NA-MIC National Alliance for Medical Image Computing ITK Workshop October 5-8, 2005...

NA-MICNational Alliance for Medical Image Computing http://na-mic.org

ITK Workshop

October 5-8, 2005

Multi-Threading

National Alliance for Medical Image Computing http://na-mic.org

ITK Workshop – Multi-Threading

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


The Concept of Multi-Tasking

Insight Toolkit - Advanced Course


The Multi-Tasking Concept

Task A

Processor

Task B Task C

The Operating System assigns processor time

to each task


Tasks have independent memory spaces

They cannot communicate with each other unless they use sockets or files



The Concept of Multi-Threading



The Multi-Threading Concept

Task A

Processor

A Threading library creates threads and

assigns processor time to each thread

T0T1

T2


Threads from a common Task have the same memory space (Shared Memory)

They can simultaneously access variables that were instantiated before creating

the threads.



The Multi-Threading in Multi-Processors

Task A

Processor 1T0

T1T2

Processor 2

Processor 3

Processor 4


Different Threading libraries are available in different platforms

Insight Toolkit – Multi-Threading Libraries

• PThreads

• SProc

• Win32 Threads


ITK Provides a class for hiding the differences between threading libraries

Insight Toolkit – Multi-Threading Libraries

itk::MultiThreader

Win32Threads

PThreads SProc


Using the itk::MultiThreader


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


#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();

}

Using the Multi-Threader


Defining the Theader Callback


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;

}


Exercise 31a


Run the Multi-Caster for different numbers of Threads


DON’T PUSH THIS BUTTON !



DON’T TRY TO WRITE TO SHARED DATA FROM

MULTIPLE THREADS



Exercise 31b


Do the wrong thing on purpose:

Attempt to write to a static variable

simultaneously from different Threads


Defining the WRONG Theader Callback


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;}


IF YOU REALLY REALLY WANT TO PUSH THIS BUTTON



Exercise 31c


Prevent the possible simultaneous

access to share memory by

introducion Mutual Exclusion


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;}


Exercise 31d


Use the itk::RealTimeClock to measure how fast (or how slow)

the Mutual exclusion


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; }


Writing a Multi-Threading Filter



Insight Toolkit – Filters Hierarchy

itk::ProcessObject

public: MultiThreader * GetThreader();

private: MultiThreader m_Threader int m_NumberOfThreads


Insight Toolkit – Filters Hierarchy

itk::ImageSource

public: virtual void BeforeThreadedGenerateData(); virtual void AfterThreadedGenerateData();

virtual void ThreadedGenerateData( const OutputImageRegionType & region, int threadId );


Insight Toolkit – GenerateData()

BeforeThreadedGenerateData

AfterThreadedGenerateData

TGD TGD TGD TGD TGD


END


NA-MIC National Alliance for Medical Image Computing ITK Workshop October 5-8, 2005...

Documents

Transcript of NA-MIC National Alliance for Medical Image Computing ITK Workshop October 5-8, 2005...