a collection of processes is to a single computer
  system
• Both run in parallel
• Both share resources
• Threads share
• The process memory
• The process files

5
Some Background

• A process may fork off a child process which can
  fork off a child process
• Each of these processes are clones of one another
  except that each has its own address space
• Resulting in a tree-like organization of
  processes
• Communication between processes is done through
  special IPC mechanisms

6
Lightweight Processes

• Threads are sometimes called lightweight
  processes because
• They have separate threads of control (as do
  processes)
• But share memory (as processes do not)

7
Use

• Suppose we are downloading an audio clip
• We could either wait until the entire clip is
  downloaded or we could divide the process into
  threads
• One thread downloads
• One thread plays
• The key notion here is that threads are only
  useful when there is a mix of activities, some of
  which may block waiting on resources
• Threads yield no performance gain when all of
  them are CPU bound, but when there is substantial
  computing and also substantial i/o, having
  threads allows these activities to overlap, thus
  speeding up the application. Tannenbaum, Modern
  Operating Systems, p. 85, (2nd edition,
  Prentice-Hall).

8
Process Creation Examples

• Parent/Child
• Creates a child process
• Overwrites the child process with the name of a
  program gotten from the command line
• Notice the sequence of execution
• Pipes
• Create a childe process
• Communicate over an IPC mechanism called a pipe.

9
Concurrent Processesthe Downside

• Processes are expensive to create
• Historically, the type of concurrency that
  computers perform today generally has been
  implemented as operating system primitives
  available only to highly experienced systems
  programmers.

10
In Steps ADA/GUI

• Used by DOD
• Made concurrency widely available
• Development of GUIs made concurrency more useful
• Now
• Multithreading is available to all .NET
  programming languages
• Also a part of Java

11
A thread object is always in one of several states

• Unstarted program creates the object and passes
  a ThreadStart delegate to the objects
  constructor
• Running program calls the objects Start
  method. Now the newly running thread can run
  concurrently with any other threads that are part
  of the process.
• Blocked thread waits on a resource
• Stopped ThreadStart delegate terminates
• Suspended threads Suspend method has been
  called
• WaitSleepJoin
• Thread encounters code it cant execute. It
  waits until another thread tells it to resume
• Thread can decide to sleep for a prescribed time
  interval
• Thread should not continue until another thread
  finishes at which time it joins the just
  completed thread and resumes.

12
Thread Scheduling

• Every thread has a priorityThreadPriorityHighest
  to ThreadPriorityLowest
• Imagine all threads in a given priority on a
  separate list.
• Each thread in a list gets a burst of processor
  time called a quantum.
• If it doesnt finished, the next thread gets a
  burst.
• When all threads at a given priority level
  complete, the scheduler turns its attention to
  the next highest priority list.

13
Examples

• Ch 14 pgm58thread
• Program creates three threads of execution, each
  with a default priority Normal.
• Each thread displays a message indicating that it
  is going to sleep for a random amount of time in
  the interval 0 to 5000 milliseconds.
• When the thread awakens, it displays its name,
  terminates and enters the Stopped state
• The application consists of
• MessagePrinter
• The class containing the method executed by the
  thread
• Computes random interval
• Displays message
• _tmain (contained in ThreadTester.cpp)
• Creates three instances of MessagePrinter
• Creates three threads
• Starts them using the delegate class ThreadStart
• Passes each the name of the method in
  MessagePrinter to execute