ITK Workshop

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ITK Workshop
Multi-Threading

• October 5-8, 2005

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ITK Workshop Multi-Threading

• Multi-Threading
• The portable Multi-Threader
• Mutual Exclusion
• ThreadedGenerateData
• BeforeThreadedGenerateData
• AfterThreadedGenerateData
• SetNumberOfThreads
• The Evil legacy of FORTRAN

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Insight Toolkit - Advanced Course

• The Concept of Multi-Tasking

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The Multi-Tasking Concept
Task A
Task B
Task C
Processor
The Operating System assigns processor time to
each task
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Insight Toolkit - Advanced Course

• Tasks have independent memory spaces
• They cannot communicate with each other unless
  they use sockets or files

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Insight Toolkit - Advanced Course

• The Concept of Multi-Threading

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The Multi-Threading Concept
Task A
T2
Processor
T1
T0
A Threading library creates threads and assigns
processor time to each thread
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Insight Toolkit - Advanced Course

• Threads from a common Task have the same memory
  space (Shared Memory)
• They can simultaneously access variables that
  were instantiated before creating the threads.

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The Multi-Threading in Multi-Processors
Task A
T2
Processor 1
T1
T0
Processor 2
Processor 3
Processor 4
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Insight Toolkit Multi-Threading Libraries

• Different Threading libraries are available in
  different platforms

• PThreads
• SProc
• Win32 Threads

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Insight Toolkit Multi-Threading Libraries

• ITK Provides a class for hiding the differences
  between threading libraries

itkMultiThreader
Win32Threads
PThreads
SProc
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Insight Toolkit - Advanced Course

• Using the itkMultiThreader

• Declare a Callback
• Instantiate an itkMultiThreader
• Set the number of Threads
• Set the callback
• Invoke SingleMethodExecute()

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Using the Multi-Threader
include itkMultiThreader.h int main( int
argc, char argv ) typedef
itkMultiThreader ThreaderType
ThreaderTypePointer threader
ThreaderTypeNew() const unsigned int
numberOfThreads atoi( argv1 )
threader-gtSetNumberOfThreads( numberOfThreads
) threader-gtSetSingleMethod(
ThreaderCallback, NULL ) threader-gtSingleMeth
odExecute()
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Defining the Theader Callback
include itkMultiThreader.h ITK_THREAD_RETURN_T
YPE ThreaderCallback( void arg ) typedef
itkMultiThreaderThreadInfoStruct
ThreadInfoType ThreadInfoType infoStruct
static_castlt ThreadInfoType gt( arg ) const
unsigned int threadId infoStruct-gtThreadID std
cout ltlt Thread ltlt threadId ltlt
stdendl ITK_THREAD_RETURN_TYPE value return
value
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Insight Toolkit - Advanced Course

• Exercise 31a

Run the Multi-Caster for different numbers of
Threads
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Insight Toolkit - Advanced Course

• DONT PUSH THIS BUTTON !

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Insight Toolkit - Advanced Course

• DONT TRY TO WRITE TO SHARED DATA FROM MULTIPLE
  THREADS

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Insight Toolkit - Advanced Course

• Exercise 31b

Do the wrong thing on purpose Attempt to write
to a static variable simultaneously from
different Threads
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Defining the WRONG Theader Callback
include itkMultiThreader.h static unsigned
long int Counter 0 ITK_THREAD_RETURN_TYPE
ThreaderCallback( void arg ) typedef
itkMultiThreaderThreadInfoStruct
ThreadInfoType ThreadInfoType infoStruct
static_castlt ThreadInfoType gt( arg )const
unsigned int threadId infoStruct-gtThreadID Cou
nter stdcout ltlt Thread ltlt
threadIdstdcout ltlt Counter ltlt Counter ltlt
stdendl ITK_THREAD_RETURN_TYPE value return
value
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Insight Toolkit - Advanced Course

• IF YOU REALLY REALLY WANT TO PUSH THIS BUTTON

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Insight Toolkit - Advanced Course

• Exercise 31c

Prevent the possible simultaneous access to share
memory by introducion Mutual Exclusion
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Defining the RIGHT Theader Callback
include itkMultiThreader.h include
itkSimpleFastMutexLock.h static unsigned long
int Counter 0 static itkSimpleFastMutexLock
mutex ITK_THREAD_RETURN_TYPE ThreaderCallback(
void arg ) mutex.Lock() Counter
mutex.UnLock() stdcout ltlt Counter ltlt
Counter ltlt stdendl ITK_THREAD_RETURN_TYPE
value return value
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Insight Toolkit - Advanced Course

• Exercise 31d

Use the itkRealTimeClock to measure how fast
(or how slow) the Mutual exclusion
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Timing the Mutual Exclusion Class
include itkSimpleFastMutexLock.h include
itkRealTimeClock.h static unsigned long int
Counter 0 static itkSimpleFastMutexLock
mutex int main() const unsigned int N
400000L itkRealTimeClock clock const
double T1 clock.GetTimestamp() for(unsigned
long i0 i lt N i) mutex.Lock()
Counter mutex.UnLock() const
double TT clock.GetTimestamp() T1
stdcout ltlt Time ltlt TT / N ltlt tdendl
ITK_THREAD_RETURN_TYPE value return value
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Insight Toolkit - Advanced Course

• Writing a Multi-Threading Filter

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Insight Toolkit Filters Hierarchy
itkProcessObject
public MultiThreader GetThreader() private
MultiThreader m_Threader int
m_NumberOfThreads
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Insight Toolkit Filters Hierarchy
itkImageSource
public virtual void BeforeThreadedGenerateData(
) virtual void AfterThreadedGenerateData()
virtual void ThreadedGenerateData(
const OutputImageRegionType region,
int threadId )
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Insight Toolkit GenerateData()
BeforeThreadedGenerateData
TGD
TGD
TGD
TGD
TGD
AfterThreadedGenerateData
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Insight Toolkit - Advanced Course

• END