Chapter 4: Multithreaded Programming

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Chapter 4 Multithreaded Programming
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Chapter 4 Multithreaded Programming

• Overview
• Multithreading Models
• Threading Issues
• Pthreads
• Windows XP Threads
• Linux Threads
• Java Threads

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Threads vs. Processes
Creation of a new process using fork is
expensive (time memory). A thread (sometimes
called a lightweight process) does not require
lots of memory or startup time.
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fork()
fork()
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pthread_create()
Process A Thread 1
pthread_create()
Process A Thread 2
Stack
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Multiple Threads

• Each process can include many threads.
• All threads of a process share
• memory (program code and global data)
• open file/socket descriptors
• signal handlers and signal dispositions
• working environment (current directory, user ID,
  etc.)

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Thread-Specific Resources

• Each thread has its own
• Thread ID (integer)
• Stack, Registers, Program Counter
• errno (if not - errno would be useless!)
• Threads within the same process can communicate
  using shared memory.
• Must be done carefully!

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Single and Multithreaded Processes
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Benefits

• Responsiveness
• Resource Sharing
• Economy
• Utilization of MP Architectures

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User Threads

• Thread management done by user-level threads
  library
• Three primary thread libraries
• POSIX Pthreads
• Win32 threads
• Java threads

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Kernel Threads

• Supported by the Kernel
• Examples
• Windows XP/2000
• Solaris
• Linux
• Tru64 UNIX
• Mac OS X

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Multithreading Models

• Many-to-One
• One-to-One
• Many-to-Many

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Many-to-One

• Many user-level threads mapped to single kernel
  thread
• Examples
• Solaris Green Threads
• GNU Portable Threads

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Many-to-One Model
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One-to-One

• Each user-level thread maps to kernel thread
• Examples
• Windows NT/XP/2000
• Linux
• Solaris 9 and later

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One-to-one Model
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Many-to-Many Model

• Allows many user level threads to be mapped to
  many kernel threads
• Allows the operating system to create a
  sufficient number of kernel threads
• Solaris prior to version 9
• Windows NT/2000 with the ThreadFiber package

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Many-to-Many Model
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Two-level Model

• Similar to MM, except that it allows a user
  thread to be bound to kernel thread
• Examples
• IRIX
• HP-UX
• Tru64 UNIX
• Solaris 8 and earlier

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Two-level Model
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Posix Threads
We will focus on Posix Threads - most widely
supported threads programming API. Solaris - you
need to link with -lpthread On many systems
this also forces the compiler to link in
re-entrant libraries (instead of plain vanilla C
libraries).
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Thread Creation

• pthread_create(
• pthread_t tid,
• const pthread_attr_t attr,
• void (func)(void ),
• void arg)
• func is the function to be called.
• When func() returns the thread is terminated.

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pthread_create()

• The return value is 0 for OK.
• positive error number on error.
• Does not set errno !!!
• Thread ID is returned in tid

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Multithreaded C program using the Pthreads API (1)

• include ltpthread.hgt
• include ltstdio.hgt
• int sum / this data is shared by the thread(s)
  /
• void runner( void param ) / the thread /
• int main( int argc, char argv )
• pthread_t tid / the thread identifier /
• pthread_attr_t attr / set of thread
  attributes /
• if( argc ! 2 )
• fprintf( stderr, "usage a.out ltinteger
  valuegt\n" )
• return -1
• if( atoi(argv1) lt 0 )
• fprintf( stderr, "d must be gt 0\n", atoi(
  argv1 ) )
• return -1

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Multithreaded C program using the Pthreads API (2)

• / The thread will begin control in this function
  /
• void runner( void param )
• int i, upper atoi( param )
• sum 0
• for( i 1 i lt upper i )
• sum i
• pthread_exit( 0 )

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Java program for the summation of a non-negative
integer (1)

• class Sum
• private int sum
• public int getSum()
• return sum
• public void setSum( int sum )
• this.sum sum
• class Summation implements Runnable
• private int upper
• private Sum sumValue
• public Summation( int upper, Sum sumValue )
• this.upper upper

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Java program for the summation of a non-negative
integer (2)

• public class Driver
• public static void main( String args )
• if( args.length gt 0 )
• if( Integer.parseInt( args0 ) lt 0 )
• System.err.println( args0 " must be gt
  0." )
• else
• // create the object to be shared
• Sum sumObject new Sum()
• int upper Integer.parseInt( args0 )
• Thread thrd new Thread( new Summation(
  upper, sumObject ) )
• thrd.start()
• try
• thrd.join()
• System.out.println( "The sum of " upper
  " is " sumObject.getSum() )
• catch ( InterruptedException ie )

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Threading Issues

• Semantics of fork() and exec() system calls
• Thread cancellation
• Signal handling
• Thread pools
• Thread specific data
• Scheduler activations

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Semantics of fork() and exec()

• Does fork() duplicate only the calling thread or
  all threads?

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Thread Cancellation

• Terminating a thread before it has finished
• Two general approaches
• Asynchronous cancellation terminates the target
  thread immediately
• Deferred cancellation allows the target thread to
  periodically check if it should be cancelled

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Signal Handling

• Signals are used in UNIX systems to notify a
  process that a particular event has occurred
• A signal handler is used to process signals
• Signal is generated by particular event
• Signal is delivered to a process
• Signal is handled
• Options
• Deliver the signal to the thread to which the
  signal applies
• Deliver the signal to every thread in the process
• Deliver the signal to certain threads in the
  process
• Assign a specific threa to receive all signals
  for the process

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Thread Pools

• Create a number of threads in a pool where they
  await work
• Advantages
• Usually slightly faster to service a request with
  an existing thread than create a new thread
• Allows the number of threads in the
  application(s) to be bound to the size of the pool

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Thread Specific Data

• Allows each thread to have its own copy of data
• Useful when you do not have control over the
  thread creation process (i.e., when using a
  thread pool)

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Scheduler Activations

• Both MM and Two-level models require
  communication to maintain the appropriate number
  of kernel threads allocated to the application
• Scheduler activations provide upcalls - a
  communication mechanism from the kernel to the
  thread library
• This communication allows an application to
  maintain the correct number kernel threads

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Pthreads

• A POSIX standard (IEEE 1003.1c) API for thread
  creation and synchronization
• API specifies behavior of the thread library,
  implementation is up to development of the
  library
• Common in UNIX operating systems (Solaris, Linux,
  Mac OS X)

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Threads Support in Solaris 2

• Solaris 2 is a version of UNIX with support for
  threads at the kernel and user levels, symmetric
  multiprocessing, and real-time scheduling
• LWP intermediate level between user-level
  threads and kernel-level threads.
• Resource needs of thread types
• Kernel thread small data structure and a stack
  thread switching does not require changing memory
  access information relatively fast.
• LWP PCB with register data, accounting and
  memory information switching between LWPs is
  relatively slow
• User-level thread only need stack and program
  counter no kernel involvement means fast
  switching. Kernel only sees the LWPs that support
  user-level threads.

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Solaris 2 Threads
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Solaris Process
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Windows XP Threads

• Implements the one-to-one mapping
• Each thread contains
• A thread id
• Register set
• Separate user and kernel stacks
• Private data storage area
• The register set, stacks, and private storage
  area are known as the context of the threads
• The primary data structures of a thread include
• ETHREAD (executive thread block)
• KTHREAD (kernel thread block)
• TEB (thread environment block)

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Linux Threads

• Linux refers to them as tasks rather than threads
• Thread creation is done through clone() system
  call
• clone() allows a child task to share the address
  space of the parent task (process)

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Java Threads

• Java threads are managed by the JVM
• Java threads may be created by
• Extending Thread class
• Implementing the Runnable interface

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Java Thread States
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End of Chapter 4