NA-MIC
National Alliance for Medical Image Computing
http://na-mic.org
ITK Workshop
Multi-Threading
October 5-8, 2005
ITK Workshop – Multi-Threading
• Multi-Threading
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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
Processor
The Operating
System assigns
processor time
to each task
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Task B
Task C
Insight Toolkit - Advanced Course
Tasks have independent memory spaces
They cannot communicate with each other
unless they use sockets or files
National Alliance for Medical Image Computing
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Insight Toolkit - Advanced Course
The Concept of Multi-Threading
National Alliance for Medical Image Computing
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The Multi-Threading Concept
Task A
Processor
T2
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
Processor 1
T2
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
itk::MultiThreader
Win32
Threads
PThreads
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SProc
Insight Toolkit - Advanced Course
Using the itk::MultiThreader
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Declare a Callback
Instantiate an itk::MultiThreader
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 itk::MultiThreader ThreaderType;
ThreaderType::Pointer threader = ThreaderType::New();
const unsigned int numberOfThreads = atoi( argv[1] );
threader->SetNumberOfThreads( numberOfThreads );
threader->SetSingleMethod( ThreaderCallback, NULL );
threader->SingleMethodExecute();
}
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Defining the Theader Callback
#include “itkMultiThreader.h”
ITK_THREAD_RETURN_TYPE ThreaderCallback( void * arg )
{
typedef itk::MultiThreader::ThreadInfoStruct ThreadInfoType;
ThreadInfoType * infoStruct = static_cast< ThreadInfoType * >( arg );
const unsigned int threadId = infoStruct->ThreadID;
std::cout << “Thread = “ << threadId << std::endl;
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
DON’T PUSH THIS BUTTON !
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Insight Toolkit - Advanced Course
DON’T 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 itk::MultiThreader::ThreadInfoStruct ThreadInfoType;
ThreadInfoType * infoStruct = static_cast< ThreadInfoType * >( arg );
const unsigned int threadId = infoStruct->ThreadID;
Counter++;
std::cout << “Thread = “ << threadId;
std::cout << “Counter = “ << Counter << std::endl;
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 itk::SimpleFastMutexLock mutex;
ITK_THREAD_RETURN_TYPE ThreaderCallback( void * arg )
{
mutex.Lock();
Counter++;
mutex.UnLock();
std::cout << “Counter = “
<< Counter << std::endl;
ITK_THREAD_RETURN_TYPE value;
return value;
}
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Insight Toolkit - Advanced Course
Exercise 31d
Use the itk::RealTimeClock 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 itk::SimpleFastMutexLock mutex;
int main()
{
const unsigned int N = 400000L;
itk::RealTimeClock clock;
const double T1 = clock.GetTimestamp();
for(unsigned long i=0; i < N; i++)
{
mutex.Lock();
Counter++;
mutex.UnLock();
}
const double TT = clock.GetTimestamp() – T1;
std::cout << “Time = “ << TT / N << td::endl;
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
itk::ProcessObject
public:
MultiThreader * GetThreader();
private:
MultiThreader
int
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m_Threader
m_NumberOfThreads
Insight Toolkit – Filters Hierarchy
itk::ImageSource
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
National Alliance for Medical Image Computing
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